Which of the following is not classified as a criteria pollutant?

Carbon dioxide is not classified as a criteria pollutant. Criteria pollutants are specific air pollutants that are regulated by the Environmental Protection Agency (EPA) due to their potential to harm human health or the environment, and they have established National Ambient Air Quality Standards (NAAQS). The criteria pollutants include substances like carbon monoxide, sulfur dioxide, nitrogen oxides, lead, ozone, and particulate matter.

Unlike these pollutants, carbon dioxide is primarily associated with global climate change and does not have short-term health effects at typical concentrations found in the atmosphere. Therefore, while it is a significant greenhouse gas contributing to climate change, it does not fall under the category of criteria pollutants that are monitored for immediate health impacts. This distinction is critical for understanding air quality regulations and environmental health standards.