About half of Americans are breathing air with unhealthy levels of pollution due to soot and smog particles on a daily basis, a new report says.

The American Lung Association released its annual study on national air quality on Wednesday, revealing that some 148 million people inhabit areas that have unhealthy amounts of pollution in the air, The Guardian reported.  

"Weather played a factor," said the study, which is the 15th yearly report from the ALA. "The warmer summers in 2010 and 2012 contributed to higher ozone readings and more frequent ozone days. Sunlight and heat create conditions that increase the risk of high ozone levels."

Smog has increased in 22 of America's 25 biggest metropolitan areas, including the cities of Los Angeles, Houston, New York and Chicago, all of which had more high-ozone days on average compared with the previous year.

The study also found that more than 27.8 million Americans, or 8.9 percent of the population, live in 17 counties that have unhealthy levels of pollutants, according to the ALA's measurements.

Risky levels of ozone and particle pollution can make people cough or wheeze and can result in asthma attacks, heart attacks and premature death. Those at the greatest risk include infants, children, teenagers and older adults as well as people who have lung diseases such as asthma, heart diseases or diabetes.

In cities like Beijing, China, pollution has reached such dangerous levels that outside activity is greatly limited on particularly bad days and driving has been restricted.

Despite the sobering ozone levels, the report on the U.S. wasn't all bad news--overall, national pollution has decreased.

"We are happy to report continued reduction of year-round particle pollution across the nation, thanks to cleaner diesel fleets and cleaner power plants," Harold Wimmer, president and CEO of the ALA, said in a statement.  "However, this improvement represents only a partial victory. We know that warmer temperatures increase risk for ozone pollution, so climate change sets the stage for tougher challenges to protect human health."

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