The "State of the Air 2014" report from the American Lung Association has identified the cities in the United States with the worst air pollution problems. 

Los Angeles, which has experienced a long history of challenges with smog, is still the location of the worst ozone problem n the country, according to the report. 

The State of the Air 2014 report states that half of all Americans live in counties that experience unhealthy levels of pollution from particles and ground-level ozone. In 17 counties, 27.8 million people, 8.9 percent of the population, live in areas where there are dangerous levels of all contaminants measured. 

Of the 25 cities shown to have the worst ozone problems in 2013, 23 of those had a greater number of high ozone days in this year's report than in the previous tally. Measurements were taken from in monitoring stations around the country. Pollution levels were measured two ways - by long-term, yearly exposure, as well as short-term 24-hour exposures. 

One in four Americans live in areas with unhealthy levels of ozone, according to the report. Around 14 percent of Americans experienced days with hazardous levels of particulate matter in the air, the association stated. 

Los Angeles and surrounding areas were found to have the worst problems, along with New York City, St. Louis, Tulsa, and Cincinnati. Philadelphia, Louisville, Oklahoma City and Chicago were also among the American cities with the worst air pollution problems. 

There is some good news in the report, as many areas seem to be improving over time. 

"Thanks to stronger standards for pollutants and for the sources of pollution, the United States has continued the long-term trends to lower ozone and particle pollution as well as other pollutants for decades. This figure from the EPA shows that, since 1970, the air has gotten cleaner while the population, the economy, energy use and miles driven increased greatly," the report stated. 

Pollution levels could get worse in with increasing global temperatures, the report warns. 

"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," Harold Wimmer, president of the American Lung Association, said.