# Reactive oxygen species
**Reactive oxygen species** (ROS) are intermediates in the reduction of molecular dioxygen (O₂) to water, including the superoxide radical (O₂•−), hydrogen peroxide (H₂O₂), and the hydroxyl radical (HO•)[1](https://goldbook.iupac.org/terms/view/13634). The term is a collective one: it covers species derived from O₂ that are more reactive than O₂ itself, both free radicals and non-radical oxidants such as hypochlorous acid and singlet oxygen, and it spans molecules whose biological roles range from redox signaling to cell damage[2](https://www.nature.com/articles/s42255-022-00591-z) • [3](https://www.nature.com/articles/s41580-022-00456-z). Because these species differ so widely in reactivity, lifetime, and function, current guidelines recommend naming the actual chemical species involved in a biological process rather than using the generic label "ROS"[2](https://www.nature.com/articles/s42255-022-00591-z) • [3](https://www.nature.com/articles/s41580-022-00456-z).
| Key fact | Detail |
|---|---|
| Definition | Intermediates in the reduction of O₂ to water: superoxide O₂•−, hydrogen peroxide H₂O₂, hydroxyl radical HO•[1](https://goldbook.iupac.org/terms/view/13634) |
| Classes | Radical ROS (•OH, ROO•, O₂•−, RO•) and non-radical oxidants (HOCl, singlet O₂, H₂O₂, peroxynitrite)[4](https://www.degruyterbrill.com/document/doi/10.1515/pac-2020-0902/html?lang=de) |
| Oxygen flux to ROS | Widely cited ~2% of oxygen consumption came from artificial isolated-mitochondria conditions; physiological basal value is ~0.2%[5](https://www.cell.com/fulltext/S0092-8674(05)00109-1) |
| Hydroxyl radical | Half-life ~1 ns, diffusion-limited rate constants of 10⁹–10¹⁰ M⁻¹ s⁻¹, reactivity confined to ~50 molecular diameters from its formation site[6](https://link.springer.com/article/10.1007/s00204-024-03696-4) |
| H₂O₂ | Not a free radical; diffuses through aquaporins more easily than through lipid membranes; removed by catalase and glutathione peroxidase[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/) |
| Mitochondrial output | Isolated rat muscle mitochondria produce ~890 pmol H₂O₂ min⁻¹ mg protein⁻¹ with succinate, falling to ~50 during intense aerobic exercise[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5641882/) |
| Antioxidant trials | Double-blinded randomized trials of beta-carotene, vitamin C, and vitamin E generally failed to influence disease activity[2](https://www.nature.com/articles/s42255-022-00591-z) |
## Definition and chemical classes
IUPAC defines ROS as the intermediates formed as molecular oxygen accepts electrons on its way to water[1](https://goldbook.iupac.org/terms/view/13634). An IUPAC Technical Report divides them into radical species, hydroxyl (•OH), peroxyl (ROO•), superoxide (O₂•−), and alkoxyl (RO•), and non-radical oxidizing species that are easily radical-convertible, such as hypochlorous acid (HOCl) and singlet dioxygen (¹O₂)[4](https://www.degruyterbrill.com/document/doi/10.1515/pac-2020-0902/html?lang=de). All oxygen radicals are ROS, but not all ROS are radicals[2](https://www.nature.com/articles/s42255-022-00591-z).
**What makes a species reactive.** Free-radical ROS carry one unpaired electron in the outer orbit and are commonly described as one-electron oxidants; non-radical species such as H₂O₂, peroxynitrite (ONOO−), and HOCl are commonly described as two-electron oxidants. Some free-radical reactions proceed through chain steps of initiation, propagation, and termination. One-electron reduction potentials provide a basis for comparing oxidizing strength, with the hydroxyl radical graded highest and nitric oxide lowest[9](https://rcastoragev2.blob.core.windows.net/a3894e83d458b636638c568b8b79cf4d/PMC6831867.pdf). Reactivity also varies enormously within the family: superoxide and H₂O₂ are selective and leave most biomolecules unscathed, whereas •OH attacks essentially everything at near diffusion-controlled rates[2](https://www.nature.com/articles/s42255-022-00591-z).
## Sources and generation mechanisms
Mitochondrial ROS generation was first reported in the early 1970s[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/). Early studies suggested that about 2% of total oxygen consumption was funneled to ROS generation, a figure that was widely cited even though the measurements were made under artificial isolated-mitochondria conditions; subsequent work under more physiological conditions reduced the basal value to about 0.2%[5](https://www.cell.com/fulltext/S0092-8674(05)00109-1). A common textbook figure that approximately 1–2% of total oxygen is converted to superoxide radicals in cells[6](https://link.springer.com/article/10.1007/s00204-024-03696-4) therefore sits well above the physiological estimate for mitochondria, and the two values should not be conflated.
Quantitative work has moved from a single "ROS burden" to individual sources. Total rates of mitochondrial H₂O₂ production in isolated rat muscle mitochondria were approximately 890 pmol H₂O₂ min⁻¹ mg protein⁻¹ with succinate, about 620 with glycerol 3-phosphate, about 200 with palmitoylcarnitine, about 180 with glutamate plus malate, and about 340 at rest, 80 during mild aerobic exercise, and 50 during intense aerobic exercise[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5641882/). The relative contributions of specific sites (site IQ, site IIF, site IF, site IIIQo) vary with bioenergetic condition[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5641882/). Recent studies have identified at least eleven individual sources of mitochondrial ROS (ISOMRs) with distinct, dynamic, and often source-specific behaviors, revising the older view of ROS as a unitary, non-specific oxidative burden[10](https://www.cell.com/molecular-cell/fulltext/S1097-2765(26)00473-9).
## Chemical kinetics and reactivity
Lifetimes and diffusion distances span many orders of magnitude. Reported values are: hydroxyl radical about 10⁻⁹ s and 4.5 nm; superoxide about 50 ms and 320 nm; hydrogen peroxide about 1 ms and 1.5 mm; peroxynitrite about 15 µs and 50 µm; nitric oxide about 1 s and 10 m[9](https://rcastoragev2.blob.core.windows.net/a3894e83d458b636638c568b8b79cf4d/PMC6831867.pdf). The hydroxyl radical's half-life in biological systems is approximately 1 ns, it reacts at diffusion-limited rate constants of 10⁹–10¹⁰ M⁻¹ s⁻¹, and its reactivity is limited to approximately 50 molecular diameters from the site of formation, so hydroxyl damage is strictly local[6](https://link.springer.com/article/10.1007/s00204-024-03696-4).
**Diffusion and speciation.** Superoxide's chemistry depends on pH. Its conjugate acid, the hydroperoxyl radical (HO₂•), dismutates about 100 times faster than O₂•− and, being uncharged, can cross membranes; the pKa of the HO₂•/O₂•− pair is 4.8, so very little superoxide is protonated in the mitochondrial matrix. Spontaneous dismutation is faster at lower pH, with k = 5 × 10⁵ M⁻¹ s⁻¹ at pH 7.0 versus 10² M⁻¹ s⁻¹ at pH 11[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/). H₂O₂, like water, diffuses more easily through aquaporins than through lipid membranes, which is one reason a comparatively unreactive species can act at a distance from its source[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/).
**Radical cross-reactions.** [Peroxynitrite](https://www.edgepedia.org/peroxynitrite), formed from nitric oxide and superoxide, is a nonradical that is protonated to cytotoxic peroxynitrous acid, causing thiol depletion, lipid oxidation, DNA strand breakage, and base nitration[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/). The rate constant of peroxynitrite formation in vivo exceeds the rate constants of nitric oxide reacting with hemoglobin (k ~ 5–6 × 10⁷ M⁻¹ s⁻¹) and of superoxide conversion by superoxide dismutase (k ~ 2 × 10⁹ M⁻¹ s⁻¹)[6](https://link.springer.com/article/10.1007/s00204-024-03696-4).
## Antioxidant defense systems
The first line against superoxide is dismutation to hydrogen peroxide by superoxide dismutase (SOD), with k = 1.6 × 10⁹ M⁻¹ s⁻¹[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/). H₂O₂ is then kept low by catalase and glutathione peroxidase[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/). Outside enzymes, radical chain propagation terminates when radicals react with phenolic antioxidants such as vitamin E, flavonoids, and ascorbic acid, or with another radical to form a covalent bond, a step called termination[9](https://rcastoragev2.blob.core.windows.net/a3894e83d458b636638c568b8b79cf4d/PMC6831867.pdf).
## Redox signaling versus oxidative damage
Rather than serving only as harmful waste products of aerobic metabolism, ROS function throughout the body as redox signaling agents that control a wide range of processes, including cell proliferation, differentiation, inflammatory responses, autophagy, and programmed cell death; whether the result is physiological or pathological depends on the balance between their production and their elimination.[11](https://link.springer.com/article/10.1007/s00204-025-04276-w) At low to moderate concentrations ROS act as signaling mediators, a state termed good stress or eustress; at moderate to high concentrations they cause damage to biomolecules[6](https://link.springer.com/article/10.1007/s00204-024-03696-4).
**Telling signaling from damage.** Because ROS have short half-lives, oxidative stress in vivo can only be evaluated indirectly. The modern framework separates signaling proxies, such as reversible cysteine oxidation, the peroxiredoxin redox state, and compartment-targeted H₂O₂ reporters, from damage proxies, such as lipid-peroxidation adducts, protein carbonyls, nitrotyrosine, DNA oxidation and strand breaks, and evidence of bioenergetic failure[6](https://link.springer.com/article/10.1007/s00204-024-03696-4) • [11](https://link.springer.com/article/10.1007/s00204-025-04276-w). According to IUPAC, oxidative stress refers to the harmful outcomes that arise when ROS production outpaces what a biological system can detoxify and remove; the surplus ROS may then injure cellular lipids, proteins, and DNA.[12](https://goldbook.iupac.org/terms/view/11982)
## By the numbers
- Oxygen-to-superoxide fraction: ~1–2% of total oxygen in cells by one review[6](https://link.springer.com/article/10.1007/s00204-024-03696-4), revised to ~0.2% of oxygen consumption for basal mitochondrial production under physiological conditions[5](https://www.cell.com/fulltext/S0092-8674(05)00109-1).
- Mitochondrial H₂O₂ output: ~890 pmol min⁻¹ mg protein⁻¹ (succinate) down to ~50 (intense exercise) in isolated rat muscle mitochondria[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC5641882/).
- Half-lives: •OH ~10⁻⁹ s, O₂•− ~50 ms, H₂O₂ ~1 ms, ONOO− ~15 µs, NO ~1 s[9](https://rcastoragev2.blob.core.windows.net/a3894e83d458b636638c568b8b79cf4d/PMC6831867.pdf).
- SOD dismutation: k = 1.6 × 10⁹ M⁻¹ s⁻¹[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/); hydroxyl radical reactions: 10⁹–10¹⁰ M⁻¹ s⁻¹[6](https://link.springer.com/article/10.1007/s00204-024-03696-4).
## Measurement and why probes mislead
Superoxide quantification with dihydroethidium (DHE) requires HPLC separation of 2-hydroxyethidium, because raw fluorescence is non-specific and cannot discriminate the superoxide-specific product from ethidium and other adducts; MitoSOX is susceptible to photobleaching, redox cycling, and DNA intercalation[11](https://link.springer.com/article/10.1007/s00204-025-04276-w). Amplex-type H₂O₂ reporters pool signals from multiple sources and can be confounded by endogenous peroxidases and scavengers, requiring catalase or catalase-mimetic negative controls[11](https://link.springer.com/article/10.1007/s00204-025-04276-w). The Nature Metabolism guidelines advise against the d-ROMS assay, TBARS, and determinations of total antioxidant activity in clinical studies, and recommend validated biomarkers in preliminary dose-ranging antioxidant studies[2](https://www.nature.com/articles/s42255-022-00591-z). The field still needs site-specific, real-time tools for detecting and quantifying individual oxidants in cells and model organisms, along with physiological O₂ levels in cell culture[3](https://www.nature.com/articles/s41580-022-00456-z).
## ROS in disease and antioxidant therapy
Many double-blinded randomized clinical trials of "antioxidants" such as beta-carotene, vitamin C, and vitamin E generally failed to influence disease activity, and in most cases the effect on oxidative damage was not measured[2](https://www.nature.com/articles/s42255-022-00591-z). Evidence that small-molecule antioxidant supplementation slows aging or increases longevity is insufficient to support the mitochondrial free radical theory of aging; the mitochondria-targeted antioxidant MitoQ failed to extend lifespan in [Drosophila melanogaster](https://www.edgepedia.org/drosophila-melanogaster)[13](https://link.springer.com/article/10.1186/2046-2395-3-4).
**Targeted antioxidants in trials.** In a 42-patient pilot double-blind, placebo-controlled randomized trial of septic shock, oral MitoQ 20 mg twice daily for 5 days improved oxidative biomarkers at day 5 versus placebo (GPx +0.14 U/mL, CAT +15.4 U/mL, SOD +0.82 U/mL, MDA −0.86 µmol/L; all P < 0.05), but did not significantly improve 28-day mortality (28.6% vs 38.1%; P = 0.513), organ recovery (33.3% vs 23.8%; P = 0.495), or vasopressor duration (66.2 vs 91.3 h, P = 0.087); the vasoactive-inotropic score showed a significant Treatment × Time interaction (P = 0.001)[14](https://www.springermedizin.de/a-pilot-double-blind-placebo-controlled-randomized-clinical-tria/51354466). Human evidence for MitoQ in sepsis remains preliminary: the trial showed biochemical redox effects but no clinical or hemodynamic outcome improvement, and participants were not selected for a mitochondrial-injury phenotype[15](https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2026.1975929/full).
In aging physical function, 4 weeks of MitoQ (250 µM in drinking water) attenuated the age-related decline in grip strength, coordination, and endurance in 27-month-old male C57BL6/N mice, with lower skeletal muscle mitochondrial superoxide and inflammation markers, but 6 weeks of MitoQ 20 mg/day showed no convincing effect on physical function in healthy older adults (N = 18, aged 60–79); exploratory subgroup analyses suggest possible effects on peak leg extension power and grip strength in participants aged 70 or older[16](https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP289428). In vascular function, chronic oral MitoQ improved flow-mediated dilation in older adults from 5.7 ± 0.4% to 7.0 ± 0.5% (interaction p = 0.0002 vs placebo), associated with a 61% reduction in mitoROS-related suppression of endothelial function[17](https://journals.physiology.org/doi/abs/10.1152/physiol.2026.41.S1.2294134?doi=10.1152%2Fphysiol.2026.41.S1.2294134).
**Crosstalk with other reactive species.** ROS pathophysiology cannot be understood without their crosstalk with reactive nitrogen, sulfur, and carbon species, which form an interconnected redox network; peroxynitrite formation from superoxide and nitric oxide diverts vasoprotective NO signaling toward cytotoxic routes, and labile iron availability conditions Fenton chemistry and ferroptosis susceptibility[11](https://link.springer.com/article/10.1007/s00204-025-04276-w). Reactive oxygen, nitrogen, and sulfur species (RONSS) together form a broad category whose members are indispensable in redox signaling but damaging when overproduced[18](https://pubs.rsc.org/en/content/articlelanding/2025/cb/d5cb00006h).
## Recent developments
Three revisions stand out. First, mitochondrial ROS biology has moved from a single burden to at least eleven individual sources (ISOMRs) with distinct, dynamic, and often source-specific behaviors[10](https://www.cell.com/molecular-cell/fulltext/S1097-2765(26)00473-9). Second, measurement guidance has become species-specific: DHE requires chromatographic separation, Amplex-type reporters need peroxidase controls, and d-ROMS, TBARS, and total antioxidant capacity are discouraged in clinical studies[11](https://link.springer.com/article/10.1007/s00204-025-04276-w) • [2](https://www.nature.com/articles/s42255-022-00591-z). Third, randomized trials of a mitochondria-targeted antioxidant have reported mixed outcomes: redox biomarkers improved in septic shock without clinical benefit[14](https://www.springermedizin.de/a-pilot-double-blind-placebo-controlled-randomized-clinical-tria/51354466), physical function did not convincingly change in healthy older adults[16](https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP289428), and vascular endothelial function improved with a 61% reduction in mitoROS-related suppression[17](https://journals.physiology.org/doi/abs/10.1152/physiol.2026.41.S1.2294134?doi=10.1152%2Fphysiol.2026.41.S1.2294134).
## References
1. [IUPAC Gold Book – reactive oxygen species](https://goldbook.iupac.org/terms/view/13634)
2. [Guidelines for measuring reactive oxygen species and oxidative damage in cells and in vivo, Nature Metabolism (2022)](https://www.nature.com/articles/s42255-022-00591-z)
3. [Defining roles of specific reactive oxygen species (ROS) in cell biology and physiology, Nature Reviews Molecular Cell Biology (2022)](https://www.nature.com/articles/s41580-022-00456-z)
4. [Methods to evaluate the scavenging activity of antioxidants toward reactive oxygen and nitrogen species, IUPAC Technical Report](https://www.degruyterbrill.com/document/doi/10.1515/pac-2020-0902/html?lang=de)
5. [Mitochondria, Oxidants, and Aging, Cell](https://www.cell.com/fulltext/S0092-8674(05)00109-1)
6. [Several lines of antioxidant defense against oxidative stress, Archives of Toxicology (2024)](https://link.springer.com/article/10.1007/s00204-024-03696-4)
7. [Mitochondrial Reactive Oxygen Species Production in Excitable Cells (PMC2842133)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2842133/)
8. [Production of superoxide and hydrogen peroxide from specific mitochondrial sites under different bioenergetic conditions (PMC5641882)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5641882/)
9. [Biochemical basis and metabolic interplay of redox regulation (mirror of PMC6831867)](https://rcastoragev2.blob.core.windows.net/a3894e83d458b636638c568b8b79cf4d/PMC6831867.pdf)
10. [Individual sources of mitochondrial ROS (ISOMRs), Molecular Cell (2026)](https://www.cell.com/molecular-cell/fulltext/S1097-2765(26)00473-9)
11. [Pathophysiology of reactive oxygen species (ROS), Archives of Toxicology (2025)](https://link.springer.com/article/10.1007/s00204-025-04276-w)
12. [IUPAC Gold Book – oxidative stress](https://goldbook.iupac.org/terms/view/11982)
13. [A midlife crisis for the mitochondrial free radical theory of aging, Longevity & Healthspan](https://link.springer.com/article/10.1186/2046-2395-3-4)
14. [A pilot double-blind, placebo-controlled, randomized clinical trial of MitoQ in the treatment of septic shock](https://www.springermedizin.de/a-pilot-double-blind-placebo-controlled-randomized-clinical-tria/51354466)
15. [MitoQ in sepsis-associated acute lung injury, Frontiers in Pharmacology (2026)](https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2026.1975929/full)
16. [MitoQ for age-associated physical dysfunction: mouse study and randomized cross-over clinical trial, Journal of Physiology](https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP289428)
17. [Chronic oral MitoQ improves vascular endothelial function in older adults, APS conference abstract (2026)](https://journals.physiology.org/doi/abs/10.1152/physiol.2026.41.S1.2294134?doi=10.1152%2Fphysiol.2026.41.S1.2294134)
18. [Navigating the dichotomy of reactive oxygen, nitrogen, and sulfur species, RSC Chemical Biology (2025)](https://pubs.rsc.org/en/content/articlelanding/2025/cb/d5cb00006h)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Reaction rates, mechanisms, and engineering › Chemical kinetics and reaction engineering*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license