Which of the following is NOT a EPA criteria air pollutant under the Clean Air Act?

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

Answer: (D)

Under the Clean Air Act, the EPA identifies a specific group of six pollutants as “criteria pollutants” for which National Ambient Air Quality Standards are set because of their widespread health and environmental impacts. Those six include ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Radon, while a major indoor air health concern due to its radioactivity, is not part of this criteria-pollutant list and is regulated through radiation protection and indoor air quality programs rather than ambient air standards. Therefore, Radon is not a criteria air pollutant under the Clean Air Act.