Areas in the US with high levels of air pollution, specifically PM 2.5, that do not meet national air quality standards are called nonattainment areas. The EPA sets these standards to protect public health and the environment. States can have stricter rules but not weaker ones. The EPA uses science-based guidelines to set limits on pollutants to ensure everyone has the same level of protection. If an area meets the health-based standards, it's called an attainment area; if it doesn't, it's a nonattainment area.