The Tulsa area includes Creek and Tulsa counties as well as parts of Osage, Wagoner and Washington counties.
Other eastern Oklahoma counties with an "F" ranking included Adair, Cherokee, Mayes, Ottawa, Pittsburg and Sequoyah.
The association said the Tulsa area cut its number of ozone days by almost 8 compared to the previous year.
The report says that nationwide, more than 4 in 10 Americans -- nearly 138.5 million people -- live in counties where ozone levels make the air unhealthy to breathe.
The Associated Press contributed to this report.
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