Benzene may seem innocuous - it is clear, colorless, and has a slightly sweet odor.
But it's one bad actor. It's a carcinogen, and you don't want to be breathing it.
Traditionally, people have thought of industrial facilities as the main threat. Data from the U.S. Environmental Protection Agency shows that the top major emitters in Pennsylvania and New Jersey are the refineries in South Philadelphia and Paulsboro.
New research out of Emory University in Atlanta suggests that how close you live to one of these facilities may affect your chances of getting a particular kind of cancer, non-Hodgkin's lymphoma.
But the qualifier is that the EPA and the Emory study looked only at major stationary sources.
The big "wow" is that, in Philadelphia, the refinery isn't the biggest source of benzene overall. It's traffic.
An EPA assessment of Philadelphia's air toxics, based on 2005 data, the latest available, shows that the area with the highest risk of cancer from benzene exposure is roughly along the Vine Street Expressway, near the Schuylkill Expressway.
Overall, only about 7 percent of the city's benzene comes from "major risks," such as the refinery. Traffic accounts for more than triple that - 26 percent, the largest piece of the city's benzene pie.
To Joseph O. Minott, head of the Clean Air Council in Philadelphia, this means "it really makes sense in urban areas to promote alternatives to the automobile, such as public transit, walking, and biking."
Other sources of benzene include the airport, construction equipment, and gas stations. About 22 percent of the benzene in the city blows in from somewhere else.
Beyond that, cigarettes are considered the dominant lifestyle-related source of benzene exposure.
In the human body, benzene causes cells to not work correctly. It can lead to anemia and damage the immune system, says the Centers for Disease Control and Prevention.
Non-Hodgkin's lymphoma, which the Emory study tracked, starts in the immune system. The National Cancer Institute estimates it will kill nearly 20,000 Americans this year.
Since the 1970s, diagnoses have increased by 3 to 4 percent a year. Researchers think part of the increase is due to things such as changes in the classification system, better diagnoses, and increases of HIV, which also attacks the immune system.
But that would account for only half the additional cases, they say. The other half? Increased diagnoses also appear to parallel expanded industrial production, which has prompted suspicion of occupational exposures.
The Emory study was one of the first to look at the risk for people who, while they may not work at a facility, live near it.
This matters. EPA statistics show that nearly 3,000 people live within a mile of the Philadelphia refinery - most of them white, most with no more than a high school diploma. Fewer than half the households have incomes topping $25,000 a year.
In a recent study in the journal Cancer, the Emory researchers found that a person's risk for the disease falls a third of a percent for every mile between the home and a major facility.
This doesn't necessarily mean you should move. Christopher Flowers, an epidemiologist at Emory who led the study, said the risk is based on mean distance calculations, and there's no way people could be sure that in moving away from one site, they're not moving toward another.
Plus, the study pointed simply to an association, not a cause. Many other factors could also increase your risk of lymphomas.
But, Flowers said, "it is something provocative that merits further exploration."
To Marilyn Howarth, an occupational and environmental physician with Penn's Center of Excellence in Environmental Toxicology, the Emory study is part of an important "process of discovery."
"There really are many things we don't know about the risks of the various sources of hazardous air pollutants in our region, but we certainly know that there are numerous sources, and they are producing known carcinogens," she said.
"We should pay more attention to the quality of the air and what we're agreeing to allow sources to add to it," she said. "Because it affects all of us."
GreenSpace: Limiting Benzene Exposure
Here are some suggestions from experts:
Don't smoke. A recent study showed that water-pipe smoking, a popular alternative to cigarettes, results in even higher exposure to benzene.
Avoid rush hour, when you'll be in heavier traffic for longer periods.
Avoid the fumes when pumping gas or filling tanks of lawn equipment.
Minimize or avoid exposure to fumes from solvents, paints, and art supplies.
SOURCES: American Cancer Society, Marilyn Howarth, Journal of the American Association for Cancer Research
"GreenSpace," about the environment and health, appears every other week, alternating with Art Carey's "Well Being" column. Contact staff writer Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or @sbauers on Twitter. Visit her blog at www.philly.com/greenspace.