The 6 US cities with the least pollution

Rob LudacerMore than 125 million Americans live in counties with dangerous levels of pollution, according to the American Lung Association's 2017 "State of the Air" report, released April 19.

Cities can take various steps to reduce vehicle and power plants emissions and protect air quality. But Janice Nolen, a lead author on the report, says that because air moves, certain places are still vulnerable to unfavorable geography and polluting neighbors.

Those same factors can render other places safe, however. The American Lung Association ranks cities based on three measures of particle pollution: Ozone levels, days with severe particle pollution, and year-round particle pollution.

Of all the cities they were able to study (Illinois, most of Tennessee, and Maine didn't have enough data for the report), only six reported no high-ozone or particle pollution days. Here they are, ranked by their year-round particle levels.