Air quality levels in Dane County earned a B grade in the American Lung Association's annual report released Tuesday.

The county received the same score last year, though the number of people at risk during hazardous air-quality days increased slightly, according to U.S. Census data.

"In general, a B is a pretty good score," said Dona Wininsky, spokesperson for the American Lung Association of Wisconsin. "You have to have a perfect score to get an A."

Four counties in Wisconsin received perfect scores, and Duluth-Superior made the report's "Cleanest U.S. Cities for Ozone Air Pollution" list, which required no observed ozone pollution in unhealthy ranges.

Seven counties, mostly located in eastern Wisconsin, received failing grades. Milwaukee County scored the worst with 22 unhealthy "orange ozone" days and one "red ozone" day. The report said Dane County had one "orange ozone" day last year.

Wininsky said the manufacturing coastline of the Midwest, including Milwaukee, is really the "trouble point" of the region. Pollution from the cities harboring millions of people can drift north to Wisconsin or to other parts of the Midwest, depending on wind and lake conditions.

Poor air quality days are usually determined by weather conditions, said Tracey Halloway, a University of Wisconsin environmental studies professor. Extremely hot weather with low wind can produce more chemical reactions in the atmosphere, leading to poor air quality.

Halloway said the weather would not play such a large role if people were not already participating in activities, such as driving cars that produce large amounts of pollution.

"Even though cars are getting cleaner, people are buying bigger cars and driving more," Halloway added.

Overall, Wininsky said the Midwest is following a national trend of improving air quality. She said it may be a direct result of the federal Clean Air Act and its various regulations, but it may also be a growing awareness of environmental issues.

"When you reduce carbon dioxide emissions for global warming, your air quality improves as well," Wininsky said. "Air quality is definitely getting better … but it hasn't gotten good enough to adequately address public health."

Most people at risk during times of poor air quality — usually occurring during extreme heat waves — include the elderly, the young and those with chronic respiratory problems like asthma.

Dane County's air quality report was "green" or good Tuesday, according to the Wisconsin Department of Natural Resources, which offers a 24-hour hotline. "Green" air quality is expected to have no health effects.