PM2.5 is a type of air pollution made up of solid particles and liquid droplets that are undetectable to the human eye. These fine particulates are just 2.5 microns or smaller in width - or dozens of times smaller than fine beach sand or a human hair.
New studies show that even low levels of PM2.5 can have serious health effects. The level considered unsafe varies by agency - the World Health Organization considers anything above 5 micrograms per cubic meter (µg/m3) to be unsafe.
To find the worst hotspots in the country, the Guardian looked at areas with levels 11.5µg/m3 or higher.