Despite years of air quality improvements, both nationwide and in this area, ozone remains a vexing problem.
Every county in the region (except for Burlington, which is an unknown because it has no air monitor) fails to meet federal health standards for the pollutant at least some of the time, according to an annual "State of the Air" report being released Wednesday by the American Lung Association.
The report ranks the broader region, from Berks County to Cape May County, 16th in the nation for ozone pollution, also called smog, and 11th for particles.
The pollution puts thousands of people with asthma, other lung diseases, and heart disease, plus the very young and the very old, at risk for serious health effects. Even people who work or exercise outdoors can be affected.
Even so, many public health and environmental advocates feel the current standard is not protective enough. The U.S. Environmental Protection Agency is considering lowering it.
Meanwhile, because hot, sunny weather contributes to ozone formation, many fear climate change will make the situation more complex and threatening.
The report is being issued one day after the U.S. Supreme Court upheld the EPA'S Cross-State Air Pollution Rule, which holds upwind states and their pollution sources accountable for the pollution that blows to other states.
It puts in place a process for New Jersey, say, to demand that Pennsylvania's coal-fired power plants install more pollution controls. But Pennsylvania also gets air pollution from states to the west and could ask for concessions there.
The lung association, which intervened in the case, applauded the decision, saying the rule was a lifesaving standard that would finally curtail "secondhand smog."
The National Mining Association, however, said it was "gravely concerned with the latitude afforded EPA" to preempt what should be a state matter.
According to the report, Philadelphia remains the most polluted county in the area - and in the state - with an average of 16.7 days a year that have unhealthful levels of ozone, according to the report, which calculated it from 2010 to 2012 data.
This compares to 10.7 days a year for the previous report.
Bucks, Chester, Delaware, Montgomery, Camden, and Gloucester Counties also saw more ozone pollution, perhaps because 2012 was hotter and sunnier.
The report also tracks particle pollution, which has improved in this area.
Ozone and particulates were the focus, in part because they most often violate health standards nationwide, and they can cause the greatest harm to health, said Kevin M. Stewart, director of environmental health for the American Lung Association of the Mid-Atlantic.
This kind of pollution can exist far from the source, such as a coal-fired power plant, that contributed to it. So it can affect large segments of the population.
Nationwide, nearly half of all people - more than 147 million - live in counties where one or both of the pollutants makes the air unhealthy to breathe, the report finds.
"Overall, it's kind of depressing," said Joseph O. Minott, executive director of the Philadelphia-based Clean Air Council.
"The fact is, a lot of people in Pennsylvania and New Jersey live in areas where there are serious air quality issues. We are not doing enough to lower the emissions that contribute to poor air quality, and that has real results in increased hospitalization, increased asthma, and increased mortality."
Ground level ozone is not only the most widespread air pollutant in the nation, but also one of the most dangerous.
It forms from gases that come out of tailpipes and smokestacks - from burning of fossil fuels - and react with heat and sunlight.
Ozone is a potent lung irritant that can affect even a healthy person's ability to breathe. But people with asthma or other lung diseases, and people who have heart disease, are especially vulnerable.
The lung association estimates that amounts to about 1.5 million people in the larger Philadelphia metropolitan area.
Ozone can exacerbate asthma, cause heart attacks, and even lead to early death, the EPA has concluded.
This week, the Air Quality Partnership, a program of the Delaware Valley Regional Planning Commission, is marking the beginning of ozone season, cooler temperatures notwithstanding. It offers daily forecasts for both pollutants at www.airqualitypartnership.org.
Particle pollution, also called soot, is a mixture of tiny solids and liquids in the air. The smallest can lodge deep in the lungs and pass into the bloodstream, carrying other pollutants with them.
It presents a long-term risk for many people, and short-term spikes can kill.
Research on the harms of both pollutants continues to mount, Stewart said. One emerging area looks at the effect of highway pollution on people who live, work, or go to school nearby.
Meanwhile, if you're looking for better air, the report ranked four areas among the cleanest in every category - Bangor, Maine; Bismarck, N.D.; Fort Myers, Fla.; and Salinas, Calif.