Air quality monitors measure concentrations of NO₂ throughout the country. EPA, state, tribal and local agencies use that data to ensure that NO₂ in the air is at levels that protect public health and the environment. Nationally, average NO₂ concentrations have decreased substantially over the years. The charts below show national and regional trends in NO₂ concentrations. For information on NO₂ standards, sources, health effects, and programs to reduce NO₂, please see our Nitrogen Dioxide Pollution page.