Roughly half of all Americans, or more than 147 million, are living with unhealthy air, according to a report that was recently released by the American Lung Association. The report, titled “State of the Air 2014,” reveals that approximately half of all Americans live in counties with ozone or particle pollution levels that are high enough to make the air unhealthy to breathe.

The 15^th annual report card for the U.S. indicates that although the country continues to experience a reduction in particle pollution, a pollutant that has recently been linked to lung cancer, poor air quality remains an important public health concern. A changing climate is also making it more difficult to protect human health.

The levels of ozone, or smog, a potent respiratory irritant and the most prominent air pollutant, were worse than in last year’s report.

National President and CEO of the American Lung Association, Harold Wimmer, said in a statement, “We are happy to report continued reduction of year-round particle pollution across the nation, thanks to cleaner diesel fleets and cleaner power plants.”

He continued, “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. We must meet these challenges head on to protect the health of millions of Americans living with asthma and chronic obstructive pulmonary disease. All of us –everyone in every family—have the right to healthy air.”

Among the report’s most significant findings, according to the organization, were:

• Nearly half of the people in the U.S. (147.6 million) live in counties with unhealthy levels of either ozone or particle pollution.
• More than 27.8 million people (8.9 percent ) in the U.S. live in 17 counties with unhealthful levels of all pollutants measured in the report.
• Twenty-two of the 25 most ozone-polluted cities in the 2014 report – including Los Angeles, New York City, and Chicago – had more high ozone days on average when compared to the 2013 report.

The most ozone-polluted city in the U.S. is Los Angeles-Long Beach, California. See how other U.S. cities rank here.