The 24th annual State of the Air report from the American Lung Association gave Hillsborough a "D" grade - the only grade lower than a "C" in Florida. It rates the level of ozone, which comes mainly from vehicle emissions.
The report said there were an average of three days per year from 2019 through 2021 in the county when ozone levels exceeded federal standards.
Jason Waters is with the air management division of Hillsborough's Environmental Protection Commission.
"Automobiles is the primary driver now. And what happens is basically the sunlight comes in and it chemically reacts in the presence of sunlight to make ozone," he said.
Click here to read about air quality in Central Florida
The report noted that Hillsborough's air quality has made dramatic improvements since the late 1990's, when as many as 38 days were flagged as high ozone. Waters credits the improvement to cleaner cars and cleaner smokestacks.
"The whole state of Florida is in attainment with the national ambient air quality standards set by (the federal Environmental Protection Agency)," Waters said. "We're a very populous state, and the entire state is in attainment with all the standards, not just ozone, but all the other main pollutants set by EPA. So that's a pretty big deal."
