Despite decades of progress, the air quality in the United States has started to decline over the past few years, according to data provided in summer 2019 by the Environmental Protection Agency. The agency recorded 15 percent more days with unhealthy air in the country in 2018 and 2017 compared to the average from 2013 to 2016.

The reasons for the recent decline in air quality remain unclear, says the agency, but may be related to high numbers of wildfires, a warming climate, and increasing human consumption patterns driven by population growth and a strong economy. The long-term outlook also remains unclear, even as politicians debate air pollution standards.

Air pollution is a mix of particles and gases that can