Los Angeles again tops the list of cities that have the worst ozone pollution in the U.S., according to a new report released Wednesday.

The city has topped the "dirty air" list in 14 of the 15 annual State of the Air reports released so far.

The report, released by American Lung Association (ALA), found that half of all Americans or 147 million people live in areas with unhealthy levels of ozone pollution.

Researchers found that the levels of particle pollution have reduced but the air in major metropolitan cities is unhealthy, mostly due to rise in ozone levels.

The report was based on data collected between 2010 and 2012. Researchers found 22 of the 25 large cities in the U.S. registered high levels of ozone or smog. The cities included Los Angeles, Houston, Washington-Baltimore, New York City, The Guardian reported.

Ozone is a gas that can be found both at the upper levels of the atmosphere and at ground levels. It is classified as good or bad depending on its location. At higher elevation, the gas acts as a protective blanket by preventing harmful radiation from reaching earth. Ground level ozone is a pollutant and is associated with several health problems.

Smog is a mixture of pollutants and ground - level ozone. The pollutants are usually produced when volatile compounds and nitrogen oxides react in the presence of sunlight. Rising temperatures have contributed to the increase in smog levels in major U.S cities.

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

Last week, California Air Pollution Control Officers' Association had released a report saying that climate change will increase smog formation and up the risk of wildfires in the region, the Los Angeles Times reported.

 "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," Wimmer added.

According to the report, Bangor, ME, Bismarck, ND, Cape Coral-Fort Myers, FL, and Salinas, Calif., were the cleanest cities in the U.S. These cities had zero days with unhealthy levels of ozone pollution.

Several cities have reduced air pollution levels. Los Angeles, for example, reduced over one-third of unhealthy ozone days in the past 15 years. Pittsburg too has lowered its particle pollution levels.

Wimmer said that the reduction in air pollution levels over the last one and half decade shows that the "tools in the Clean Air Act work."

"The Clean Air Act has been proven to deliver tremendous health benefits," said Wimmer in the news release. "Congress must allow the Clean Air Act to continue to protect our health and ensure that the EPA and the states have adequate funding to monitor and protect the nation from air pollution and new threats caused by increased temperatures."