In the 2015 report, six cities set local records for the most days with dangerously high levels of particle pollution, including the San Francisco and Visalia, Calif., metropolitan areas. "We've been doing this report for 16 years, and generally speaking, you'll see a variation in pollution levels from one year to the next, but almost never do you set new records because we've made huge improvements in reducing pollution." Nolen went on to explain that these cities, all in the Western United States, had just begun to experience drought conditions — that are still ongoing — when these data were collected from 2011 through 2013. "When we get into next year's report and the year after, I think we'll see even higher levels in these areas."Several of the most polluted cities, notably Pittsburgh and Cleveland, are former manufacturing and production strongholds.
While these areas are no longer the industrial strongholds of their heydays, enough industrial and manufacturing production remains for the cities to maintain their standings among the most polluted places in the country.
The number of area residents with asthma, including the number of adults and the number of pediatric cases of the disease, also came from the ALA.Long term particle pollution is measured in micro-grams per cubic meter (ug/M3).The short-term particulate measure represents how many days of extreme air pollution occur, and usually reflects how extreme conditions such as fire or drought are creating abnormally high levels of unhealthy particles.The Central Valley is surrounded by mountains on three sides, which traps the pollution from the San Francisco Bay Area in the valley.Five of the 10 most polluted cities, including parts of the San Francisco metro area, are in the California Central Valley.