At a glance
What matters most
- 01
NO₂ is one of a group of reactive gases called nitrogen oxides.
- 02
Fuel combustion from vehicles, power plants and equipment is a major source.
- 03
NO₂ can irritate airways and worsen respiratory disease, especially asthma.
- 04
NO₂ also helps form ground-level ozone and particulate pollution.
Where NO₂ comes from
Nitrogen dioxide forms when fuel burns at high temperature. Outdoor sources include cars, trucks, buses, power plants and off-road equipment. Indoors, poorly vented gas appliances and other combustion sources can contribute.
Traffic-related concentrations can vary sharply over short distances and by time of day, so a city-wide value may not represent every roadside location.
Health and environmental effects
Breathing elevated NO₂ for short periods can irritate the respiratory system. People with asthma, children and older adults may be more susceptible to coughing, wheezing or difficulty breathing.
NO₂ and other nitrogen oxides participate in reactions that form ozone, particles, acid rain and haze. Reducing combustion emissions can therefore affect several pollutants at once.
How to interpret an NO₂ reading
- Check the unit: monitoring services commonly use parts per billion or micrograms per cubic metre.
- Match the reading to the correct averaging period before comparing it with a guideline.
- Use the pollutant sub-index, not concentration alone, when interpreting US AQI.
- Treat a live reading as current information, not a formal compliance determination.
Frequently asked questions
Is NO₂ the same as NOx?
No. NOx is a broader group of nitrogen oxides. NO₂ is one member and is commonly used as an indicator for the group.
Can traffic raise NO₂?
Yes. Cars, trucks and buses are important combustion sources, and concentrations can be higher near busy roads.
Does NO₂ create PM2.5?
Nitrogen oxides can react in the atmosphere and contribute to nitrate particles, which can be part of PM2.5.
Primary sources
This reference was prepared from the following primary public-health and regulatory sources: