While the nation's air quality has improved in the past few years, the most recent State of the Air 2011 report from the American Lung Association found that just over half of the population lives in areas with unsafe levels of air pollution.

According to the report, 50.3 percent of Americans live in 366 counties that have exceedingly high levels of either ozone or particle pollution. In addition, about one in 17 people - a total of 18.5 million - live in counties with unhealthy amounts of ozone, short-term and year-round particle pollution.

However, the organization said there is some good news as well. More than half of the country's most smog-polluted cities reported decreased smog levels. Plus, all but two of the 25 most polluted cities that experience year-round particle pollution improved since the last report, with 19 of them reporting their best levels to date.

Air quality can have a serious impact on human health. For instance, the U.S. Environmental Protection agency says particle pollution - a mixture of components such as acids, metals, organic chemicals and dust particles - can damage the heart and lungs with heavy exposure, which may lead to serious illnesses such as lung cancer and cardiovascular diseases.