air pollution were seen in Los Angeles and some Southern states, including Alabama, Arkansas and Oklahoma.
Shortened life expectancy was highest in places where poor people lived and lowest in wealthy areas.
The acceptable PM2.5 level, set by the U.S. Environment Protection Agency, is 12 micrograms per cubic meter of air (ug/m3).
In 1999, the highest levels of PM2.5 were in Fresno County, Calif., at 22.1 ug/m3. By 2015, the highest levels were in Tulare County, Calif., at 13.2 ug/m3.
The lowest level in 2015 was in
