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.