A more thorough explanation:

An air-quality-control region is a designated area where multiple communities share a problem with air pollution. These regions are often located across several states or within a large area of a single state. The Environmental Protection Agency (EPA) designates these regions for monitoring and improving the quality of the air.

For example, the Los Angeles air basin is an air-quality-control region that includes several counties in Southern California. The region has a significant problem with smog due to high levels of vehicle emissions and industrial pollution. The EPA has designated this area as a non-attainment area, meaning it does not meet federal air-quality standards.

Another example is the Ohio River Valley air-quality-control region, which spans across six states in the Midwest. This region has a problem with sulfur dioxide emissions from coal-fired power plants. The EPA has implemented regulations to reduce these emissions and improve air quality in the region.

These examples illustrate how air-quality-control regions are designated to address specific air pollution problems in a coordinated and comprehensive manner. By bringing together multiple communities and states, the EPA can develop and implement effective strategies to improve air quality and protect public health.