Air quality in the United States is a stark contrast between a overall pollution levels that have continually dropped in recent years, but the presence of the serious irritant ozone, or smog, which is more widespread than it was in 2013.

According to the American Lung Association's 15th annual "State of the Air" report, half of the American population lives in counties where pollution from smog and particulates have left the air unhealthy to breathe.

"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, National President and CEO of the American Lung Association, said in a news release. "However, this improvement represents only a partial victory."

Warmer temperatures, he said, "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."

Air pollution, said ALA officials, represents an increased health threat to certain groups of people in particular, including: infants, children, teenagers and older adults; anyone with lung diseases like asthma or chronic obstructive pulmonary disease, more commonly referred to as COPD; individuals with heart disease or diabetes; people with low incomes; and, pretty much anyone who works or exercises outside.

Data included in the yearly report card shows:

  • Nearly half of the people in the United States (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 United States 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.
  • Thirteen of the 25 cities with the worst year-round particle pollution reached their lowest levels yet, including Los Angeles, Atlanta, Pittsburgh and Bakersfield.