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KNOXVILLE — Knoxville ranks 12th in the country in the number of unhealthy smog days per year, according to a report from the American Lung Association.

Several factors contribute to the area’s air being so polluted, a University of Tennessee civil and environmental engineer said Thursday.

Dr. Wayne Davis said coal fired power plants and the area’s natural topography combine to make smog worse.

“A large part of the problem is that we have a fairly large percentage of our power produced by coal. We also have topography bounded on the West by the Cumberland Plateau and on the East by the Smokies,” Davis said. “That tends to create a little bit more of a valley effect that might enhance the formation of ozone because some of the pollutants get trapped in that area.”

Davis said motor vehicle traffic is another contributor to local concentrations of ozone and smog formation.

“We have our fair share of mobile sources such as automobiles,” Davis said. “You couple together large emissions from coal fired power plants and mobile sources and you have a formula that starts creating ozone formation on hot days.”

Davis said another reason for the area’s low air quality ranking is that it has more air monitoring stations than other regions.

“A lot of areas do not have that many monitors located in the vicinity,” Davis said. “Knox, Sevier, and Blount counties happen to have more monitors than most areas, and many are installed by the National Park system. That means that we will have documentation when the ozone has been quite high.”

The “State of the Air 2000” report also rated air pollution in Nashville as 18th worst in the nation, and Memphis 23rd.