Not that anyone should panic.
"We're not saying that if you reside in one of these areas, you need to pack up and move, or seal your windows shut," the business magazine's website states.
Just because there are Superfund sites nearby, air quality sometimes is unhealthful, or dangerous chemicals are processed at local plants doesn't mean residents face serious daily risks, Forbes explains.
Propelling the Greater Philadelphia area - not just the city itself - to the top spot was having more than 50 Superfund sites, as well as the nation's worst water quality, according to Forbes.
By some measures, though, Philadelphia was far less risky than some other cities.
Bakersfield, Calif., No. 2 overall, had the highest total of "unhealthy air quality days" in 2009 - 43, as opposed to just 2 for Philadelphia.
Salt Lake City, No. 9 overall, topped the "toxic release inventory" list, with 130 million pounds of hazardous chemicals being handled by local industries. The Philadelphia area's total: 11.3 million.
What's the difference? Oh, about 60,000 tons.
Not that the statistic implies exposures or mishandling, by the way.
Also, the Philadelphia metro region includes Cecil County, Md., and New Castle County, Del., as well as the city and its suburban counties in Pennsylvania and New Jersey.
The rest of the Forbes list in descending order: Fresno, Calif.; New York City; Baton Rouge, La.; Los Angeles; Houston; St. Louis; and Riverside-San Bernardino, Calif.
The claim that Philadelphia's "water quality" is the nation's worst comes from a website, Sperling's Best Places, that scores Philadelphia at 1 on a scale of 1 to 100, based on the EPA's "complex method of measuring watershed quality using 15 indicators."
But what's in the ground isn't the same as what comes out of the tap, which is well-regulated, as the city's website points out:
