Nitrogen dioxide
Nitrogen dioxide is a reddish-brown reactive gas from combustion that can irritate airways and help form ozone and particle pollution.
What nitrogen dioxide is
Nitrogen dioxide is a reactive gas made of one nitrogen atom and two oxygen atoms. It is part of a larger family called nitrogen oxides, often written as NOx. In outdoor air, NO2 is both a pollutant on its own and an ingredient in the chemistry that forms ground-level ozone and some particle pollution.
How it forms
NO2 forms during high-temperature combustion when nitrogen and oxygen in air react. Cars, trucks, buses, ships, power plants, industrial boilers, construction equipment, and other engines can emit nitrogen oxides. Indoors, gas stoves, heaters, fireplaces, and other unvented or poorly vented combustion sources can raise NO2 levels.
Traffic and street-level exposure
Nitrogen dioxide often varies sharply near roads because vehicle exhaust is a major source. Concentrations can be higher close to busy streets, freight corridors, tunnels, ports, garages, and intersections. This makes NO2 useful for understanding traffic-related pollution, even when broader citywide air quality looks moderate.
Health effects
Breathing elevated NO2 can irritate the airways and worsen respiratory symptoms. People with asthma, children, and older adults are often more vulnerable. Short-term exposure can contribute to coughing, wheezing, and breathing difficulty, while repeated exposure to traffic-related pollution can add to broader respiratory health risks.
Ozone and particle chemistry
NO2 participates in sunlight-driven reactions that form ground-level ozone. Nitrogen oxides can also help form nitrate particles, a component of fine particulate matter. This means reducing NO2 and related emissions can help with multiple air quality problems, not just the concentration of NO2 itself.
Indoor air
Indoor NO2 matters because people spend much of their time inside. Gas cooking appliances and other fuel-burning devices can increase indoor levels, especially when ventilation is poor. Range hoods that vent outdoors, appliance maintenance, and avoiding unvented combustion sources can reduce exposure.
Monitoring and AQI
Air agencies monitor nitrogen dioxide outdoors and report it as part of the Air Quality Index when relevant. NO2 data also support pollution planning, transportation studies, environmental justice analysis, and satellite observations that show how emissions vary across cities and industrial regions.
Why it matters
Nitrogen dioxide matters because it sits at the intersection of combustion, traffic, indoor air, respiratory health, and atmospheric chemistry. Tracking NO2 helps communities understand where pollution comes from, who is exposed, and how changes in vehicles, buildings, fuels, and ventilation can improve air quality.