Indoor environment factor · measured in µg/m³ or ppb (1 ppb ≈ 2 µg/m³)
Ozone (O₃)
Also: O3, ground-level ozone
A reactive form of oxygen with three atoms instead of two. Beneficial in the stratosphere, at ground level it irritates the airways and eyes and reacts with indoor compounds to form further pollutants, formaldehyde and ultrafine particles among them.
Why it matters indoors
Ozone at ground level forms when sunlight drives reactions between nitrogen oxides and volatile organic compounds, mainly from traffic. It is the main component of summer smog. Indoors it is short-lived, but its reactivity is exactly the problem: it attacks the carbon–carbon double bonds in terpenes from cleaning products, in skin oils and in soft furnishings, producing formaldehyde, other aldehydes and ultrafine particles.
Where it comes from indoors
Infiltration of outdoor air on high-ozone days is the main route. Indoor sources include ionisers and ozone-generating “air purifiers”, electrostatic precipitators, laser printers and copiers, and germicidal ultraviolet lamps of the wrong wavelength.
Health effects
Ozone inflames the airway lining. Short-term exposure causes cough, chest tightness and reduced lung function, and increases hospital admissions for asthma and respiratory disease; long-term exposure is associated with the development of asthma and with respiratory mortality. Effects begin below the guideline value in sensitive individuals, particularly children exercising outdoors on high-ozone days.
Exposure limits
| Jurisdiction | Value | Averaging time | Source |
|---|---|---|---|
| World Health Organization | 100 µg/m³ | 8-hour daily maximum | WHO AQG 2021 |
| World Health Organization | 60 µg/m³ | peak season (average of daily 8-hour maxima over the six highest months) | WHO AQG 2021 |
| European Union (ambient) | 120 µg/m³ | 8-hour daily maximum, target value | Directive (EU) 2024/2881 |
Slovak, UAE and Hong Kong values are being compiled in the indoor air quality limits table.
How it is measured
Reference monitors use ultraviolet absorption. Compact electrochemical or metal oxide sensors track ozone at ambient levels, with cross-sensitivity to nitrogen dioxide that good monitors correct for.
What the studies report
12 of 42 Findings — those with the strongest study design.
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| — | mean 20 | — · conference room | field experiment in a closed conference room of an office suite in New York: far-UVC lamps at 222 nanometres were run at single-lamp and at higher intensity while ozone, particle number and particle size were measured continuously in the room and in the adjacent open lounge over experiments lasting up to ten daytime hours | F-1258 McNeill Effects of germicidal far-UVC on ozone and particulate matte… (2025) | machine |
| — | difference 10 | — · conference room | field experiment in a closed conference room of an office suite in New York: far-UVC lamps at 222 nanometres were run at single-lamp and at higher intensity while ozone, particle number and particle size were measured continuously in the room and in the adjacent open lounge over experiments lasting up to ten daytime hours | F-1259 McNeill Effects of germicidal far-UVC on ozone and particulate matte… (2025) | machine |
| — | difference 3 | — · conference room | field experiment in a closed conference room of an office suite in New York: far-UVC lamps at 222 nanometres were run at single-lamp and at higher intensity while ozone, particle number and particle size were measured continuously in the room and in the adjacent open lounge over experiments lasting up to ten daytime hours | F-1261 McNeill Effects of germicidal far-UVC on ozone and particulate matte… (2025) | machine |
| — | emission-rate 1040 | — · office | measurement study in office spaces in Denmark: germicidal ultraviolet lamps with a peak emission at 222 nanometres were run in the rooms while ozone, ultrafine particles and volatile organic compounds were measured continuously before, during and after operation, under unoccupied conditions and with one and with two people present, with the ozone production rate also measured against the ultraviolet path length | F-1262 Influence of Germicidal UV (222 nm) Lamps on Ozone, Ultrafin… (2024) | machine |
| — | emission-rate 17 | — · office | measurement study in office spaces in Denmark: germicidal ultraviolet lamps with a peak emission at 222 nanometres were run in the rooms while ozone, ultrafine particles and volatile organic compounds were measured continuously before, during and after operation, under unoccupied conditions and with one and with two people present, with the ozone production rate also measured against the ultraviolet path length | F-1263 Influence of Germicidal UV (222 nm) Lamps on Ozone, Ultrafin… (2024) | machine |
| Low birth weight | RR 1.055 (1.034–1.077) per 1 | pregnant women and their term births · city (ambient) | cohort of 113,382 births in Shandong Province, China, between 2016 and 2022: ozone, particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide and temperature during pregnancy estimated at the mother's address and related to term low birth weight and small for gestational age with logistic regression, Cox proportional hazards models and a mediation analysis through gestational hypertension | F-1294 Gestational Hypertension as a Mediator of Prenatal Ozone Exp… (2026) | machine |
| Small for gestational age (SGA) | RR 1.037 (1.026–1.048) per 1 | pregnant women and their term births · city (ambient) | cohort of 113,382 births in Shandong Province, China, between 2016 and 2022: ozone, particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide and temperature during pregnancy estimated at the mother's address and related to term low birth weight and small for gestational age with logistic regression, Cox proportional hazards models and a mediation analysis through gestational hypertension | F-1295 Gestational Hypertension as a Mediator of Prenatal Ozone Exp… (2026) | machine |
| Infertility | difference 52 | women aged 15 to 60 · city (ambient) | cohort of women aged 15 to 60 in Taiwan drawn from the National Health Insurance Research Database between July 2003 and December 2013: concentrations of sulfur dioxide, carbon monoxide, ozone, particulate matter, nitrogen oxides, total and non-methane hydrocarbons and methane estimated at the insurance registration address and related to incident infertility per standard deviation of ten-year exposure | F-1296 Long-term exposure to ambient air pollutants and female infe… (2025) | machine |
| — | OR 0.72 (0.50–1.04) per 1 | women undergoing in vitro fertilisation · city (ambient) | prospective cohort of women undergoing in vitro fertilisation at a Boston clinic: time-varying exposure to nitrogen dioxide, fine particulate matter, black carbon and ozone estimated for four windows of each cycle and related to failure of the cycle with logistic regression | F-1297 Gaskins Time-Varying Exposure to Air Pollution and Outcomes of in Vi… (2019) | machine |
| — | median 0.12 | — · dwelling | cross-sectional measurement study in Regina, Saskatchewan, Canada: indoor and outdoor air pollutants measured in 106 homes in winter and 111 homes in summer of 2007 (71 homes in both seasons), with air exchange rates, housing characteristics and questionnaires as predictors | F-0697 Héroux Predictors of indoor air concentrations in smoking and non-s… (2010) | machine |
| Emission test chamber method | mean 14 | — · test chamber | chamber study at the National Institute of Standards and Technology: a freshly cut Douglas fir stood for seventeen days in an environmentally controlled chamber and the volatile organic compounds it emitted were measured continuously with a proton transfer reaction mass spectrometer, with ozone injected on several of the days to observe the indoor chemistry it starts | F-1237 Poppendieck et al. Jingle bells, what are those smells? Indoor VOC emissions fr… (2024) | machine |
| — | mean 4.3 | two adult inhabitants of the house · home | eight-week summer campaign in a normally occupied single-family house in California with two adult inhabitants: space- and time-resolved measurements of ozone and of volatile organic compounds by proton transfer reaction time-of-flight mass spectrometry in the living space, in the crawlspace and outdoors, with the indoor source strength of each compound regressed against the indoor ozone concentration | F-1241 Observing ozone chemistry in an occupied residence | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
How Coridum measures it: Coridum devices with an ozone channel use an electrochemical sensor referenced to WHO's 8-hour guideline.
Identifiers
- Molecular formula
- O₃
- IUPAC name
- trioxygen
- CAS Registry Number
- 10028-15-6 · PubChem · CTD
- InChIKey
- CBENFWSGALASAD-UHFFFAOYSA-N
- MeSH
- D010126
- SMILES
[O-][O+]=O
Frequently asked questions
Where does indoor ozone come from?
Mostly from outside on sunny, still days, entering through windows and ventilation. Indoor sources are ionising air purifiers, some 'ozone generators' sold as cleaners, laser printers and photocopiers, and ultraviolet lamps.
Are ozone air purifiers safe?
Devices that deliberately generate ozone are not recommended for occupied spaces. Ozone at effective disinfecting concentrations is well above health limits, and it reacts with common indoor compounds to create secondary pollutants.
Why is indoor ozone usually lower than outdoors?
Ozone is so reactive that it is consumed by surfaces, furnishings and other gases within minutes of entering. Indoor levels are typically 20–70 % of outdoor levels — but the reactions themselves produce other pollutants.
Sources
- WHO global air quality guidelines: particulate matter (PM₂.₅ and PM₁₀), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. World Health Organization, 2021.
- Directive (EU) 2024/2881 on ambient air quality and cleaner air for Europe.
- Observing ozone chemistry in an occupied residence. Proceedings of the National Academy of Sciences of the United States of America. doi:10.1073/pnas.2018140118 CKB 0236
- The impact of clothing on ozone and squalene ozonolysis products in indoor environments… (2019). Communications Chemistry. doi:10.1038/s42004-019-0159-7 CKB 0165
- Weschler Human skin oil - a major ozone reactant indoors (2023). Environmental Science: Atmospheres. doi:10.1039/d3ea00008g CKB 0215
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Ozone (O₃). Coridum Knowledge G-0007, 16 September 2026. https://coridum.com/k/G-0007/