Hamilton County passed this year's air quality exam, but the area's not impressing anyone with its GPA.
The county earned a "C" for high particle pollution days and an "F" in high ozone pollution days. Air was unhealthy for residents with such conditions as asthma for 41 days in 2007 through 2009, the most recent data available for the American Lung Association's annual State of the Air report.
But the area is improving. Last year the county passed the annual air quality test, but with 56 unhealthy days and a "D" in high particle pollution.
Hamilton County improvements are noticeable. Compare this year's results to those of some notable Chattanooga neighbors: Atlanta was the 23rd most ozone polluted city in America, Birmingham was 21st and Knoxville came in at 18th.
Compared to those areas, Chattanooga, which didn't show up in the Top 25 rankings for cities by ozone, year-round particle pollution or short-term particle pollution, doesn't look quite as bad, and officials say air quality only is getting better.
"The air is cleaner than it's ever been," said Amber Boles, spokeswoman for the local Air Pollution Control Bureau.
And Hamilton isn't the only American county looking cleaner. Most of last year's most polluted American cities also improved.
"That's good news," ALA President Charles Connor said, but "air pollution remains a very real public health threat."
According to Connor, residents in half the country live in areas with dangerous pollution levels, sometimes dangerous enough to shorten lives.
"So let's be clear," he said. "Air pollution kills."
Exposure to the air pollutants in the report has been shown to increase emergency room visits, hospitalizations related to asthma and to increase hospital treatment for pneumonia and chronic obstructive pulmonary disease.
"Ozone irritates body tissue," said Norman Edelman, chief medical officer of the ALA. Some of the effects of inhaling the gas are "very much like getting a bad burn on the inside of your lung."
He said even short-term exposure to pollutants can be deadly. It's especially risky for children, he said, who run around outside and breathe in more pollution relative to their size.
But Boles said the area's air is improving, and locals can breathe a little easier.
She attributes the improvements to this point to regulations on brush burning, programs to reduce gas station pollution and air quality public awareness campaigns.
The largest single source of local pollution is cars, she said, and people can make sure the area's improvement trend continues by taking small steps such as carpooling, using mass transit and combining errands.
"If they can just do all of those little things it will really make a big difference," she said.