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PHILADELPHIA (CBS) — A new report by environmental activists says the Philadelphia region has had more air quality problems than almost any other large metropolitan region in the country, potentially harming the health of millions of people.

Bad air is easy to spot in Los Angeles when the smog settles in. But a clear day can be bad too. This year, May 2 looked beautiful in Philadelphia despite an elevated ozone warning.

“Even one day with polluted air is too many,” said Kelly Flanigan, global warming solutions associate with PennEnvironment Research and Policy Center. The group announced the results of their air pollution study at the Fairmount Water Works Thursday.

According to the study, in 2016, 6 million people in the Philadelphia region, which includes Camden and Wilmington, lived through more than three months of elevated air pollution: 111 days. Only the Los Angeles metro region had more days that Philadelphia: 138.

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“The two biggest pollutants are smog, which comes from ground-level ozone, and particulate matter, which is basically very, very tiny particles in the air that can get trapped in your lungs and create a lot of health problems,” Flanigan said.

The largest source of pollution, the study says, comes from cars and trucks that burn fossil fuels like gasoline and diesel, but everything from wildfires to agricultural dust can send particulates hundreds of miles.

Dr. Walter Tsou, executive director of Philadelphia Physicians for Social Responsibility, says Philadelphia’s pollution poses “significant health implications, particularly for those with borderline respiratory statuses.”

The Philadelphia region is not alone. The study authors say 73 million Americans experienced more than 100 days of degraded air quality in 2016, which can lead to everything from sore throats to asthma attacks to premature death.

It’s not just an outdoor problem. Polluted air, the report says, gets inside and combines with indoor contaminants, causing air quality problems inside buildings and homes.

The report’s authors are calling for better regulations and laws to improve and protect air quality.

The study was conducted jointly by PennEnvironment Research and Policy Center, Frontier Group, and PennPIRG. To read the full report, go here.