A new report from the American Lung Association showed that the air quality in the United States has significantly improved compared to 14 years ago. But almost half of the American population is still exposed to unhealthy air.

"We are happy to report that the state of our air is much cleaner today than when we started the 'State of the Air' report 14 years ago," National President and CEO of the American Lung Association Harold Wimmer wrote in a news release. "Even in parts of the country that experienced increases in unhealthy days of high ozone and short-term particle pollution, they still have better air quality compared to a decade ago. But the work is not done, and the Environmental Protection Agency must continue the work necessary to achieve the promise of the Clean Air Act; healthy air that is safe for all to breathe."

Among all cities in the country, four made it to the "cleanest" list: Bismarck, N.D., Cape Coral-Fort Myers, Fla., Palm Bay-Melbourne-Titusville, Fla., and Rapid City, S.D. were all able to maintain healthy levels of ozone and low levels of particle pollution.

Los Angeles topped the list as the worst city exposed to air pollution.

Despite the reported improvement, the study highlighted that 42 percent, or 131.8 million, of the American population is still exposed to air pollution. Though that number seems high, it has actually improved compared to last year's report. Researchers believe that the improvement is a result of more people using cleaner diesel fuels and the shift of energy source from coal-powered power plants.

Scientists recommend that the government step in and support a proposal from the Environmental Protection Agency to reduce tailpipe and smokestack pollution and further improve air quality nationwide.

"We have seen time and again that the Clean Air Act delivers tremendous health benefits," Wimmer continued. "Congress must ensure that the provisions under the Clean Air Act are protected and are fully enforced. EPA and the states must have adequate funding to monitor and protect the nation from air pollution."

Air pollution is a serious health threat - excess levels of smog can cause wheezing, coughing, ashtma attacks, heart attacks and premature deaths.

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