The American Lung Association says Denton, Dallas and Tarrant counties failed its annual review of air quality.
The group analyzes at state air quality data each year to compile the State of the Air report.
Dr. Sven Wust, an allergist in Denton, said the "F" grade Denton County received for ozone levels is not shocking. But he said it was concerning for Denton County residents with respiratory diseases.
"If you're inhaling poor quality air and it causes a flair up in those conditions, it can actually close off that area, and you can have an asthma attack or a COPD attack, and it can threaten your life," he said.
Ozone levels continue to improve but experts say it may take a while before they are good enough to earning a passing grade in the American Lung Association's annual report.
John Kuruvilla, a professor at the University of North Texas, monitors air quality around the state. He has a team of graduate students that study air quality in Denton. They said the failing grade is not solely Denton County's fault.
"You really don't have a critical amount of emissions in the Denton County region. However, you have quite a bit of emissions in the counties south and southeast of here," he said.
Wust agreed with Kuruvilla.
"Denton County is located downwind from major metropolitan areas, including Dallas, Fort Worth and cities further to the south," Wust said.