This year, 2 of Central Illinois' most densely population counties made the list.
Peoria and McLean Counties scored a "D-rating" for ozone.
Ozone is a colorless gas composed of atoms of oxygen.
Some ozone is actually good.
In the upper atmosphere there's the "Ozone Layer" which protects us from the sun's harmful UV Rays.
At the surface ozone is created by car emissions, power plants and refineries.
Ozone at the surface is bad for the respiratory system.
"Ozone makes people with asthma and lung problems have more issues", says Dr. Bryan McVay, who specializes in pulmonary diseases.
McVay says there isn't really anything you can do directly to protect yourself from ozone because it is all around.
There are standards in place from the Environmental Protection Agency like the "Clean Air Act" to limit the amount of pollution cars, power plants and refineries can emit.
To review the whole report from the American Lung Association click here.
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