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

The correct answer is methane, as it is not classified as a criteria pollutant under the Clean Air Act. The Clean Air Act specifically identifies six air pollutants that are considered criteria pollutants due to their widespread presence in the atmosphere and their potential to harm public health and the environment. These include carbon monoxide, lead, sulfur oxides, nitrogen dioxide, particulate matter, and ozone.

Methane, while a significant greenhouse gas that contributes to climate change and is monitored for its environmental impact, does not fall under the same regulatory framework as the criteria pollutants. It is primarily addressed under different regulations related to greenhouse gas emissions rather than as a direct threat to air quality via the criteria pollutant standards. This distinction is essential in environmental studies and policy discussions, highlighting the different sources and impacts of various pollutants on air quality and climate.