Yael Abraham, Ph.D.

Independent Chief Scientific Consultant

Singlet Oxygen Low-Dose Technology (patented in the U.S., South Korea, China, India, Mexico, Europe, and other countries worldwide)

Email: [email protected]

Abstract

This review examines the role of singlet oxygen (¹O₂) as a hormetic agent capable of activating adaptive stress responses without causing oxidative damage. We outline the molecular mechanisms involved, including Nrf2–Keap1 activation, MAPK signaling, and autophagy regulation, and discuss evidence from both endogenous processes and novel non-irradiative gas-phase generation technology. Observational case reports in asthma, COPD, sleep disorders, anemia, and chemotherapy-related symptoms are presented as preliminary support for potential therapeutic applications. The review identifies current knowledge gaps and highlights the need for controlled clinical trials to determine optimal dosing, safety, and long-term efficacy.

Keywords

Singlet oxygen, oxidative stress, hormesis, Nrf2, autophagy, inflammation, respiratory disease, immune modulation

1. Introduction

Redox biology has revolutionized our understanding of how organisms manage internal and environmental challenges. Reactive oxygen species (ROS), once viewed exclusively as harmful metabolic byproducts, are now recognized as essential signaling molecules that modulate adaptive stress responses, immune regulation and tissue repair. Among these ROS, singlet oxygen (¹O₂), an electronically excited form of molecular oxygen, plays a unique dual role. At high concentrations, it induces oxidative damage and cytotoxicity; however, at controlled low levels, it may act as a hormetic trigger, initiating protective pathways that enhance cellular function, immune regulation, and homeostasis.

Although singlet oxygen shares functional similarities with other reactive oxygen species such as superoxide and hydrogen peroxide, it has been comparatively understudied as a physiological signaling molecule. This is largely due to its extremely short lifespan, high reactivity, and the difficulty of delivering it in a controlled manner. It is known that singlet oxygen is generated endogenously in various biological contexts, including photoactivation in chloroplasts, mitochondrial respiration, and specific enzymatic reactions.^, In these settings, it can modulate key processes such as inflammation, apoptosis, and redox-sensitive gene regulation. Importantly, low-level exposure to singlet oxygen has been shown to enhance antioxidant defenses, influence immune cell behavior, and promote metabolic resilience, all hallmarks of a hormeticresponse.

While the hormetic properties of other ROS are well established, the specific signaling and systemic effects of low-dose singlet oxygen exposure in humans remain insufficiently studied. Recent advances in non-irradiative generation of singlet oxygen, particularly through catalytic interactions between air and transition metals, allow for safe, continuous, and non-invasive delivery of singlet oxygen gas at biologically relevant concentrations. These developments overcome many of the limitations associated with traditional photosensitizer-based systems, such as photobleaching, dose inconsistency, and tissue penetration constraints. As a result, they open new avenues for research into singlet oxygen’s preventive and regulatory effects in human health, independent of therapeutic photodynamic contexts.

This review aims to synthesize current understanding of low-dose singlet oxygen as a hormetic modulator, highlighting its effects on systemic and organ-specific functions, particularly in immune regulation and stress response pathways. In this first part of the review, we focus on the mechanistic foundations of singlet oxygen signaling, emerging clinical observations, and the biological plausibility of its preventive potential. By integrating experimental insights with real-world exposure data, we propose a framework for singlet oxygen as a non-pharmacological, redox-based intervention that complements the body’s innate regulatory networks, inviting further empirical validation and exploration.

2. Singlet Oxygen: Properties and Generation

2.1. Chemistry and Electronic States of Oxygen

Oxygen constitutes approximately 20% of atmospheric air and is vital for aerobic life. In its most stable form, dioxygen (O₂) exists as a diradical in the ground triplet state (³Σg⁻), characterized by two unpaired electrons in separate degenerate orbitals (Figure 1-a). This electronic configuration imposes spin restrictions on reactions with singlet-state molecules (which most biological molecules are) but allows reactivity with radicals.

Excitation of molecular oxygen leads to two singlet states: ¹Δg and ¹Σg⁺ (Figure 1b and 1c, respectively). The higher-energy ¹Σg⁺ state is extremely reactive but short-lived, rapidly relaxing to the lower-energy ¹Δg state, commonly referred to as singlet oxygen (¹O₂). In the gas phase, isolated singlet oxygen (¹Δg) has a relatively long lifetime (~72 minutes), but this decreases significantly with increased probability of collisions in liquids or under higher pressure/temperature. Under these conditions, the lifetime can reduce to mere seconds in the gas phase. In solutions, the lifetime of singlet oxygen is even shorter, ranging from microseconds to nanoseconds, depending on the solvent properties.

Figure 1 Occupation of molecular orbitals in oxygen at different energetic states: (a) triplet ground state, ³_g^–, (b) Most stable singlet state, ¹_g, (c) Highest energy, short- lived singlet state, ¹Σ_g⁺.

2.2. Singlet Oxygen Generation in Liquid & Gas Phase

Singlet oxygen can be artificially produced through a range of methods, with most traditional techniques relying on liquid-phase systems. One of the most established ischemical generation, which involves reactions such as the decomposition of trioxidane in water or reaction of hydrogen peroxide with sodium hypochlorite (Figure 2). This method is effective in liquids but produces reactive byproducts and has short diffusion range.

Figure 2 Examples for chemical reactions to produce singlet oxygen, ¹O₂: (a) decomposition of trioxidane in water; (b) reaction of hydrogen peroxide with sodium hypochlorite.

Another common method is photosensitization, in which light-excitable compounds (photosensitizers) such as porphyrins transfer energy to ground-state triplet oxygen, producing singlet oxygen via a type II energy transfer mechanism (Figure 3).This light-driven process is the foundation of photodynamic therapy (PDT), a clinically validated modality used in the treatment of certain cancers, microbial infections, and dermatological disorders. Despite its efficacy, photosensitized singlet oxygen production requires localized light exposure, often limiting treatment to surface tissues or necessitating invasive delivery systems. Moreover, outcomes are influenced by complex pharmacokinetics, oxygen availability, and potential phototoxicity. Other methods include Plasma-induced excitation which generates mixed ROS, including singlet oxygen, without photosensitizers and laser/microwave excitation which is used primarily in specialized high-energy applications.

Figure 3 Energy diagram illustrating singlet oxygen (¹O₂) that is generated from ground state triplet oxygen (³O₂) via energy transfer from the excited state of a photosensitizer to the oxygen molecule upon irradiation. 

To expand singlet oxygen applications beyond solution-phase systems, alternative gas-phase generation strategies have been developed.

2.2.1. Singlet Oxygen Generation in Gas Phase

Gas-phase singlet oxygen has a longer intrinsic lifetime and greater diffusion capacity compared with singlet oxygen generated in liquid or solution-constrained environments.⁶ These properties enable non-invasive delivery across larger biological or environmental surfaces and remove the need for liquid media, photonic targeting, or pharmacokinetic considerations. 

The most common method is photosensitization, in which immobilized dyes (e.g., porphyrins, Rose Bengal) absorb light and transfer energy to ground-state oxygen.Plasma-based methods employ non-thermal plasma to directly excite molecular oxygen, generating a mixture of reactive oxygen species (ROS) that includes singlet oxygen, without the need for light or sensitizers. Laser or microwave excitation is primarily used in high-energy or defense-related contexts. More recently, heterogeneous photocatalysis has been applied, where solid catalysts (e.g., doped TiO₂, ZnO) generate singlet oxygen from atmospheric oxygen under UV or visible light.

A novel non-irradiative catalytic approach produces gas-phase singlet oxygen by passing ambient air through metal-based substrates engineered to have controlled oxygen affinity. Here, oxygen molecules are transiently adsorbed and excited to the singlet state before release, without fully oxidizing the metal surface. The yield depends on the metal alloy composition and airflow parameters, enabling controlled low-dose output suitable for continuous operation.

In contrast, irradiative photosensitized systems integrate light sources to activate immobilized sensitizers, producing singlet oxygen via triplet–singlet energy transfer. While effective in controlled environments, these systems can face challenges in long-term or low-dose use, including photobleaching, sensitizer degradation, sensitivity to humidity and temperature, and the need for precise optical alignment. Such factors may limit their scalability for continuous, unattended clinical or domestic operation.

Although photosensitizer-based gas-phase systems are well established for laboratory disinfection and air treatment, non-irradiative catalytic platforms offer a light-independent alternative with fewer maintenance requirements. Their application in therapeutic contexts remains relatively new, and their physiological implications are explored in subsequent sections of this review.

2.3. Natural Generation in Plants and Animals

Singlet oxygen is a natural product of various biological processes. For instance, chlorophyll, that is the green pigment essential for photosynthesis in leaves, can function as a natural photosensitizer, especially under high light intensity. At these conditions,singlet oxygen is formed, which, in turn, acts as a signaling molecule, triggeringphotoinhibition, which is a protective mechanism that prevents damage to the photosynthetic apparatus from excessive light exposure.

In mammalian cells, singlet oxygen is continuously generated through multiple light-independent pathways. In non-photosensitized tissues, enzymatic systems such as myeloperoxidase (MPO) in neutrophils and eosinophils generate singlet oxygen during immune responses. This occurs through MPO-catalyzed reactions involving hydrogen peroxide and chloride ions, yielding hypochlorous acid (HOCl), which can decompose into singlet oxygen.

Beyond immune cells, singlet oxygen is formed in multiple intracellular compartments, such as peroxisomes, endoplasmic reticulum, and, most prominently, mitochondria. Mitochondria, the organelles responsible for energy (ATP) production through aerobic respiration, use oxygen as an electron source. Mitochondrial respiration inherently produces ROS as byproducts, including hydrogen peroxide (H₂O₂), hydroxyl radicals (OH•), superoxide anion radicals (O₂•⁻) and singlet oxygen.

3. Oxidative Stress, Homeostasis, and Hormesis

3.1. Dose-Dependent Effects and Thresholds of Toxicity

The biological activity of singlet oxygen is defined by a delicate balance between concentration and exposure duration. The constant production of singlet oxygen in the body leads to a baseline concentration estimated to be around 10⁻¹³ M under neutral conditions, which is far below the threshold for cellular damage. In contrast, concentrations reaching approximately 10⁻⁸ M can damage cell membranes, while local concentrations of 10⁻⁵–10⁻⁴ M are associated with irreversible oxidative damage and cell death.

Cellular and tissue damage occurs when ROS, including singlet oxygen, oxidize electron-rich sites, such as double bonds and thiol groups, in key biomolecules (Table 1).Critical cellular targets include pigments and antioxidants (e.g., chlorophyll, hemoglobin), proteins (e.g., enzymes, structural and transport proteins, receptors), lipids (e.g., membrane phospholipids), and nucleic acids. Damage to these macromolecules disrupts cellular integrity and function, contributing to pathological conditions.

Table 1 Chemical activity of singlet oxygen with various biomolecules.^22,26–28

Biomolecule Type	Key Reactive Groups	Reaction Type	Example Products	Biological Outcomes
Lipids (PUFAs)	Allylic C=C double bonds	Ene reaction	Lipid hydroperoxides (e.g., 13-HPODE)	Membrane damage, lipid peroxidation chain reactions, formation of signaling aldehydes (e.g., 4-HNE), inflammation
Proteins	Trp, His, Met, Cys	Oxidation of side chains	Methionine sulfoxide, N-formylkynurenine, 2-oxo-histidine	Loss of enzyme activity, altered protein folding, proteasomal degradation, redox signaling
Nucleic Acids	Guanine (mainly)	Base oxidation	8-oxo-7,8-dihydroguanine (8-oxoG)	Mutagenesis, transcriptional errors, impaired DNA replication, contributes to age-related diseases and cancer
Cholesterol	Allylic hydrogens, Δ5 double bond	Addition/Oxidation	7-hydroperoxycholesterol, 25-hydroxycholesterol	Altered membrane structure, LXR pathway modulation, potential atherogenesis, implicated in inflammatory diseases
Pigments and antioxidant(e.g., carotenoids, tocopherols)	Conjugated π-systems	Physical/chemical quenching	Inert or less-reactive byproducts	Impair function and protection from oxidative stress

The concept of oxidative stress arises when reactive oxidative species like singlet oxygen accumulate beyond the buffering capacity of cellular antioxidant systems. The potential for cellular and tissue damage underscores the importance of preventing ROS from reaching harmful levels. It has been found that at low concentrations, ROS essentially act as signalling molecules, as they activate cellular stress-response pathways that include the upregulation of antioxidant defences and mechanisms to repair existing damage.¹ In this way, ROS play a crucial role in supporting cellular health and adaptability. By acting as signalling molecules, they not only alert the system to potential threats but also help initiate processes that restore balance and promote resilience in the face of environmental and physiological challenges. For Instance, moderate levels of singlet oxygen have been shown to enhance mitochondrial activity and stimulate energy metabolism and boost ATP production. Instead of causing damage, this mild oxidative exposure serves as a signal that ramps up the cell’s bioenergetic capacity and can activate protective pathways, effectively priming the cell’s redox defenses.

3.2. The Plasma Membrane as a Redox-Sensitive Platform for Singlet Oxygen Signaling

At sub-toxic concentrations, singlet oxygen is increasingly recognized not merely as a byproduct of oxidative metabolism but as a finely tuned signaling mediator that operates at the cellular interface. These effects have been identified across diverse biological systems, including plants^,, bacteria and mammals¹. One of its primary sites of action is the plasma membrane where it chemically modifies specific lipids and proteins, effectively acting indirectly like receptors that, in turn, trigger or inhibit processes within the cell.^30, Among the most reactive are polyunsaturated fatty acids (PUFAs) in phospholipids, whose oxidation produces secondary messengers like 4-hydroxynonenal (4-HNE), known to activate redox-sensitive transcription factors such as Nrf2 and NF-κB. Furthermore, cholesterol, a major component of lipid rafts, is readily oxidized by singlet oxygen to form cholesterol hydroperoxides and oxysterols, such as 7-ketocholesterol and 25-hydroxycholesterol. These modulate lipid raft dynamics and membrane signaling. These oxidized derivatives influence membrane fluidity and can engage Liver X Receptors (LXRs) and Toll-like receptors (TLRs), amplifying inflammatory or metabolic responses. Additionally, singlet oxygen can oxidize cysteine and methionine residues on membrane proteins, modulating ion channels and receptor conformations. Together, these modifications convert the plasma membrane into a dynamic redox-sensitive signaling platform, with singlet oxygen acting as an initiator of downstream adaptive or immunological responses, leading to a form of stress-response hormesis

3.3. Stress-Response Hormesis: Definition, Mechanisms, and Relevance to Singlet Oxygen

Hormesis is broadly defined as a biphasic dose-response phenomenon where low levels of an otherwise harmful agent trigger beneficial adaptive responses, while high levels cause damage. Within this framework, stress-response hormesis refers specifically to the cellular and systemic activation of protective pathways in response to low-level stressors that mildly perturb homeostasis. Such responses are typically transient, self-limiting, and aimed at enhancing cellular resilience, repair capacity, and metabolic adaptability.

The hormetic process is evolutionarily conserved across species and encompasses various stressors including xenobiotic exposure, heat shock, caloric restriction andexercise, hypoxia, and mild oxidative stress. These stressors induce cytoprotective mechanisms such as upregulation of antioxidant enzymes (e.g., superoxide dismutase, catalase), enhanced DNA repair, autophagy, mitochondrial biogenesis, and immune modulation.^151, For example, transient increases in ROS generated during physical exercise or ischemic preconditioning are known to improve cardiovascular and neurological outcomes through activation of redox-sensitive pathways like Nrf2, HIF-1α, and AMPK .

What distinguishes singlet oxygen as a stress-response hormetic agent is its non-accumulative, non-reactive decay mechanism. Singlet oxygen naturally and spontaneously relaxes back to its ground state triplet oxygen, often releasing energy in the form of phosphorescence or heat, without necessarily undergoing a chemical reaction. This sets it apart from other ROS such as superoxide or hydrogen peroxide, which must be enzymatically detoxified or can diffuse and cause unintended chain reactions. This inherent reversibility reduces the risk of cumulative oxidative damage during therapeutic applications, particularly in ambient gas-phase exposures, where its effects are limited in both time and space.

Additionally, as singlet oxygen’s reactivity can give rise to oxidized lipid-derived mediators that activate redox‑sensitive pathways such, this bypasses the need for singlet oxygen itself to accumulate intracellularly. 

In summary, singlet oxygen acts as a prototypical agent of stress-response hormesis: it is transient, does not accumulate, preferentially activates adaptive signaling pathways, and avoids direct chemical interaction unless at elevated concentrations. These characteristics make it a promising modality for non-pharmacological preconditioning, immune modulation, and metabolic resilience. The mechanisms through which singlet oxygen exerts its hermetic effect will be elaborated upon in the following sections.

4. Hormetic Mechanisms Induced by Singlet Oxygen

Singlet oxygen exerts concentration-dependent effects on biological systems. At elevated levels, it contributes to oxidative damage and cell death. However, at sub-toxic concentrations, singlet oxygen triggers a hormetic response, activating intrinsic cellular defense mechanisms by upregulation of antioxidant enzymes, redox-sensitive signaling, immune modulation, and autophagic processes. This hormetic response promotesresilience, repair, and redox homeostasis.

This chapter outlines the molecular pathways and systems influenced by low-dose singlet oxygen, providing a mechanistic foundation for its therapeutic potential.

4.1. Nrf2-Keap1 Pathway and ARE Gene Activation

One of the primary oxidative stress factors activated by singlet oxygen is Nrf2 (Nuclear factor erythroid 2–related factor 2), which plays a critical role in regulating the antioxidant defense system through the Nrf2-Keap1 pathway. Nrf2 is a transcription factor that regulates the expression of various antioxidant and cytoprotective genes. Under normal conditions, Nrf2 is bound to its inhibitor, Keap1 (Kelch-like ECH-associated protein 1), which promotes its degradation. However, when cells are exposed to singlet oxygen or other ROS, cysteine residues in Keap1 become oxidized. This oxidation disrupts the interaction between Nrf2 and Keap1, freeing Nrf2 to move into the nucleus. Inside the nucleus, Nrf2 binds to antioxidant response elements (ARE) in the promoter regions of specific genes, stimulating the production of Phase II detoxification and antioxidant defense enzymes and other various enzymes and proteins involved in cellular protection, metabolism, redox balance, and stress responses. 

Major downstream Nrf2 targets include:

4.1.1. Glutathione synthesis

Nrf2 upregulates the catalytic (GCLC) and modifier (GCLM) subunits of glutamate-cysteine ligase, which is the rate-limiting enzyme in glutathione (GSH) biosynthesis.^, GSH is a tripeptide that acts as a redox buffer that neutralizes hydrogen peroxide, lipid peroxides, and electrophilic agents via enzymatic and non-enzymatic reactions. In addition, it maintains cellular proteins in their reduced thiol form, preventing aberrant disulfide cross-linking that can impair enzymatic activity and cellular signaling.By preserving this thiol-disulfide balance, GSH supports proper protein folding, redox-sensitive signal regulation, and cytoskeletal organization.

Importantly, Nrf2-dependent enhancement of GSH synthesis also contributes to tissue repair mechanisms. Elevated GSH levels facilitate fibroblast migration, reduce oxidative delay in re-epithelialization, and accelerate wound closure under inflammatory conditions. Thus, GSH not only buffers oxidative stress but orchestrates a redox environment conducive to tissue regeneration.

While GSH constitutes a primary redox buffer, Nrf2 orchestrates a broader detoxification and redox-regulatory network, as outlined below.

4.1.2. Detoxification

In addition to enhancing glutathione synthesis, Nrf2 drives the expression of a wide array of Phase II detoxification enzymes that help neutralize reactive intermediates and limit their downstream damage. Unlike redox buffers such as GSH that directly scavenge reactive oxygen species, these enzymes act primarily by converting electrophilic and pro-oxidant species into more stable and excretable forms, thereby preventing ROS amplification and preserving cellular integrity.

• NAD(P)H quinone oxidoreductase 1 (NQO1) catalyzes the two-electron reduction of quinones to hydroquinones, thereby preventing redox cycling and ROS amplification.

• heme oxygenase-1 (HO-1) degrades pro-oxidant heme into biliverdin, free iron, and carbon monoxide; biliverdin and its product bilirubin act as potent endogenous antioxidants, while carbon monoxide exhibits anti-inflammatory effects.

• Peroxiredoxins (Prxs) catalyze the reduction of peroxides and modulate redox signaling by buffering fluctuations in H₂O₂ levels.

Together, these detoxification enzymes form a complementary defense layer that acts upstream of or in concert with antioxidant systems like glutathione. They expand the cell’s capacity to disarm a broad spectrum of harmful metabolites arising from xenobiotic metabolism, lipid peroxidation, or chronic inflammation. By converting reactive substrates rather than simply neutralizing ROS, this arm of the Nrf2 response plays a vital role in restoring redox balance under sustained or complex stress conditions.

4.1.3. Redox regulation and lipid-derived mediators

Beyond antioxidant buffering and detoxification, Nrf2 regulates enzymes that fine-tune redox-sensitive signaling networks, modulate inflammation through lipid mediators, and safeguard proteostasis in cells under stress. These systems are particularly critical in long-lived or non-dividing cells, including muscle fibers and neurons, where persistent oxidative fluctuations can disrupt protein function and immune homeostasis.

A key component of Nrf2’s redox regulation is thioredoxin reductase 1 (TXNRD1), which maintains thioredoxin in its reduced form, enabling it to reduce disulfide bonds in proteins, thus repairing them, and ensuring redox signal fidelity.^,Superoxide dismutases (SOD1–3), also under Nrf2 control, catalyze the dismutation of superoxide radicals into hydrogen peroxide, which a less reactive species subsequently detoxified by catalase or glutathione peroxidase. These systems help maintain intracellular redox gradients, allowing cells to sense and adapt to oxidative cues without triggering damage.

In addition, Nrf2 influences the metabolism of lipid-derived signaling molecules through enzymes such as arachidonate 15-lipoxygenase (ALOX15). ALOX15 converts polyunsaturated fatty acids into bioactive mediators like 15-HETE and lipoxins, which resolve inflammation, promote macrophage polarization toward anti-inflammatory phenotypes, and regulate redox-sensitive immune responses. This lipid signaling arm of the Nrf2 pathway helps balance the inflammatory phase of stress responses with timely resolution and tissue restoration.

To maintain proteostasis during oxidative stress, Nrf2 also upregulates genes encoding specific proteasome subunits, enhancing both 20S and 26S proteasome activity.This supports the clearance of misfolded or oxidatively modified proteins, particularly under conditions where autophagy is impaired or ATP is limited. The ATP-independent 20S core is especially crucial for degrading damaged proteins in energy-stressed cells.

Moreover, Nrf2 intersects with the cellular chaperone machinery to reinforce proteome stability. It sustains redox conditions favorable for protein folding and coordinates with heat shock proteins (HSPs) such as HSP70, assists in protein folding and prevents aggregation under stress. HSP70 also activates activating transcription factor 4 (ATF4), a regulator of the integrated stress response that promotes survival and redox balance. This redox–chaperone axis is particularly important in long-lived cells, such as neurons, where oxidative injury can destabilize protein structure and impair cellular function.

Through these interconnected pathways, which encompase redox signaling fidelity, resolution-phase lipid mediators, and stress-adaptive protein quality control, Nrf2 equips cells to withstand prolonged oxidative challenges without resorting to indiscriminate ROS scavenging. These mechanisms are especially vital in sensitive tissues such as the nervous and cardiovascular systems, where maintaining structural and signaling integrity under oxidative stress is essential for long-term cellular resilience.

4.1.4. Mitochondrial regulation

Nrf2 plays a pivotal role in preserving mitochondrial function during oxidative stress by supporting transcriptional programs that regulate mitochondrial biogenesis, redox homeostasis, and organelle quality control. These functions enable cells to sustain energy production while minimizing mitochondrial-derived oxidative damage. One key axis involves Nrf2’s regulation of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a master regulator of mitochondrial biogenesis. PGC-1α co-activates nuclear respiratory factors (NRF1 and NRF2/GABPA), which promote the expression of mitochondrial transcription factor A (TFAM) and other genes involved in mitochondrial DNA replication and respiratory chain assembly. This coordination supports mitochondrial renewal and metabolic flexibility in stress-challenged cells.

Under mild oxidative conditions, Nrf2 activity supports mitochondrial integrity by enhancing the expression of enzymes such as SOD2 and GPX4, which are critical in detoxifying mitochondrial ROS and preventing lipid peroxidation. It also works in conjunction with redox-sensitive regulatory proteins like DJ-1 (PARK7), which stabilizes Nrf2 by inhibiting its Keap1-mediated degradation and shields mitochondria from oxidative injury. DJ-1 thus serves as a redox sentinel that bridges Nrf2 activity with mitochondrial protection. This contributes not only to energetic resilience but also to redox signal compartmentalization, enabling cells to respond proportionally to transient mitochondrial stress.

Beyond antioxidant defense, Nrf2 influences mitochondrial quality control via indirect crosstalk with AMP-activated protein kinase (AMPK) and autophagy pathways (that will be elaborated upon later in this review). This includes modulation of mitophagy, the selective degradation of damaged mitochondria, which prevents ROS leakage and preserves metabolic homeostasis. In highly active and stress-sensitive tissues such as the brain, heart, and liver, these pathways converge to align mitochondrial function with the cell’s adaptive redox state.

Through these mechanisms, Nrf2 ensures that mitochondrial bioenergetics remain resilient under oxidative stress, not only by protecting against damage but by actively integrating mitochondrial health into the cell’s broader adaptive response network.

4.1.5. Anti-Inflammatory and Cytoprotective Roles of Nrf2

Beyond its antioxidant and metabolic functions, Nrf2 plays a critical role in shaping the immune response by modulating inflammation at the transcriptional level. Upon activation, Nrf2 suppresses pro-inflammatory signaling pathways, notably nuclear factor kappa B (NF-κB), and induces the expression of anti-inflammatory mediators that restore immune balance.

Among its downstream targets, heme oxygenase-1 (HO-1) and ferritin serve dual roles as antioxidant and immunoregulatory proteins. HO-1 not only degrades pro-oxidant heme but also generates carbon monoxide and biliverdin, which exert anti-inflammatory effects by inhibiting cytokine production and leukocyte recruitment. Ferritin sequesters free iron, reducing oxidative inflammation and dampening tissue damage in inflammatory processes.

Through these and related mechanisms, Nrf2 activation leads to the downregulation of pro-inflammatory cytokines such as IL-6, TNF-α, and IL-1β. This immune modulation limits excessive inflammation, curtails immune cell hyperactivation, and preserves tissue function across a range of pathological contexts, including:

• Autoimmune diseases: Suppresses cytokine overproduction (so called “cytokine storms”) and limits tissue injury in disorders such as lupus and multiple sclerosis.

• Metabolic syndromes: Attenuates chronic low-grade inflammation associated with insulin resistance, diabetes, and obesity.

• Pulmonary disorders: Mitigates airway inflammation in asthma, chronic obstructive pulmonary disease (COPD), and acute lung injury.

• Neurodegenerative diseases: Modulates microglial reactivity and reduces neuroinflammation in conditions like Alzheimer’s and Parkinson’s disease.

By integrating redox regulation with immune restraint, Nrf2 acts as a central node in the cytoprotective response to both oxidative and inflammatory stress. This dual role underpins its emerging relevance in therapeutic strategies aimed at promoting resilience in redox-sensitive tissues.

4.2. MAPK Signaling and Oxidative Stress Sensors

Beyond the Nrf2-Keap1 pathway, low level singlet oxygen can activate several additional signaling pathways involved in stress response, inflammation regulation, cell death and repair mechanisms. These pathways ensure that cells maintain homeostasis, limit damage, and initiate adaptive processes during oxidative stress. 

For example, the p53 tumor suppressor protein, a key regulator of DNA repair and apoptosis, is activated in response to oxidative stress, helping cells manage or eliminate damage. Similarly, the MAPK family consists of several key protein kinases, including extracellular signal-regulated kinase (ERK1/2), c-Jun N-terminal kinase (JNK), and p38 MAPK, which are activated in response to various stress signals, including singlet oxygen. It was found these kinases are activated through upstream stress sensors, including oxidative modifications of receptor proteins, mitochondrial ROS signaling, and changes in redox balance.

These processes are critical for managing oxidative damage to DNA, lipids, and proteins, and initiate appropriate adaptive or apoptotic responses depending on stress intensity and duration.

4.3. Autophagy and Cell Fate Regulation

Singlet oxygen plays a complex and context-dependent role in cellular stress responses, with the capacity to promote either cell survival or cell death. This duality is mediated through its ability to induce autophagy or apoptosis, two distinct yet interconnected processes that are often activated sequentially in response to stress. Autophagy, a highly conserved mechanism present in all eukaryotes, from unicellular organisms to mammals. This mechanism is responsible for selectively degrading oxidized and damaged intracellular components, such as proteins and organelles (particularly mitochondria), via the lysosomal pathway. This process is essential for preserving cellular integrity and function during mild stress.

4.3.1. Oxidative Stress Intensity as a Determinant of Cell Fate

The outcome of oxidative stress is not binary but graded, depending on its intensity, duration, and localization.^35,, At low to moderate levels, oxidative stress initiates adaptive responses, including Nrf2 activation and the induction of autophagy, enabling cells to restore homeostasis. In contrast, sustained or intense oxidative insults can overwhelm these defenses, tipping the balance toward apoptosis or necrosis.

Importantly, singlet oxygen can directly oxidize signaling molecules that govern this threshold. For example, it modifies cysteine residues on redox-sensitive kinases and phosphatases, leading to the activation of ASK1, a MAP3K that mediates stress responses including autophagy and apoptosis. Similarly, singlet oxygen promotes the disruption of Bcl-2–Beclin-1 complexes via oxidative modifications of Bcl-2, releasing Beclin-1 and enabling autophagy initiation.^81,82,

In addition, singlet oxygen can influence the hypoxia-inducible factor 1 (HIF-1) pathway, which plays a pivotal role in cellular responses to low oxygen levels. While the HIF-1 pathway is primarily known for regulating genes that adapt to hypoxic conditions, it also intersects with autophagy mechanisms.

​These immediate oxidative modifications allow singlet oxygen to act as a first messenger in stress adaptation, influencing the balance between repair and cell death independently of transcription. This reinforces the concept of oxidative stress as a rheostat, where controlled oxidation promotes survival pathways, while excess shifts the balance toward irreversible damage.

4.3.2. Subtypes of Autophagy

The primary forms of autophagy implicated are macroautophagy, mitophagy, and chaperone-mediated autophagy (CMA), each targeting distinct cellular liabilities within the cell.

Macroautophagy (often just referred to as autophagy) is responsible for bulk degradation of damaged or oxidized cytoplasmic material. Upon low-level oxidative stress, singlet oxygen can trigger macroautophagy by oxidizing redox-sensitive regulators, enabling the sequestration of dysfunctional proteins and organelles into autophagosomes. Although traditionally viewed as non-selective, macroautophagy often targets specific substrates based on oxidative modification patterns.^81,

Mitophagy, a specialized subset of macroautophagy, mitophagy, specifically eliminates damaged or dysfunctional mitochondria. Mitochondria are central to cellular energy production. However, as organisms age, mitochondrial DNA (mtDNA) accumulates mutations, leading to decreased efficiency in the electron transport chain and increased production of ROS. The increased oxidative stress may further damage cellular components, thus accelerating aging and age-related diseases, including neurodegenerative disorders, cardiovascular diseases, and metabolic syndromes. This underscores the importance of inducing mitophagy to remove these compromised organelles and prevent further ROS production. Two mitophagy pathways have been identified. In one pathway, JNK influences mitophagy through the modulation of PINK1/Parkin Pathway, as JNK activation can promote the stabilization of PINK1 on the outer membrane of depolarized mitochondria, facilitating the recruitment of Parkin, an E3 ubiquitin ligase. Parkin ubiquitinates mitochondrial proteins, marking the mitochondria for autophagic degradation.

Independently of the PINK/Parkin pathway, mitophagy can also occur through the phosphorylation of FUNDC1. JNK phosphorylates FUNDC1, a mitochondrial receptor, which enhances its binding affinity for LC3 or other autophagy proteins, facilitating the engulfment of damaged mitochondria by autophagosomes.

Chaperone-Mediated Autophagy (CMA) is a form of autophagy that selectively degrades soluble cytosolic proteins containing the KFERQ-like pentapeptide motif. Under oxidative stress, oxidized proteins are recognized by chaperones and translocated directly into lysosomes for degradation. This process helps in the removal of oxidatively damaged proteins, thereby maintaining protein quality control. JNK signaling has been implicated in modulating this pathway as it may influence the expression of chaperone proteins under mild oxidative conditions, thereby enhancing the selective degradation of oxidized cytosolic proteins. While the direct involvement of JNK in CMA regulation is less clear, oxidative stress may upregulate the expression of LAMP-2A, the lysosomal receptor essential for CMA substrate translocation. Increased LAMP-2A levels enhance the cell’s capacity to degrade oxidized proteins via CMA. In addition, stress conditions can affect the activity of Hsc70,⁹⁴ the cytosolic chaperone that recognizes and delivers substrates to LAMP-2A. Modulation of Hsc70 activity influences the efficiency of CMA.

Together, these autophagic pathways offer an orchestrated response that favors selective clearance and functional recovery rather than indiscriminate degradation, a hallmark of the hormetic action of singlet oxygen.

4.3.3. Biological Implications of Autophagy

Autophagy pathway activation has been shown to prevent stress-induced tissue and organ injury. Beyond its immediate cytoprotective function, autophagy contributes to a wide range of systemic adaptations relevant to stress resilience, aging, immunity, and tissue homeostasis.

4.3.3.1. Systemic Functions: Immunity, Inflammation, and Aging

Autophagy plays a multifaceted role in immune homeostasis. It mitigates chronic inflammation by degrading damaged organelles and oxidized macromolecules, thereby limiting the passive release of damage-associated molecular patterns (DAMPs) such as mitochondrial DNA, ATP, and oxidized proteins. This suppression of DAMP leakage helps prevent uncontrolled inflammasome activation and cytokine release. Conversely, under certain stress conditions, autophagy may also actively facilitate the secretion of select DAMPs, such as HMGB1, through non-classical pathways- initiating a context-dependent immunomodulatory feedback loop that can further enhance autophagy.

In addition, autophagy functions as a key regulator of immune responses. It plays a crucial role in defending against intracellular pathogens by sequestering and degrading them through a process known as xenophagy, as well as contributing to antigen processing and presentation via MHC class II molecules, supports T cell development and homeostasis, and helps shape adaptive immune responses. 

Through these mechanisms, autophagy serves as a multi-layered system that integrates microbial defense, inflammation control, and immune regulation, highlighting its potential as a therapeutic target for infectious, inflammatory, and autoimmune diseases.

In the aging context, basal autophagy levels decline, contributing to the accumulation of cellular damage. Mild oxidative signals such as singlet oxygen can act as hormetic triggers to transiently restore autophagic activity, improving stress resistance and supporting tissue regeneration. Autophagy also preserves stem cell function across multiple tissues, including muscle, liver, and hematopoietic systems, by maintaining redox and mitochondrial integrity.

4.3.3.2. Nervous System: Maintaining Proteostasis and Synaptic Health

Neuronal cells are especially vulnerable to oxidative damage due to their high metabolic demands and limited regenerative capacity. Autophagy helps remove dysfunctional mitochondria (mitophagy) and protein aggregates, maintaining proteostasisand preventing excitotoxicity. In the context of singlet oxygen exposure, selective autophagy helps prevent the accumulation of neurotoxic proteins such as α-synuclein and phosphorylated tau, processes implicated in neurodegenerative diseases^, including Parkinson’s and Alzheimer’s. Moreover, autophagy contributes to neural development and synaptic plasticity, influencing learning and memory processes.¹⁰⁵ Dysregulation of autophagy with aging or chronic oxidative stress can exacerbate excitotoxicity and synaptic loss, while controlled activation may confer neuroprotection.

4.3.3.3. Skin: Barrier Maintenance and Oxidative Detoxification

The skin can be viewed as a dynamic immuno-cutaneous ecosystem, where autophagy within skin cells plays a crucial role in preserving immune balance. As the body’s largest interface with the environment, the skin is chronically exposed to environmental stressors such as ultraviolet (UV) radiation, pathogens, and oxidants,stressors that can damage keratinocytes, lipids, and extracellular matrix proteins.Autophagy plays a central role in removing harmful intracellular components, maintain cellular homeostasis, and prevent inflammation and premature aging.

A wide range of resident immune cells inhabit the skin, including Langerhans cells, dermal dendritic cells, macrophages, mast cells, and tissue-resident T cells. These immune cells are in ongoing communication with keratinocytes and other structural cells of the skin, collaboratively detecting and responding to pathogens, tumors, allergens, and autoantigens through intercellular signaling involving DAMPs and pathogen-associated molecular patterns (PAMPs) .

• Keratinocytes, the predominant cells of the epidermis, are particularly active in this process. In response to UV exposure, oxidative stress, or physical injury, they release DAMPs such as HMGB1, ATP, IL-1α, uric acid, and heat shock proteins. They also express pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors, to identify microbial PAMPs like bacterial lipopolysaccharide (LPS), viral RNA, or fungal β-glucans.¹¹⁰

• Dermal fibroblasts also serve as key immune modulators. They detect both DAMPs and PAMPs in response to tissue stress, contributing to immune cell recruitment and activation. Importantly, fibroblasts exhibit considerable heterogeneity, with specific subpopulations involved in immune surveillance, epithelial–immune cell communication and sustaining chronic skin inflammation.

• Melanocytes, while less studied in this context, can respond to PAMPs by producing cytokines and chemokines, and may also release DAMPs under oxidative stress, a process implicated in inflammatory skin conditions such as vitiligo.

It has been shown that autophagy supports the function of key skin cells types, including keratinocytes, melanocytes, and immune cells, by regulating inflammation, promoting cell differentiation, and protecting against infection. In dermal fibroblasts, autophagy plays a critical role in modulating responses to DAMPs and PAMPs, limiting excessive immune activation through the degradation of damaged organelles and the regulation of cytokine release.¹¹² In keratinocytes and epidermal stem cells, autophagy helps preserve skin renewal and barrier function, thereby reducing pathogen penetration and lowering the activation threshold of resident immune cells. In melanocytes, autophagy contributes to pigmentation homeostasis by regulating melanin distribution.

These functions highlight autophagy’s essential role in skin wound healing across all key phases: hemostasis, inflammation, proliferation, and remodeling. It promotes keratinocyte proliferation and migration, drives fibroblast activation and differentiation, supports new blood vessel formation (angiogenesis), and helps clear damaged organelles and inflammatory mediators, supporting efficient tissue repair and re-epithelialization. Additionally, autophagy contributes to immune balance by regulating macrophage and neutrophil activity and supporting resolution of inflammation.

When autophagy is disrupted, it can lead to cellular senescence, chronic inflammation, impaired wound healing, and increased risk of pathological scarring or skin cancers. Dysregulated autophagy is also associated with immune-related skin disorders, including atopic dermatitis, psoriasis, and alopecia areata. ¹⁰⁸

It can be concluded that the controlled induction of autophagy may offer a more effective, targeted approach for managing chronic skin conditions. It is also a promisingtherapeutic target for enhancing wound healing outcomes, treating skin aging and related disorders. 

4.3.3.4. Lung: Adaptive Remodeling and Inflammation Control

Autophagy plays a multifaceted role in lung injury and repair by supporting tissue-specific responses such as alveolar regeneration, redox homeostasis, and immune modulation. In particular, autophagy is essential for alveolar type 2 (AT2) progenitor cell–mediated regeneration following lung injury. By promoting glucose, limiting lipid accumulation, and reducing oxidative stress, autophagy supports AT2 proliferation and epithelial barrier restoration. 

In acute lung injury (ALI), autophagy interacts with the Nrf2 Nrf2 pathway toenhance antioxidant defense: by degrading the Nrf2 inhibitor Keap1, autophagy facilitates Nrf2 stabilization, contributing to protection against oxidative inflammation and preserving alveolar function. However, as in other tissues, the context and extent of autophagy determine its protective vs. detrimental effects

This balance becomes especially relevant in sepsis-induced lung injury, where autophagy maintains epithelial barrier integrity and regulates immune cell activity. Excessive or prolonged autophagic activation under systemic inflammatory stress may contribute to tissue damage and impaired resolution.

Autophagy also contributes to the pathogenesis of chronic pulmonary diseases:

• In chronic obstructive pulmonary disease (COPD), impaired autophagy disrupts mitochondrial clearance, while excessive autophagy can lead to epithelial cell death and persistent inflammation. Both patterns contribute to tissue destruction and reduced regenerative potential. ^,

• In idiopathic pulmonary fibrosis (IPF), deregulated autophagy in epithelial and mesenchymal cells is associated with aberrant fibroblast activation and extracellular matrix accumulation, promoting fibrosis. ^,

• In asthma, autophagy modulates immune cell recruitment and cytokine release, influencing airway inflammation and remodeling. Dysregulated autophagy has been linked to enhanced airway hyperresponsiveness and resistance to corticosteroids. ^119,

Taken together, these findings underscore the context-dependent roles of autophagy in pulmonary health. Targeting autophagic signaling in a cell-type- and disease-specific manner may open new therapeutic avenues for both acute and chronic lung diseases.

4.3.3.5. Cardiovascular System: Cardioprotection and Vascular Homeostasis

Autophagy plays a critical role in maintaining cardiovascular health by supporting the turnover of damaged cellular components, particularly under stress conditions such as ischemia. In cardiac tissues, this process reduces myocardial injury, facilitates tissue remodeling, and promotes recovery after oxidative or mechanical insults. ¹³⁸

When autophagy is defective or insufficient, cardiomyocytes accumulate dysfunctional mitochondria, misfolded proteins, and oxidative byproducts, which are hallmarks of cardiac aging and pathology. These impairments contribute to the progression of cardiomyopathies and heart failure. Mitophagy, the targeted removal of damaged mitochondria, is especially vital in the heart, where cells contain a high density of mitochondria and are constantly subjected to high oxidative and metabolic demands. By clearing depolarized mitochondria, mitophagy limits excessive ROS production and protects against ischemia–reperfusion injury.

In the vascular system, autophagy also serves as a key homeostatic mechanism. Endothelial cells rely on basal autophagic activity to preserve nitric oxide signaling, limit vascular inflammation, and prevent atherogenesis. When this regulatory process is impaired, either by aging, chronic inflammation, or external oxidative stimuli such as singlet oxygen, vascular dysfunction and pro-atherogenic changes may ensue. ¹²⁴

Importantly, the Nrf2–autophagy axis integrates redox-sensitive signaling with lipid metabolism and immune regulation, offering an additional layer of protection against oxidative and inflammatory stressors. Through this pathway, low-level singlet oxygen may promote adaptive remodeling and preserve vascular tone without triggering damaging oxidative overload.

Together, these findings position autophagy as a central mechanism in cardiovascular resilience, mediating both cardiac and vascular adaptations to oxidative stress, including that induced by singlet oxygen exposure.

4.3.3.6. Skeletal Muscle: Quality Control and Adaptation to Stress

Autophagy maintains skeletal muscle homeostasis by removing dysfunctional proteins and organelles, particularly mitochondria, which is essential for preserving muscle mass, contractile performance, and metabolic adaptability. Mitophagy, the selective degradation of damaged mitochondria, plays a particularly critical role in muscle fibers, where high oxidative load and metabolic turnover necessitate constant mitochondrial quality control. Impairments in this process have been associated with various myopathies, including sarcopenia, muscular dystrophies, and cancer-associated cachexia, as accumulation of dysfunctional mitochondria and protein aggregates leads to fiber atrophy and impaired regeneration.^,

During physiological stress such as endurance exercise or caloric restriction, autophagy is transiently upregulated to promote muscle fiber remodeling, improve mitochondrial efficiency, and mobilize intracellular substrates for energy production. This dynamic regulation involves key redox-sensitive signaling hubs, including AMPK and the Nrf2 pathway, which coordinate the transcriptional and post-translational activation of autophagy machinery in response to oxidative fluctuations. Notably, exercise-induced production of singlet oxygen and other reactive oxygen species may act as a hormetictrigger to fine-tune autophagic activity, enhancing long-term muscle resilience and metabolic health. Understanding the precise balance of autophagic signaling in skeletal muscle may therefore hold therapeutic relevance for age-related muscle decline and metabolic syndromes.

4.3.3.7. Endocrine System: Hormone Regulation and Organelle Turnover

Within endocrine tissues, autophagy plays a dual role in general cellular maintenance and in regulating hormone synthesis and secretion. In peptide-producing cells such as those of the pituitary gland, a specialized autophagic process known as crinophagy enables the degradation of excess secretory granules, thus preventing hormone hypersecretion and contributing to hormonal balance.

In steroidogenic cells of the adrenal cortex and testes, autophagy selectively targets mitochondria and smooth endoplasmic reticulum, which are organelles central to steroid biosynthesis. This regulatory role ensures metabolic efficiency and prevents oxidative damage, especially under stress conditions.

Additionally, autophagy in endocrine tissues is modulated by oxidative signals, including ROS such as singlet oxygen, which can trigger redox-sensitive autophagy pathways to adjust hormonal output. Dysfunctional autophagy in endocrine glands has been associated with various pathologies, including diabetes, infertility, and hormone-secreting tumors, due to disrupted secretory or steroidogenic functions.^132,

These findings suggest that targeted modulation of autophagy in endocrine tissues could represent a novel therapeutic approach to hormone-related disorders and metabolic diseases.

4.3.3.8. Pregnancy: Placental Development and Immune Tolerance

Autophagy plays a vital role in pregnancy by regulating trophoblast survival, supporting placental development, maintaining immune tolerance, and adapting to the dynamic metabolic demands of both mother and fetus. During early gestation, trophoblasts invade the uterine lining under low-oxygen conditions; here, autophagy facilitates cellular survival and promotes cytotrophoblast differentiation into the invasive phenotype essential for placental formation.

Autophagy also contributes to maternal–fetal immune tolerance by modulating inflammatory cytokine signaling, promoting tolerogenic dendritic cells, and regulating T cell activation. These effects reduce the risk of immune-mediated rejection of the semi-allogeneic fetus and help maintain immunological equilibrium at the maternal-fetal interface.

Additionally, under periods of maternal nutritional stress, autophagy enables intracellular recycling of macromolecules to maintain fetal nutrient supply and energy balance. Dysregulated autophagy, whether insufficient or excessive, has been implicated in various pregnancy complications, including preeclampsia, intrauterine growth restriction (IUGR), and gestational diabetes mellitus (GDM), highlighting its protective function in sustaining placental and fetal health.

4.3.3.9. Liver: Metabolic Regulation and Tissue Regeneration

The liver relies on autophagy for maintaining metabolic balance, detoxification, and redox homeostasis. ^129,  Through lipophagy, autophagy facilitates the degradation of intracellular lipid droplets, contributing to lipid turnover and preventing hepatic steatosis. Simultaneously, mitophagy ensures the removal of dysfunctional mitochondria, protecting hepatocytes from oxidative stress and limiting ROS production. This mitochondrial quality control also supports energy-efficient regeneration of liver tissue following injury, ensuring adequate ATP production while minimizing oxidative damage.^129, Dysregulation of hepatic autophagy has been implicated in liver diseases such as non-alcoholic fatty liver disease (NAFLD), alcoholic hepatitis, and fibrosis, underscoring its critical role in hepatocellular protection and regeneration.

4.3.4. Concluding Synthesis: Autophagy as a HormeticMediator of Singlet Oxygen Signaling

Across a wide spectrum of tissues and physiological systems, autophagy emerges as a central regulatory process that integrates metabolic signals, oxidative cues, and stress adaptation. The evidence compiled in this chapter reveals a unifying theme: low-level oxidative stress, particularly that induced by singlet oxygen, can serve as a hormeticstimulus, activating autophagic pathways that support cellular renewal, immune modulation, and tissue repair.

In barrier tissues such as the skin and lungs, autophagy preserves epithelial integrity, controls inflammation, and enhances resilience to environmental insults. In skeletal and cardiac muscle, it maintains mitochondrial quality and metabolic homeostasis, particularly during physiological challenges like exercise or ischemia. In the liver, it coordinates lipid turnover and regeneration, while in the endocrine system and placenta, autophagy modulates hormonal synthesis and immune tolerance. Even during pregnancy, autophagy protects the maternal-fetal interface and contributes to placental development.

These diverse functions are governed by overlapping molecular mechanisms, including mitophagy, lipophagy, and redox-sensitive signaling via Nrf2, AMPK, and mTOR, yet are finely tuned to the needs of each tissue context. The dual role of autophagy in both cytoprotection and cell death is especially notable: the intensity and duration of singlet oxygen exposure may dictate whether autophagy serves a survival-promoting or pathological role, reflecting the delicate threshold between adaptive and maladaptive responses.

Understanding this context-dependence and threshold sensitivity is crucial for therapeutic modulation. Pharmacological or photo-induced activation of singlet oxygen may, under controlled conditions, be harnessed to stimulate beneficial autophagy, offering novel treatment avenues in inflammatory, degenerative, metabolic, and fibrotic diseases.

In summary, autophagy is not merely a stress response- it is a homeostatic amplifier of hormetic signaling. Through its ability to decode singlet oxygen cues into tissue-specific adaptive programs, autophagy bridges environmental sensing with intracellular resilience, affirming its relevance as both a biomarker and target of oxidative medicine.

5. Temporal dynamics of oxidative stress response

​The adaptations to oxidative stress unfold in a time-dependent manner⁸³ and can result in long-lasting physiological benefits (Table 2). Early after oxidative insult, typically within minutes, NF-κB is rapidly activated, leading to a transient pro-inflammatory response that primes the system for defense and cellular recruitment.Within 1–3 hours, these early signals stimulate transcriptional programs, notably via Nrf2, promoting transcription of phase II detoxification enzymes and antioxidant defenses, such as HO-1 and NQO1. As these protective proteins accumulate, they provide an enhanced state of stress resilience lasting up to 48 hours. ^83,,

Table 2 Timing and duration of defence responses to moderate oxidative stress^83,140–142

Response Type	Trigger	Onset (Approx.)	Duration
NF-κBactivation	ROS	Minutes	1–3 hours
Nrf2 activation	4-HNE, ROS	1–3 hours	6–48 hours (longer term)
Autophagy induction	Persistent ROS	Several hours	Long-lasting (days)
Cellular repair	Nrf2/ARE genes	After Nrf2 signal	Hours–days

During this intermediate phase, lipid peroxidation by-products like 4-hydroxynonenal (4-HNE) act as secondary messengers that stably modify proteins and influence cell signaling well beyond the initial oxidative event.¹⁴¹ This extended adaptive phase is further supported by the induction of autophagy, typically induced several hours after exposure, contributing to cellular maintenance by clearing damaged organelles.¹⁴⁰These processes are complemented by cellular repair mechanisms, activated to restore redox balance and tissue integrity. 

Together, these temporally orchestrated responses to moderate oxidative stress, such as that induced by singlet oxygen, foster a sustained hormetic effect that contributes to cellular preconditioning, enhanced metabolic regulation, and long-term resilience to stress

The preceding sections have outlined the dual nature of singlet oxygen and its capacity to activate adaptive stress responses, enhance mitochondrial function, and modulate immune and redox signalling pathways when delivered at low, sub-toxic concentrations. These mechanistic insights support the biological plausibility of therapeutic benefits observed under controlled oxidative exposure. In this context, preliminary clinical observations, including case reports and practitioner experiences, suggest that low-level ambient singlet oxygen exposure may be associated with measurable improvements in diverse physiological domains, including respiratory function, hematopoiesis, immune regulation, and neurobehavioral responses. Although these reports are observational and uncontrolled, they offer physiologically plausible insights that are broadly consistent with the hormetic and redox-mediated processes described in earlier sections. However, these findings should be interpreted cautiously and underscore the need for systematic clinical validation. This section presents a summary of selected individual patient reports and clinical observations describing subjective or objective functional improvements following low-dose Singlet Oxygen Therapy (SOT), administered via a patented device that generates low-dose singlet oxygen without irradiation.¹⁸

5.1. Effects on Sleep Quality and Sleep Disorders

​One of the earliest and most consistently reported effects by individuals undergoing SOT, including both healthy subjects and those with chronic conditions, is a subjective improvement in sleep quality. These improvements include reports of faster sleep onset, longer uninterrupted sleep, and fewer nocturnal awakenings. Such effects are notable given the central role sleep plays in physiological repair, cognitive functioning, and immune modulation.

​Sleep is closely tied to redox biology. During restful sleep, antioxidant systems are restored, and metabolic byproducts, including ROS, are effectively neutralized. Conversely, sleep deprivation has been shown to elevate oxidative stress in the brain and peripheral tissues, disrupt redox-sensitive signaling pathways, and increase the production of pro-inflammatory cytokines such as TNF-α and IL-6. These inflammatory mediators can also impair sleep regulation, potentially reinforcing a feedback loop of oxidative stress, immune dysregulation,¹⁴³ and poor restorative capacity. This may contribute to increased risk for chronic diseases such as cardiovascular and neurodegenerative disorders.

Clinical Case 1: Remission of Obstructive Sleep Apnea Without CPAP

A 69-year-old male diagnosed with mild-to-moderate obstructive sleep apnea (OSA) in 2018 (AHI 7.0; SpO₂ nadir 86%) was unable to tolerate continuous positive airway pressure (CPAP) therapy. Beginning in 2020–2021, he initiated nightly use of a passive, low-dose singlet oxygen-emitting device during sleep. Over the following years, he experienced a reported improvement in sleep quality and energy, coinciding with modest weight loss (BMI decreased from ~29 to 24.7). 

A follow-up sleep study in 2025 revealed normalized breathing parameters (AHI 6.8, mean SpO₂ 93%, nadir 87%, sleep efficiency 91.4%), and CPAP was deemed unnecessary. Although causality cannot be established, the temporal association raises the possibility that SOT contributed to improved upper airway function and respiratory stability during sleep. This represents a rare instance of OSA remission without CPAP, surgery, or pharmacological intervention.

Proposed Mechanisms: Evidence from preclinical and clinical studies suggests that moderate, transient oxidative stimuli, such as those induced by singlet oxygen, can restore redox balance through hormetic mechanisms that recalibrate circadian rhythms and stress responses. This may include the resetting of dysfunctional sleep-wake cycles and attenuation of hyperarousal states commonly associated with insomnia and anxiety. ^,

​Users commonly describe experiencing noticeable improvements within the first24 hours of singlet oxygen exposure. These rapid effects may reflect immediate neurophysiological adaptations via redox-sensitive systems involved in melatonin synthesis, hypothalamic signaling, or neuronal hyperexcitability. While speculative, the observed restoration of sleep continuity may suggest an indirect role for singlet oxygen in modulating neuroimmune interactions and systemic stress resilience.

5.2. Snoring alleviation

​Snoring and sleep-disordered breathing are widespread issues, affecting over 40% of the population at some point in their lives with varying severity. These conditions disrupt sleep architecture by limiting airflow and preventing entry into deeper, restorative sleep phases. Snoring typically results from partial obstruction of the upper airway, often due to relaxed throat muscles, excess soft tissue, nasal congestion, or tongue positioning, leading to vibratory noise as air passes through narrowed passages. 

In otherwise healthy individuals, snoring may present as a benign nuisance, typically impacting bed partners. However, in more severe presentations such as obstructive sleep apnea (OSA) or asthma, repeated airway collapse can result in intermittent hypoxia, oxidative stress, and increased cardiovascular and metabolic risks.

Clinical Observations: Passive Snoring Reduction in Bed Partners

​Anecdotal reports from users of SOT have described significant reductions in snoring frequency and intensity, including among individuals not directly using the device. In over 90 reported cases, individuals receiving SOT for unrelated conditions, noted that their bed partners, many of whom were habitual snorers, experienced diminished or absent snoring during the treatment period. 

One particularly striking case involved an asthma patient who began nightly SOT; shortly thereafter, his wife, previously a consistent snorer, ceased snoring entirely. This effect occurred without her direct use of the device, raising the possibility that passive exposure to singlet oxygen-enriched air may influence upper airway function.

Proposed Mechanisms: Several biological mechanisms could plausibly explain these effects. Low-level oxidative stress induced by singlet oxygen may elicit hormeticresponses that promote subtle increases in upper airway muscle tone, thereby reducing the likelihood of collapse during sleep. It may also attenuate inflammation, a key contributor to airway narrowing in conditions such as allergic rhinitis and sinus congestion. Given that OSA and snoring are associated with chronic inflammation and oxidative stress in the upper airway, the redox-balancing effects of singlet oxygen may help reduce tissue swelling, improve airflow, and lower snoring severity. 

​While these observations are mechanistically consistent with findings described in Sections 5 and 7, they remain preliminary. Controlled clinical studies are needed to determine the extent and reproducibility of SOT’s impact on snoring and upper airway physiology.

5.3. Alleviation of Chronic Nightmares and Post-Traumatic Stress Symptoms 

Chronic nightmares, that are distressing, recurrent dreams that disrupt sleep, affect approximately 2–6% of adults and are especially common in individuals with post-traumatic stress disorder (PTSD), anxiety, or depression.  Despite their impact on sleep quality and mental health, nightmares are frequently underdiagnosed and undertreated. They are rarely spontaneously reported by patients and often overlooked by clinicians, despite well-documented associations with increased risk of suicide, insomnia, and emotional dysregulation.^, This persistent clinical neglect has contributed to diagnostic and therapeutic gaps, leaving many individuals untreated despite significant sleep disruption and psychological distress. 

Clinical Case: Resolution of Trauma-Associated Nightmares Following SOT

A woman who experienced daily trauma-related nightmares for eight years following an intraoperative awareness event reported immediate and lasting resolution of her symptoms after beginning SOT. Her symptoms, including dissociative aftereffects, reportedly ceased within the first nights of exposure, enabling sustained sleep restoration for the first time in years.

Proposed Mechanisms: These effects may be mediated by redox-sensitive neural pathways involved in fear processing and REM regulation. Singlet oxygen-induced low-level oxidative stress may activate antioxidant and anti-inflammatory responses in limbic structures such as the amygdala and prefrontal cortex. These adaptations could stabilize neural activity and reduce hyperarousal, thereby improving REM sleep continuity and reducing nightmare intensity.

Although mechanistically plausible and consistent with preclinical literature on redox-neural interactions, these observations remain anecdotal. Controlled studies are required to confirm therapeutic efficacy in nightmare disorders or trauma-related sleep disturbances.

5.4. Enhancing Exercise Recovery and Performance 

​A recent study by Hsieh et al. reported enhanced exercise performance and physiological resilience following exposure to low-dose singlet oxygen energy (SOE). Participants using a passive, non-invasive singlet oxygen-generating device during exercise showed improvements in cardiorespiratory function, muscle recovery, and subjective vitality.

Rather than attributing these effects solely to SOE, as the writers do, they may reflect generalizable redox-based mechanisms consistent with known physiological adaptations to exercise. Physical activity induces transient oxidative stress that activates hormetic signaling pathways, including Nrf2-mediated antioxidant responses and mitochondrial biogenesis via AMPK and PGC-1α. These responses are part of mitohormesis, a process by which low-level mitochondrial stress improves energy metabolism, mitochondrial quality control (via fusion/fission and mitophagy), and resistance to subsequent challenges.

During recovery, moderate ROS exposure stimulates AMPK signaling, facilitating glucose uptake, fatty acid oxidation, and mitochondrial renewal via upregulation of PGC-1α. In parallel, increased expression of antioxidant enzymes, such as superoxide dismutase and heme oxygenase-1, mitigates inflammation and protects muscle tissue from oxidative damage. These established processes likely underlie the recovery-related benefits observed by Hsieh et al.

​Thus, while the SOE device appears to confer tangible benefits, these improvements are interpretable within the broader framework of redox-based adaptive physiology, as discussed in Sections 5 and 7. Future studies comparing singlet oxygen-based delivery to other hormetic interventions, using biomarkers and controlled exercise protocols—could provide a clearer understanding of its relative efficacy.

5.5. Impact on Respiratory Health: Asthma, COPD, and Airway Inflammation

5.5.1. Asthma

Asthma is a chronic inflammatory airway disease characterized by bronchial hyperresponsiveness, reversible airway obstruction, and recurrent symptoms such as wheezing, coughing, and breathlessness. Common triggers include allergens, exercise, infections, and environmental irritants. During exacerbations, airway inflammation leads to bronchial constriction, edema, and mucus production, reducing airflow and impairing gas exchange.

Repeated asthma attacks increase the risk of airway remodeling and irreversible airflow limitation. Conventional therapies, including corticosteroids and bronchodilators, are effective but often carry side effects, and many patients remain symptomatic despite optimized pharmacological management. Therefore, there is a growing interest in adjunctive therapies aimed at reducing attack frequency, minimizing reliance on synthetic drugs, and enhancing endogenous healing responses.

Observational Reports of Singlet Oxygen Therapy in Asthma

Self-reported observations suggest that singlet oxygen-enriched air may confer both acute relief and longer-term benefits for asthma patients. Users have described rapid alleviation of breathlessness during attacks, as well as a gradual reduction in attack frequency and medication dependence. Notably, several individuals who previously required prophylactic steroids for seasonal asthma reported discontinuing their medication after consistent nightly SOT. Some also reported improvements in symptoms associated with airway remodeling.

Preliminary Clinical Study

In a small observational study conducted in Spain by Prof. M. Fegricio, two groups of 25 asthma patients were evaluated over 30 and 60 days. The treatment group used a low-level ambient singlet oxygen generator daily, while the control group received a sham device. Patient-reported outcomes (Error! Reference source not found.) showed significant improvements in sleep quality and energy in the treatment group, alongside a reduction in weekly asthma attacks (from 1.08 to 0.48 attacks/week). Sixteen patients discontinued all pharmacologic treatment by the study’s end, while no such changes occurred in the control group.

Figure 4 Change in the average rating of energy levels and sleep quality of 25 asthma patients, with (left graph) and without (right graph) daily usage of low-level ambient singlet oxygen generator over a period of 60 days.¹⁶⁷

Proposed Mechanisms: While preliminary and based on observational data, these findings are consistent with the mechanistic rationale discussed in Sections 5 and 7. Specifically, the controlled oxidative signal generated by singlet oxygen may activate redox-sensitive anti-inflammatory and epithelial repair pathways. These processes could improve airway tone and mucosal integrity, thereby lowering susceptibility to bronchoconstriction. However, controlled trials are essential to validate these effects and rule out placebo influences or seasonal fluctuations.

5.5.2. Chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD), a progressive respiratory disorder encompassing chronic bronchitis and emphysema, is characterized by persistent airflow limitation, mucus overproduction, and alveolar destruction. As the third leading cause of death in the United States, COPD currently has no cure, only supportive treatments are available, aimed at slowing disease progression and managing symptoms.

Patients with advanced COPD often experience debilitating breathlessness, frequent exacerbations, and, eventually, dependence on supplemental oxygen. Without a lung transplant, this condition ultimately proves fatal. In this context, low-level singlet oxygen exposure has been anecdotally associated with perceived improvements in breathing and function.

Several users have reported enhanced respiratory ease and a reduction in oxygen dependence within days of initiating therapy. A few individuals with advanced disease described regaining the ability to work and travel, improvements that warrant systematic investigation. In some cases, spirometry conducted after a month of use revealed increases in forced expiratory volume (FEV₁) and improved FEV₁/FVC ratios, outcomes not commonly observed in late-stage COPD with non-invasive methods.

These preliminary findings align with earlier observational reports, such as a small, unpublished study in which patients with moderate-to-severe COPD underwent a four-week regimen of singlet oxygen inhalation therapy. Participants demonstrated modest improvements in FEV₁, peak expiratory flow, and a reduction in symptom burden and bronchodilator use. Although the study lacked a control group and peer review, its outcomes parallel known physiological mechanisms described in Sections 5 and 7, including redox-modulated inflammation control, epithelial repair, and enhanced mitochondrial turnover.

The following case reports provide individual examples of patients with moderate to severe COPD who reported respiratory improvements following routine use of SOT.

Clinical Case 1: Functional and Spirometric Improvement in a Patient with Severe COPD Following SOT

A 67-year-old male with a 20-year smoking history (approx. 1 pack/day) was diagnosed with severe COPD. Despite smoking cessation, his respiratory condition progressively worsened, with persistent wheezing, dyspnea, and pulmonary congestion that impaired basic activities such as walking and showering.

Initial treatment included corticosteroids (prednisone) and antibiotics (cephalexin), though his lung function continued to deteriorate. FEV₁ declined from 52% to 35% predicted within a year. At this point, physicians recommended long-term oxygen therapy and pulmonary rehabilitation. In April 2017, the patient initiated daily SOT. Within days, he reported subjective improvement in breathing and energy levels. Over the following weeks, he resumed travel and returned to work. Spirometry one month into therapy indicated an increase in FEV₁ from 35% to 58% predicted, with further improvement (63%) over the next year. No changes in standard medications were recorded during this time.

Although causality cannot be established from a single case, the timing and magnitude of improvement support further investigation into singlet oxygen as a potential adjunct therapy in COPD management.

Clinical Case 2: Remission of COPD With Discontinuation of Inhaled Therapy Following Passive Singlet Oxygen Exposure

A 57-year-old female with heavy smoking history was diagnosed in 2020 with moderate COPD, confirmed by spirometry (FEV₁ ~83%; FEV₁/FVC <80%). She experienced progressive dyspnea, wheezing, and exertional fatigue that interfered with her occupation as a children’s entertainer.

In 2021, the patient began nightly SOT. During the first week, she experienced productive coughing with green sputum, possibly reflecting a mucolytic or detoxification response. Over the next two months, she noted substantial improvements in breathing and physical stamina. Wheezing fully resolved within three months. Initially prescribed four inhaled medications, including corticosteroids and bronchodilators, the patient gradually discontinued them over a six-month period. By year’s end, she remained asymptomatic and off all inhalers. Pulmonary function tests in 2023 and 2025 showed normalized spirometry (FEV₁ >95%; FEV₁/FVC ≥83%). A pulmonologist concluded that the patient no longer met diagnostic criteria for COPD. The patient has since resumed working and other physical activities, including running and long walks, without any recurrence of respiratory symptoms. She currently reports full respiratory health.

This case represents an unusual example of long-term COPD remission, both in symptoms and lung function, after stopping all standard medications. The improvement occurred alongside daily use of low-dose singlet oxygen, suggesting a probable connection. The recovery pattern supports further research into how this approach may reduce airway inflammation and improve mucus clearance. Taken together, these observations support further clinical investigation into SOT as an adjunctive treatment for COPD, particularly in patients unresponsive to conventional therapies.

5.5.2.1. Healthcare Cost Implications of Asthma and COPD

In addition to its profound clinical burden, COPD imposes a substantial financial toll. An analysis done in 2020 estimated that respiratory diseases cost the U.S. healthcare system $170.8 billion annually. Asthma accounted for the highest costs, followed by COPD. For asthma, prescription drugs represented the largest share (48%), while for COPD, hospitalizations (28.8%) and prescriptions (28.5%) were primary cost drivers.

Most COPD-related spending was concentrated in adults over 45, with nearly 70% covered by public insurers. In contrast, asthma-related costs were more evenly distributed across age groups.

These data highlight the potential value of adjunct therapies that can reduce exacerbation rates, hospitalizations, and medication use. If low-dose SOT is validated in future clinical studies, it may represent a cost-effective strategy that leverages endogenous repair pathways rather than relying solely on pharmacological suppression.

5.5.3. Pneumonia 

Pneumonia remains a significant global health concern, affecting over 500 million individuals annually and accounting for approximately 4 million deaths, which is about 7% of all global mortality. It is the leading infectious cause of death in children under five, particularly in regions with limited access to medical care, such as South Asia and sub-Saharan Africa.

The condition arises when an infection, which can be bacterial, viral, or fungal, triggers inflammation in the lungs, leading to alveolar fluid accumulation, impaired gas exchange, and respiratory distress. Diagnosis and treatment are often complicated by uncertainty around the causative pathogen and the rise in antimicrobial resistance, which can hinder effective, targeted therapy.

Observational Reports of SOT Use in Pneumonia

Several users of singlet oxygen therapy (SOT) have anecdotally reported significant symptomatic relief within 1–3 days of starting therapy. These effects included:

• Improved breathing comfort

• Reduced coughing and phlegm production

• Relief of chest tightness or discomfort

• Increased physical energy and vitality

Some individuals also noted a shortened recovery timeline, which is typically protracted in pneumonia, often lasting several weeks.

Proposed Mechanisms: The reported improvements in respiratory function and recovery following singlet oxygen therapy may be explained by its capacity to modulate redox-sensitive biological pathways. Low-dose oxidative stimulation is known to reduce local inflammation, promote epithelial repair, and enhance mucociliary clearance, key processes in the resolution of pneumonia. Additionally, by supporting immune regulation and restoring redox balance in the lung microenvironment, singlet oxygen may facilitate more efficient pathogen clearance and tissue recovery. These mechanisms, detailed in Section 5, offer a biologically plausible framework for understanding how SOT could accelerate symptom resolution and shorten recovery time in individuals with pneumonia.

5.5.4. Excess Phlegm and Mucociliary Function 

Excessive phlegm production is a common and distressing symptom across a broad spectrum of respiratory conditions, including asthma, COPD, pneumonia, allergic rhinitis, chronic exposure to tobacco smoke and other conditions. While mucus is essential for trapping pathogens and particulates, abnormal accumulation or viscosity can impair airflow, reduce oxygenation, and heighten infection risk. In severe cases, it contributes to hypoxemia, atelectasis, and respiratory failure.

Conventional therapies, which include expectorants, mucolytics, bronchodilators, and physical airway clearance techniques, often offer limited relief and may be impractical or burdensome, especially in patients with reduced mobility or chronic illness. Moreover, these treatments do not address upstream dysregulation in inflammatory or redox pathways that often underlie mucus hypersecretion.

Clinical Observations of Mucus Reduction Following SOT

Following SOT, users have consistently reported improvements in mucus-related symptoms, including reductions in phlegm volume and viscosity. These reports span a variety of respiratory conditions, including asthma, COPD, pneumonia, allergies, and chronic smoking-related bronchitis. Notably, some individuals described the expulsion of thick, dark mucus during early exposure, interpreted as a “detoxification phase,” followed by an enduring sense of airway clearance and improved breathing.

Clinical Case: Discontinuation of Airway Suctioning in a Non-Communicative Patient

non-communicative adult with a severe brain injury required routine nightly suctioning to prevent aspiration due to excessive phlegm production. Following SOT, the need for suctioning diminished and ultimately ceased. His mother and caregiver reported improved sleep and overall well-being for both herself and the patient.

Proposed Mechanisms: These effects are consistent with redox-modulatory mechanisms described in Section 5. Low-dose singlet oxygen likely reduces local airway inflammation, downregulating the inflammatory pathways that drive excess mucus production. It may also stimulate epithelial repair¹¹⁶ and enhance ciliary activity, both of which are crucial for effective mucociliary clearance. Additionally, redox-sensitive modulation of mucus viscosity may lower its viscosity, facilitating easier expectoration.

Taken together, these effects suggest that singlet oxygen may help restore the mucus production-clearance cycle, particularly in individuals with impaired mucociliary function or chronic inflammation. As a non-invasive, low-burden approach, this therapy could complement existing treatments or offer relief to patients with refractory symptoms.

5.5.5. Biological Basis for Low-Level Singlet Oxygen Beneficial Effect for Respiratory Diseases

The beneficial respiratory effects of low-dose singlet oxygen stem from its ability to induce mild, transient oxidative stress, triggering a hormetic cellular response. This controlled redox perturbation activates protective signaling pathways without overwhelming endogenous antioxidant defenses.

Key mechanisms include the upregulation of antioxidant enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, modulation of pro- and anti-inflammatory mediators, and promotion of epithelial repair and regeneration.¹¹⁶ These responses collectively restore redox balance, attenuate tissue inflammation, reduce bronchoconstriction, and enhance mucociliary clearance,^180,181 all of which are essential for resolving respiratory infections and managing chronic airway disorders.

Additionally, singlet oxygen exposure has been proposed to stimulate autophagy and mitochondrial biogenesis, contributing to enhanced metabolic resilience and immune competence within pulmonary tissues. These redox- and autophagy-mediated adaptations, as described in Sections 5 and 7, offer a plausible mechanistic foundation for the observed improvements in respiratory function.

​Taken together, these findings support low-dose singlet oxygen as a non-invasive and physiologically adaptive modality for promoting respiratory health, particularly in conditions characterized by chronic inflammation, redox imbalance, and epithelial dysfunction.

5.6. COVID-19 and Related Pulmonary Inflammatory Conditions

​The COVID-19 pandemic drew global attention to the devastating consequences of dysregulated immune responses in viral pneumonia, where excessive cytokine release, oxidative stress, and alveolar damage contribute to acute respiratory distress syndrome (ARDS) and long-term pulmonary complications such as fibrosis and reduced oxygen diffusion capacity. In the absence of curative antiviral therapies for severe cases, clinical management has relied primarily on supportive interventions such as mechanical ventilation and ECMO, both of which associated with high morbidity and long-term impairment of lung function.

5.6.1. Informal Reports of SOT During COVID-19

While no controlled clinical trials have yet assessed the efficacy of low-dose SOT in COVID-19 management, a number of anecdotal reports describe apparent improvements in respiratory function, oxygenation, and symptom resolution following its use. The following case summaries are included for observational context:

Clinical Case 1: Rapid Symptom Resolution and Functional Recovery

A 15-year-old patient with confirmed COVID-19 presented with high fever (~40 °C), significant labored breathing, and loss of taste and smell. Following the initiation of SOT, the patient reported improved respiratory comfort, restoration of ambulation, and return of olfactory and gustatory function.

Clinical Case 2: Recovery from Severe Respiratory Distress 

A 36-year-old recording artist experiencing severe respiratory symptoms during acute COVID-19 infection, including significant pulmonary involvement and breathing difficulties, reported progressive respiratory improvement and resumption of full functional capacity after initiating SOT.

Clinical Case 3: Prevention of Hospitalization and Respiratory Recovery

​A 39-year-old woman with acute deterioration following COVID-19 infection, including loss of taste and smell, nausea, marked weakness, and episodes of respiratory distress with declining oxygen saturation, requiring emergency medical attention. After initiating SOT, she described phlegm expectoration and steady improvement in breathing and energy, with eventual discontinuation of supplemental oxygen and symptom resolution. Her family members, who also began SOT, reported similar improvement.

Proposed Mechanisms: These reports, while anecdotal and uncontrolled, align mechanistically with the proposed redox-modulating and immunoregulatory effects of low-dose singlet oxygen described in Sections 5 and 7. Their observational consistency underscores the need for rigorous clinical trials to investigate its therapeutic potential in acute viral respiratory illnesses.

5.7. Long COVID

Post-acute sequelae of SARS-CoV-2 infection (PASC), or long COVID, is increasingly recognized as a multifactorial syndrome characterized by persistent symptoms lasting weeks to months beyond the resolution of acute illness. According to the World Health Organization, manifestations may include fatigue, dyspnea, cognitive dysfunction (“brain fog”), autonomic instability, and multisystem involvement, often with delayed onset.

Emerging evidence links these symptoms to chronic low-grade inflammation, endothelial dysfunction, and mitochondrial dysregulation, aligning closely with the redox imbalances and impaired energy metabolism described in Section 3. Long COVID pathophysiology includes microvascular injury, immune dysregulation, and altered oxygen delivery, which are all factors contributing to persistent oxidative stress and impaired cellular bioenergetics.

Low-dose singlet oxygen exposure has been anecdotally associated with improvements in these domains. Observational reports describe functional gains in respiratory endurance, cognitive clarity, and fatigue reduction, suggesting that singlet oxygen may support microcirculatory recovery, mitochondrial adaptation, and modulation of chronic inflammation.

Proposed Mechanisms: These outcomes are consistent with the hormetic mechanisms discussed in Sections 5 and 7, particularly NRF2 activation, mitochondrial biogenesis, and autophagy induction. Although formal validation is required, the absence of systemic toxicity and the feasibility of continuous use make SOT a candidate for further research in post-viral syndromes.

5.8. Anemia and Blood Disorders 

​Anemia is a condition in which there is a reduced number of red blood cells or hemoglobin molecules, which are responsible for transporting oxygen throughout the body. Consequently, reduced oxygen supply can impair tissue and organ function. In 2019, around 25% of the global population was affected by anemia (over 1.5 billion people), with the highest prevalence observed in children under five, reaching nearly 50%.

​One of the most common forms is anemia of chronic disease, also known as anemia of inflammation. This type arises in the context of chronic illnesses such as autoimmune disorders, cancer, or kidney failure, and is characterized by impaired erythropoiesis and disturbed iron metabolism. The resulting anemia may further impair recovery by limiting oxygen delivery to tissues and suppressing immune competence.

Anecdotal reports suggest that exposure to low-level singlet oxygen may support red blood cell production, including in cases of longstanding or treatment-resistant anemia. 

Clinical Case: Hematologic Improvement in a Patient with Refractory Anemia

A non-communicative adult patient with dementia, thyroid dysfunction, cardiac disease, and severe anemia (hemoglobin <8 g/dL) had not responded to conventional treatments for several years. After two months of SOT, hemoglobin levels increased to 10 g/dL and reached 10.2 g/dL after six months—levels not previously attained under standard care. Additionally, improvements in thyroid and renal function were also reported.

Proposed Mechanisms: Mild oxidative stimulation may activate redox-sensitive transcription factors, supporting hematopoiesis and counteracting inflammation-induced suppression of red blood cell production. In particular, singlet oxygen may influence the hypoxia-inducible factor (HIF) pathway, which regulates erythropoietin (EPO) production in the bone marrow. Ferritin and iron metabolism may also be modulated through oxidative signaling, contributing to more effective erythropoiesis.^, In parallel, the mitigation of chronic inflammation via redox modulation may relieve inhibitory signals that suppress bone marrow activity in anemia of inflammation.

While these mechanisms are theoretically consistent with observed improvements, further research is essential to determine whether low-dose singlet oxygen exposure has a reproducible effect on hematologic parameters, and under which conditions it may be clinically relevant.

5.9. Mitigating Cancer Treatment Side Effects and Enhanced Recovery 

​Chemotherapy remains one of the most commonly used and effective treatments for various types of cancer, administered in an estimated 50–60% of cases during some phase of treatment. While its efficacy is well established, chemotherapy is frequently accompanied by debilitating side effects, including nausea, fatigue, alopecia, depression, and anemia, which can jeopardize patient compliance and survival. Reducing the severity of these adverse effects is essential to improving both treatment tolerance and outcomes.

​Exposure to low levels of singlet oxygen has been anecdotally reported to reduce toxicity and improve treatment tolerance in some individuals. The following clinical observations are presented alongside plausible biological mechanisms, though they require controlled validation.

Clinical Case 1: Cancer treatment side-effects: Myelosuppression, Nausea, Fatigue, and Depression

A 50-year-old patient undergoing chemotherapy for lymphoma presented with profound exhaustion, anemia (Hb 11.2 g/dL), breathlessness, and depression. Following the initiation of SOT, the patient reportedly experienced a rapid improvement in energy and his general mood. After one month, hemoglobin levels increased to 12.6 g/dL. Subsequent rounds of chemotherapy were better tolerated, with notably reduced nausea and fatigue.

Proposed Mechanisms: These effects may result from hormetic activation of protective cellular pathways, including Nrf2-mediated antioxidant responses,⁴⁸ mitochondrial biogenesis, enhanced ATP production and improved redox homeostasis. Modulation of inflammatory signaling, and improved mitochondrial function could account for the observed improvements in fatigue and emotional resilience.

Clinical Case 2: Radiation-Induced Neuropathy and Sensory Disturbances

A patient with a history of extensive radiation therapy developed peripheral neuropathy characterized by numbness and an atypical persistent “sharp smell,” consistent with radiation-induced oxidative injury and microvascular compromise, which promote neuronal apoptosis, demyelination, and fibrotic entrapment of nerves. After initiating SOT, the patient reported a complete resolution of both sensory disturbances.

Proposed Mechanisms: These outcomes may reflect activation of redox-sensitive neuroprotective pathways, including restoration of mitochondrial redox balance, Nrf2-regulated antioxidant responses, and modulation of glial-driven neuroinflammation (e.g., JNK and ERK signaling). These mechanisms are consistent with those previously discussed in Sections 5 and 7.

5.10. Ocular Surface Health and Relief from Dry Eye Symptoms

​Dry eye disease is a chronic disorder characterized by insufficient tear production or excessive tear evaporation, resulting in ocular discomfort, surface inflammation, and visual disturbances. Its pathophysiology often involves immune-mediated processes, including macrophage activation and the release of pro-inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), which amplify epithelial damage and perpetuate symptoms.

​Conventional therapies, such as artificial tears, corticosteroids, and immunosuppressants like cyclosporine—primarily aim to stabilize the tear film and suppress inflammation. More recently, antioxidant and redox-regulating strategies have emerged as promising adjuncts, given growing evidence that oxidative stress is a key contributor to dry eye pathology.

Anecdotal user reports suggest that exposure to low-level, non-irradiative singlet oxygen may alleviate symptoms of both chronic dry eye and allergic periorbital irritation. Users have described reduced ocular dryness and irritation, as well as rapid relief from itching, redness, and swelling commonly associated with allergic conjunctivitis. These improvements were sometimes observed within hours to days of initiating therapy.

Proposed Mechanisms: These effects may be mediated by the redox-sensitive and immune-modulatory pathways discussed earlier in the review. Singlet oxygen, when delivered at hormetic doses, likely induces mild oxidative preconditioning, which downregulates inflammatory cascades and stabilizes epithelial function. Specifically, suppression of mast cell degranulation and eosinophilic infiltration may reduce the release of histamine, leukotrienes, IL-4, IL-5, and IL-13, thereby lowering vascular permeability and sensory nerve activation.⁷³ Additionally, SOT may restore epithelial barrier integrity through Nrf2-driven antioxidant responses (Section 5.2), increasing resistance to allergens and environmental irritants. These mechanisms may also contribute to broader immune tolerance at the mucosal surface, as previously observed in respiratory and gastrointestinal contexts (Section 7).

Taken together, these observations support the possibility that singlet oxygen may offer a non-pharmacological, mechanism-based intervention for both dry eye and allergic periorbital conditions. Rather than masking symptoms, it may address underlying biological dysfunction, illustrating a potential translational application of redox-hormeticprinciples.

5.11. Future Research

In parallel to growing anecdotal observations presented above, academic research into singlet oxygen as a therapeutic agent is also emerging. A 2024 doctoral dissertation by Grimwood at the University of Derby laid the groundwork for a randomized, double-blind controlled trial investigating the physiological and psychological effects of nightly low-dose singlet oxygen exposure in individuals with COPD. While clinical results were not yet reported at the time of submission, the study involved over 180 participants and applied validated tools such as the CAT, ESS, and PSQI. Its rigorous patient-centered design and emphasis on redox-modulating therapies underscore the increasing legitimacy of this research direction and the need for peer-reviewed, controlled evaluations of clinical outcomes.

6. Ruling Out the Placebo Effect: Evidence from Non-Conscious and Indirect Exposure

One of the major challenges in evaluating novel therapeutic interventions, particularly those involving subtle physiological modulation, is distinguishing true biological effects from placebo responses. Placebo responses are mediated through central nervous system circuits involving expectation, emotional modulation, and dopamine-linked reward pathways. These mechanisms can produce transient symptom relief, but do not activate cellular repair pathways, alter redox signaling, or regenerate damaged tissue.

In the case of singlet oxygen-enriched air, several cases presented in this review provide strong evidence that the physiological benefits associated with low-dose singlet oxygen are not merely psychological. These observations, particularly in individuals who were unaware of the intervention or incapable of expectancy-based responses, provide strong evidence for a biological, rather than psychological, basis for the effects.

As previously described in the section on mucociliary function, a severely brain-injured young man, fully bedridden and non-communicative, experienced a sustained and documented reduction in the need for routine suctioning following the introduction of the singlet oxygen-enriched air generator. The cessation of routine suctioning, previously required nightly to prevent choking, occurred without any awareness or behavioral change on the part of the patient, offering compelling support for a direct physiological effect of the therapy on mucociliary function.

A similar conclusion can be drawn from the case reported under anemia and blood disorders, where an elderly man with dementia and chronic illness exhibited a sustained rise in hemoglobin levels after exposure to singlet oxygen-enriched air. Given his cognitive impairment and lack of knowledge regarding the intervention, the improvement in anemia, thyroid, and renal parameters could not have been driven by expectancy effects.

In the context of oncological support, the case of a terminal lymphoma patient, documented under supporting cancer treatment and recovery, showed an unexpectedly rapid return of verbal communication, appetite, and increased energy shortly after the initiation of low-level ambient singlet oxygen generator use. Although this patient may have been marginally aware of environmental changes, the timing, magnitude, and multifaceted nature of the improvements, including enhanced chemotherapy tolerance and rising hemoglobin levels, suggest physiological mechanisms were at play.

Further support comes from multiple reports involving young children and infants, discussed under respiratory conditions and excess phlegm, who experienced improvements in breathing, mucus clearance, and sleep quality. Given their developmental stage and limited awareness, placebo responses are not a plausible explanation in these cases.

These observations, taken together across multiple populations and physiological systems, indicate that the benefits of singlet oxygen exposure are not the result of suggestion or expectation. Instead, they reflect a pattern of redox-modulated physiological responses consistent with hormetic activation of repair and defense pathways.

7. Singlet Oxygen: A Pathway to Regulated Preventive Therapy

Low-dose exposure to singlet oxygen represents a fundamentally different therapeutic strategy compared to conventional pharmaceuticals. While most drugs act by targeting specific receptors or pathways, singlet oxygen functions mainly through hormesis, which is a mild oxidative stimulus that activates broad, evolutionarily conserved defense systems. These include autophagy, upregulation of antioxidant enzymes, and metabolic adaptation. ¹⁰⁵

Such pathways are self-limiting, highly regulated, and sensitive to context. This enables singlet oxygen to maintain physiological balance without the risks of chronic overstimulation or suppression often associated with pharmacological agents. These characteristics make it especially relevant to preventive medicine, where early modulation of redox signaling can reduce inflammation, enhance cellular resilience, and delay disease progression. ¹²³

Moreover, unlike many drug-based interventions, the protective responses triggered by singlet oxygen are not heavily influenced by individual genetic polymorphisms or metabolic profiles. This broad compatibility increases its potential applicability across diverse populations.^{Error! Bookmark not defined.} Together, these features position singlet oxygen as a novel, non-pharmacological tool for modulating redox-sensitive biological networks to prevent disease onset and maintain long-term physiological equilibrium.

8. Conclusions

This review highlights the emerging role of singlet oxygen as a hormetic agent capable of inducing protective and adaptive cellular responses when delivered at low concentrations. Historically regarded primarily as a cytotoxic species, singlet oxygen is now recognized for its ability to modulate key signaling pathways that regulate antioxidant defenses, autophagy, and inflammation.

By activating regulatory circuits such as the Nrf2-Keap1 axis, MAPK signaling, and JNK-mediated autophagy, mild oxidative stress initiated by singlet oxygen promotes detoxification, cellular repair, and long-term resilience to environmental and physiological challenges.

These mechanistic insights are supported by case observations presented in this review, which document improvements in a wide array of conditions, including asthma, COPD, sleep disturbances, anemia, and chemotherapy-related fatigue, following exposure to singlet oxygen-enriched air. The singlet oxygen-enriched air generator, which enables safe and non-invasive delivery of singlet oxygen under ambient conditions, represents a promising tool for translating these physiological mechanisms into therapeutic and preventive applications.

Together, the evidence supports the view that low-dose singlet oxygen can serve as a valuable adjunct in regenerative, anti-inflammatory, and resilience-enhancing interventions—laying the groundwork for future clinical validation and wider integration into health-supportive strategies.