Which of the following is NOT a criteria pollutant under the NAAQS?

Arsenic is not classified as a criteria pollutant under the National Ambient Air Quality Standards (NAAQS). The NAAQS, established by the Environmental Protection Agency (EPA), defines criteria pollutants as those that are widespread and harmful to public health and the environment, and for which the EPA has set specific standards to regulate their concentrations in the ambient air.

The criteria pollutants include ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter, and lead. These pollutants have established health-based standards due to their prevalence and the significant health risks they pose to the general population. Arsenic, while a hazardous substance with toxicity concerns primarily related to water quality and soil contamination, does not fall under the NAAQS criteria pollutants list because it is not typically found in the atmosphere at levels that necessitate such regulation.

Understanding the classifications of pollutants is crucial within the environmental engineering field, as it influences regulatory practices, public health policies, and environmental protection measures.