Which of the following is a criteria air pollutant?

• A. Water vapor
• B. Methane
• C. Ozone
• D. Carbon dioxide

Answer: (C)

Criteria pollutants are the pollutants for which the EPA has established National Ambient Air Quality Standards because they pose clear health and environmental risks. The list includes ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Ozone is one of these regulated pollutants because, at ground level, it irritates the respiratory system, worsens asthma, and can reduce lung function. It forms when nitrogen oxides and volatile organic compounds react in sunlight, so controlling ozone involves limiting those precursors.

Water vapor, methane, and carbon dioxide are not on the EPA’s criteria pollutant list. Water vapor fluctuates naturally and isn’t controlled as a primary ambient standard; methane and CO2 are addressed as greenhouse gases related to climate change rather than as criteria pollutants.