Pollutant Guide
Nitrogen Dioxide
By Jason Curtis · 3 min read · Updated 2026-05-21
What it is
Nitrogen dioxide (NO2) is a reddish-brown gas with a sharp, biting odor. It belongs to a group of gases called nitrogen oxides (NOx), which also includes nitric oxide (NO). The two interconvert in the atmosphere, but NO2 is the form most directly linked to health effects.
NO2 also reacts with VOCs in sunlight to produce ozone, and with ammonia and water to form fine particulate nitrates. It contributes to acid rain.
Where it comes from
The dominant outdoor source is fuel combustion at high temperatures: cars (especially diesel trucks and buses), power plants, ships, and heavy equipment. Concentrations are highest within about 300 meters of busy roads, freeways, and ports.
Indoors, the main sources are gas stoves and ovens, gas or kerosene space heaters, fireplaces, and tobacco smoke. A gas burner running for an hour in a poorly ventilated kitchen can push indoor NO2 above outdoor health standards. Recent studies link gas-stove use to roughly 13% of childhood asthma cases in the US.
Health effects
NO2 irritates the airways. Short-term exposure (30 minutes to a few hours) can worsen asthma, trigger wheezing and coughing, and increase emergency-room visits for respiratory symptoms. Children near busy roads have higher rates of asthma and reduced lung development.
Long-term exposure is associated with new asthma cases in children, more frequent respiratory infections, and increased risk of cardiovascular and respiratory mortality. People with asthma, COPD, or heart disease are most sensitive.
How it's measured and typical levels
NO2 is measured in parts per billion (ppb) or micrograms per cubic meter (µg/m³). One ppb equals about 1.88 µg/m³ at standard conditions.
Key benchmarks:
- WHO annual guideline: 10 µg/m³ (about 5 ppb)
- WHO 24-hour guideline: 25 µg/m³
- US EPA NAAQS, 1-hour: 100 ppb (98th percentile, averaged over 3 years)
- US EPA NAAQS, annual: 53 ppb
Background rural levels are 1 to 5 ppb. US urban averages typically run 10 to 25 ppb, with hourly spikes of 50 to 150 ppb near major roads at rush hour. Indoor peaks above a gas stove can reach 200 to 500 ppb.
What you can do
If you cook with gas, run the range hood every time you use it, and make sure it vents outside (many do not). Open a window. Consider replacing gas burners with induction over time; a portable induction burner is a cheap way to test the change.
Outdoors, avoid running, biking, or pushing a stroller along the curb of a busy road during rush hour. Move a few blocks back from highways when possible. HEPA filters do not capture NO2; activated carbon helps modestly. The best indoor strategy is source control plus ventilation.
If your home is within 500 feet of a freeway or busy truck route, an HVAC fresh-air filter with both HEPA and activated carbon is worth considering.
Sources
This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.