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Nitrogen Dioxide (NO₂): Sources, Health Effects and AQI

Nitrogen dioxide (NO₂) is a reactive gas produced mainly when fuels burn. Road traffic, power generation and combustion equipment are important sources, and short-term exposure can irritate airways and aggravate asthma.

AQI.news Editorial TeamUpdated 6 min read

Key facts

  • NO₂ is one of a group of reactive gases called nitrogen oxides.
  • Fuel combustion from vehicles, power plants and equipment is a major source.
  • NO₂ can irritate airways and worsen respiratory disease, especially asthma.
  • 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.

Sources

This reference was prepared from the following primary public-health and regulatory sources:

  1. Basic Information about Nitrogen DioxideUnited States Environmental Protection Agency
  2. WHO Global Air Quality GuidelinesWorld Health Organization