Nitrogen Dioxide (NO₂)
Also known as: NO₂
What is NO₂?
Nitrogen dioxide is a highly reactive gas that forms from emissions from cars, trucks, buses, power plants, and off-road equipment. NO₂ is also a precursor to ozone formation and particulate matter.
Sources
Primary sources include motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. NO₂ can also form indoors from gas stoves, kerosene heaters, and tobacco smoke.
Health Effects
Short-term exposure can irritate airways in the human respiratory system and aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms, hospital admissions, and visits to emergency rooms. Long-term exposure may contribute to the development of asthma and potentially increase susceptibility to respiratory infections.
Protecting Yourself
The most reliable way to reduce your exposure is to monitor the Air Quality Index before spending extended time outdoors. On days when the AQI for NO₂ is elevated, consider moving strenuous exercise indoors and keeping windows closed if outdoor air is worse than indoor air.
Sensitive groups — including children, older adults, and people with asthma or heart disease — should take extra precautions at lower AQI thresholds than the general public. High-quality air purifiers with HEPA and activated carbon filters can significantly reduce indoor concentrations. You can track current NO₂ levels for any US city using the search on this site, which pulls directly from EPA AirNow monitoring stations.
EPA Standard
Annual average standard set by the EPA to protect public health.
AQI Scale
US Cities Most Affected by NO₂
Cities with the most days where NO₂ was the primary pollutant driving AQI.