Nitrogen dioxide (NO2) is a harmful gas produced by combustion processes in gas appliances, vehicle exhaust, and industrial sources that degrades indoor air quality. NO2 is a reddish-brown gas with a sharp, acrid odor and is classified as a criteria air pollutant by the EPA. It forms when nitrogen oxide emissions react with atmospheric oxygen.
Sources and Indoor Concentrations
Primary indoor sources include gas stoves, furnaces, water heaters, and attached garages. The EPA establishes 1-hour NO2 standards at 100 parts per billion and annual standards at 53 parts per billion. Poorly vented gas appliances can generate concentrations exceeding 1,000 parts per billion in enclosed spaces.
Health Effects and Mitigation
Short-term exposure to high NO2 levels irritates airways and reduces respiratory function, particularly in people with asthma. Children and elderly populations face elevated risk. Mitigation strategies include proper appliance venting, regular maintenance of combustion equipment, increased outdoor air ventilation, and air quality monitoring. HVAC systems equipped with appropriate filtration can help reduce NO2 concentrations in conditioned spaces.