Casper and Cheyenne are among the best cities for air quality in the United States, but other Wyoming communities are lagging.

That's according to the American Lung Association's State of the Air report issued Wednesday. The report is issued every three years.

Wyoming's two largest cities both lead the nation in having low ozone levels and air particle pollution.

On the other hand, Sublette County scored a D for ozone pollution days with eight air quality warnings issued from 2015 through 2017.

Sublette County has had issues in the past with four high ozone days from 2012 through 2014, said Carrie Nyssen, the American Lung Association Senior Director of Advocacy for Wyoming. That's due to oil and gas activity in the area.

"Interestingly enough, it's wintertime ozone. In the rest of the nation, when you think of ozone pollution, it's normally in the warm summer months," Nyssen said.  "When you have fugitive emissions coming off the gas drilling equipment, and you have the snow and sunshine coming off the snow, that's the perfect recipe to create ozone."

Nyssen said ozone is a chemical that's formed from organic compounds being exposed to sunlight. An odorless chemical, ozone is similar to "having a sunburn on your lungs." It can also harm the central nervous system.

Two Wyoming counties also received low marks in air particle pollution with Campbell and Sheridan counties each earning a D.

But, Nyssen said, that's largely due to an influx of wildfires in the western United States over the past few years.

Air particles in the report are defined as 1/30th the thickness of a hair follicle.

"Our natural defenses help us keep out the larger particles so they don't get into our bodies, but these small particles get through our bodily defenses and they go into our lungs where they can cause inflammation," Nyssen said.

In 2013 the World Health Organization determined air particles can cause lung cancer. They can also exacerbate conditions like asthma and COPD.

All of the particle pollution days were in the summer months, which coincides with the 2015 wildfire season — the worst on record.

"I would say those are definitely attributable to wildfires," Nyssen said. "I was somewhat shocked by the impact that the wildfires had, not only in states where the wildfires were occurring but also in Wyoming."

The U.S. Environmental Protection Agency puts out an air quality index which can be found here. 

Nyssen said when their air quality dips, the best thing to do is stay indoors.