Which of the following is not a criteria air pollutant for which the EPA must set National Ambient Air Quality Standards?

• A. Nitrogen dioxide
• B. Atmospheric ozone
• C. Carbon monoxide
• D. Radon

Answer: (D)

National Ambient Air Quality Standards are set for pollutants that are common outdoors and have proven health or welfare effects. The pollutants commonly designated as criteria pollutants include ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Radon, on the other hand, is a radioactive gas that mostly affects indoor air and is not addressed by outdoor ambient air standards. It is managed through indoor air quality programs, testing, and mitigation guidance rather than NAAQS. So radon is not a criteria air pollutant for which the EPA must set National Ambient Air Quality Standards.