Can nitrogen dioxide and ozone act synergistically? Some laboratory and animal studies found effects greater than those from either gas alone, but the result depends on the exposure and the health outcome measured. The evidence does not establish a universal synergistic effect in people breathing ordinary outdoor air.
What does “synergy” mean here?
A synergistic effect means the combined response to two pollutants is greater than the response expected by adding their individual effects. That comparison is specific to a measured outcome and experimental conditions: a result for one tissue change or health endpoint does not prove synergy for every effect, dose, or person.
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Combined effects can also be additive, less than additive, or different across outcomes. The word “synergy” therefore describes a finding under particular conditions—not an inherent, constant property of nitrogen dioxide (NO2) and ozone (O3).
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How are NO2 and ozone connected in the air?
NO2 is one of the nitrogen oxides, or NOx. The U.S. Environmental Protection Agency (EPA) says NO2 forms quickly from emissions including those from vehicles, power plants, and off-road equipment. NOx contributes to the formation of ground-level ozone and fine particles. EPA’s NO2 overview describes the pollutant, while its Integrated Science Assessment for Oxides of Nitrogen evaluates the wider evidence.
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That atmospheric relationship helps explain why the pollutants may occur in related pollution settings. It does not, by itself, show that breathing them together produces a biological effect greater than the sum of their separate effects. Atmospheric formation and toxicological interaction are different questions.
What did the laboratory and animal studies find?
Older controlled studies provide evidence that combined exposure can change biological responses, but they do not establish a single result that applies across conditions or translates directly into human risk.
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| Study and design | Exposure and sequence | Outcome and finding |
|---|---|---|
| Goldstein, 1976; in vitro study | Concentration, duration, and exposure sequence affected the response; exact values are not stated in the EPA HERO record. | Responses ranged from protective to synergistic depending on conditions and endpoint. This illustrates why synergy cannot be treated as an invariant property of the gases. |
| Ehrlich et al., 1977; mouse study | Mixtures included 1.5–5.0 ppm NO2 and 0.05–0.5 ppm ozone. The study compared a single mixture exposure with repeated daily exposures; the EPA HERO record summarizes the study. | A single mixture exposure produced an additive effect. Repeated daily exposure suggested synergy in reduced resistance to respiratory infection. This is animal evidence, not a measure of human risk. |
| 1984; Swiss Webster mouse study | 4.8 ppm NO2 and 0.45 ppm ozone, 8 hours daily for 7 days, in intermittent exposures, as reported in the EPA HERO record. | For many measured lung parameters, combined exposure caused changes greater than and statistically significant compared with exposure to either gas alone. The authors characterized the combined effects as more than additive and possibly synergistic. |
The concentrations and schedules in the animal experiments are controlled experimental conditions, not descriptions of ordinary ambient exposure. Their findings show that interactions are possible in particular models; they do not quantify what happens to people exposed to typical outdoor concentrations.
Do human studies prove the combination is more harmful?
No. A 2024 controlled human exposure abstract involved 22 healthy adults, but participants were exposed to ozone at 300 ppb or NO2 at 500 ppb separately for two hours, alongside clean air—not to both pollutants together. The abstract reported responses modified by sex, but because it did not test simultaneous combined exposure, it cannot demonstrate NO2–ozone synergy. See the EPA Risk Assessment Portal record for Pulczinski et al.
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An observational study examined mixtures containing ozone, NO2, and PM2.5 across 85 U.S. counties. Some mixture categories were associated with higher cardiovascular-admission rates, according to the EPA HERO study record. Because the exposure mixtures included fine particles as well as both gases, the study cannot isolate an NO2–ozone interaction or establish that the two gases caused the association.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What does the evidence establish—and what remains unknown?
- Established: Some controlled experiments have found greater-than-additive responses to combined NO2 and ozone for particular endpoints and conditions.
- Not established: That the gases always interact synergistically, or that one pollutant invariably makes the other more harmful.
- Not quantified by these studies: The added human health risk from breathing both pollutants together under ordinary ambient conditions.
- Not supplied by these findings: A combined NO2–ozone regulatory threshold. EPA’s assessments address pollutant evidence; they are not a combined-exposure threshold.
Assessment status matters when interpreting current summaries. EPA identifies its 2016 Oxides of Nitrogen Integrated Science Assessment as the last final assessment on the NOx assessment page, which also lists a July 2026 external review draft. That draft is not final. EPA identifies its 2020 assessment as the latest evaluation on its ozone assessment page.
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What are the known health effects of ozone and NO2?
The uncertainty about a combined effect does not mean either pollutant is harmless. EPA describes ozone as a powerful oxidant that can irritate the airways. Its listed effects include respiratory symptoms, lung-function decrements, and airway inflammation. People with asthma, children, older adults, and people active outdoors are among those EPA identifies as at greater risk from ozone exposure. Details are available in EPA’s pages on ozone health effects and ozone effects in the general population.
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EPA’s NO2 overview also identifies people with asthma, children, and older adults as groups generally at greater risk from NO2-related health effects. These known vulnerabilities should not be read as proof of a specifically quantified extra risk from the combination.
For day-to-day exposure decisions, check current local air-quality information and follow public-health guidance for your area. The studies summarized here do not provide a combined-pollutant exposure limit for personal use.
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