The region's refineries are behemoths, enriching the economy, but also ranking among the region's top air polluters.
They emit a brew of contaminants, from chemicals that cause cancer to those that help make smog.
They've had difficulty meeting emissions limits, chalking up 143 "formal enforcement actions" in the last five years and more than $12 million in penalties. All the companies are under federal consent decrees to upgrade their equipment.
Last Sunday's explosion and fire at Sunoco's Marcus Hook oil refinery was one of those signature events, like a spill, a reminder of the potential for major accidents at such facilities.
But day in and day out, area refineries have a significant impact.
This region has six of the East Coast's eight major refineries, some dating nearly to the dawn of the petroleum age.
With less than 2 percent of the nation's population surrounding them, the plants account for about 7 percent of the nation's refining capacity.
The plants are cheek by jowl with major population centers. One is in Philadelphia's city limits.
Every day, on average, large tankers plying the Delaware River bring 40 million gallons of crude - nearly four times the amount spilled in the 1989 Exxon Valdez accident - to be processed here.
Not counting management and related jobs, the refineries employ about 5,200 people and add $6.6 billion to the gross regional product.
"They are very big players, both economically and in terms of their environmental footprint," said Joseph Minott, executive director of the Clean Air Council, a regional nonprofit.
"While the product that they make also produces pollution when used, it's a product that is necessary for society right now," he said. "The best we can hope for is that the plants are run properly, that they have the proper air-pollution controls, and that they don't have explosions."
The industry, both nationwide and regionally, touts its progress.
Sunoco, which owns three of the region's refineries, had its best year in 2008 in meeting limits on water and air releases, spokesman Jeffrey R. Peters said.
Yet environmental groups contend that emissions nationwide remain underreported and, in some cases, unregulated. They also say the regulations that do exist are not protective enough of public health.
"The bottom line is we really don't know enough about what's being emitted from some of the largest refineries and about what the neighbors and workers are actually breathing," said the Environmental Integrity Project's Eric Schaeffer, who resigned as a regulator in the Environmental Protection Agency in frustration over President George W. Bush's attempts to weaken the Clean Air Act.
Sunoco's Philadelphia refinery, the largest in the region and seventh-largest in the nation, is surely one of the most urban.
About 321,000 people live within three miles of the refinery, and the only reason the population is not double that or more is due to the unpopulated swaths of the Schuylkill and the Delaware River, the airport, the Navy Yard, and other industrial facilities, including the 1,400-acre refinery itself.
The refinery, then under different ownership, was founded in 1866, just seven years after the nation's first oil well was drilled in Western Pennsylvania.
Marcus Hook's refinery was another early entrant. In 1901, just months after the Sun Co. was incorporated by Joseph N. Pew - whose four children later founded the Pew Charitable Trusts - he bought 82 acres on the Delaware for the plant.
The nation had roughly 8,000 cars at that point.
Now, the nation has more than 135 million cars. And its 150 refineries meet 95 percent of the national demand for the kinds of products they produce.
The last one was built in 1976. Any increase in production has been handled by expansions, said Bill Bush, spokesman for the American Petroleum Institute.
He said it made more economic sense. And, perhaps not surprising, "it's hard to site a new plant."
While refineries affect the environment through the disposal and discharge of water, their greatest impact by far is air pollution.
Refineries emit two broad kinds. The first - "criteria" pollutants - are lead, carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, and extremely fine particles.
Generally, these substances travel far. Some contribute to ground-level ozone, or smog, which is a concern for those with respiratory ailments and heart conditions.
This region does not meet federal standards for two of these substances: smog and particulate matter.
The refineries and electric-power generators are major emitters, although other sources, from automobiles to lawn mowers, also contribute significantly.
The other category is hazardous air pollutants, including carcinogens such as benzene. Again, refineries are top emitters.
These chemicals don't travel as far as the other pollutants, so they affect local communities more, prompting concerns about environmental justice.
Compared with the rest of the city, the population within a mile of the Philadelphia refinery is overwhelmingly poorer and less-educated. It includes more minorities and has a larger proportion with health vulnerabilities - the very young and the very old.
In 2000, the EPA embarked on a national-refinery initiative, partly aimed at reducing emissions. While the agency didn't necessarily have documented violations to compel the improvements it wanted, refiners agreed to it to avoid long investigations and potentially hefty fines.
All the companies with refineries in this region eventually consented to agreements.
Sunoco, for instance, anticipates spending $350 million this year in its three regional plants. The upgrades will reduce emissions and produce fuels that have lower emissions when used in vehicles, the company said.
Still, it's not enough, critics say.
"The issue with refineries is that they're both very dirty and very complicated," said Ann Alexander, an attorney with the Natural Resources Defense Council. "A refinery has a huge number of moving parts. Any one of them can be a source of emissions."
Some are counted, often under the category of "fugitive" emissions in an online EPA reporting database, and some are not, she said.
Also, flarings - burning off vapors that can't be used anywhere else - and emissions from malfunctions and start-ups may be uncounted.
"Refineries get permitted for limited parts of their process, with the assumption that nothing is going on," Alexander said. "Then there are all kinds of additional stray components that don't get counted when they emit."
Environmentalists also worry these problems will be ignored; refineries, they say, may be falling off the agenda, replaced by global warming.
"When people talk about pollution today, it's about electric power plants or cars," said Frank O'Donnell of the national nonprofit Clean Air Watch. "Refineries are often the forgotten polluter."
As a result, he said, "these old dinosaurs are just going to keep getting older and creakier."
At the Marcus Hook refinery, the upgrades agreed to in the consent decree won't be completed until 2013.
The plant has received 13 notices of violation during the last two years, but they were for a variety of problems, signaling nothing endemic, said Francine Carlini, air program manager for the Southeastern Office of the Pennsylvania Department of Environmental Protection.
The latest penalty, announced this month, was $762,150 for emitting twice the permitted limit of particulate matter and four times the permitted level of ammonia for more than a year.
Two days later, the explosion occurred. Sunoco's Peters said the company was cooperating with investigators, who have yet to determine how or why it started.