An American Lung Association report on the nation's air quality has turned up a puzzling blip: In this heavily urbanized region, comparatively rural Chester County has the highest annual average for fine-particle pollution - the sooty stuff that carries chemical pollutants and lodges deep in the lungs.
However, the county still meets air quality standards for the pollutant.
It's one of many seeming dichotomies found in the report, which is to be released today.
Overall, the air we breathe is getting much better.
Especially in Gloucester County, which made the list of the nation's cleanest counties for short-term particle pollution levels. It was one of four New Jersey counties making the list for the first time in the 12 years that the association has produced this kind of report.
Then again, Gloucester was the only New Jersey county that didn't post fewer bad-air days for ozone.
The wider Philadelphia region - 13 counties that the federal government has combined into one statistical area - is the cleanest it has been during the 12 years of reports when it comes to ozone and short-term particle pollution.
However, this large urban area, which stretches from central New Jersey to Berks County to Cecil County, Md., remains bad enough to rank among the nation's 25 most polluted areas. Most counties still get an "F" from the lung association for ozone pollution.
According to the report, more than 40 percent of people in the United States live in areas where air pollution continues to threaten their health. Ozone and particle pollution can cause wheezing and coughing, asthma attacks, heart attacks, and premature death.
Nonetheless, "we're making steady progress in cutting dangerous pollution from the air as a result of cleanup efforts required under the Clean Air Act," said Deb Brown, president and CEO of the American Lung Association of the Mid-Atlantic.
Officials said the report is evidence of headway in cleaning up major pollution sources, including coal-fired power plants, diesel engines and inefficient vehicles.
"Bottom line, we've made a lot of improvements, and we have a long way to go," said Joseph O. Minott, head of the Clean Air Council, based in Philadelphia.
He said the report was particularly important now "because this past year has been a little rough on air issues. We had the Obama administration move away from tightening the ozone standards . . . and there's this relentless attack by some members of Congress that are attempting to block further air improvements."
Why Chester County should stand out for annual levels of particle pollution is "a complicated question," said Kevin Stewart, director of environmental health for the association's Mid-Atlantic chapter.
Burning fossil fuels - in power plants and automobiles and diesel engines - is an obvious source, he said. So are industrial boilers. But officials also recognize "it can be as simple as open burning" on a residential property, he said. And the wind can change everything.
Chester County's levels "may be a mixture of local sources plus transportation plus what it's downwind from," Stewart said.
Larry Ragonese, a spokesman for the New Jersey Department of Environmental Protection, said that "there's no doubt New Jersey has air issues." But he said the state has been making steps that will produce cleaner air.
The state has successfully challenged emissions from a coal-fired power plant upwind in Pennsylvania. Other steps include retrofitting diesel buses and construction equipment, replacing or upgrading dry cleaning equipment, and requiring home heating oil that has a lower sulfur content.
The analysis was prepared with data that the states reported to the U.S. Environmental Protection Agency. To smooth out annual variations - ozone is worse during hot, sunny summers than in cooler, cloudy summers - the association averaged data for three years, 2008 to 2010.
The report, covering the entire nation, does not offer insight into air pollution from natural gas drilling in Pennsylvania, partly because some counties where drilling is occurring are rural and have no air monitoring stations.
Katherine Gresh, spokeswoman for the Pennsylvania Department of Environmental Protection, said agency officials would review the report.
The state is creating an air pollution inventory for natural gas drilling, and companies are required to report their data. The deadline for that has passed, but some companies have been granted extensions, she said.
The lung association's Stewart said: "We are eagerly looking forward to what that initial inventory shows."