More than half the apartments with the coal tar-sealed lots contained dust with levels of PAHs that would increase the risk of cancer if ingested by preschoolers, the researchers said. They came to this conclusion by comparing their results to a 2008 study that estimated those risks based on lab tests on animals. The increased risk means one additional child in 10,000 would develop cancer if exposed to that level of toxins over a lifetime.

Although adults are at risk from toxic pollutants in house dust, young children are especially vulnerable, studies have shown. That’s because they have a higher metabolic rate, they get a bigger dose per pound of body weight, their organs are still developing and they play on or near floors where carpets concentrate and retain toxics. Stanford University researchers have recorded children putting their hands on contaminated surfaces, such as floors, and then into their mouths up to 60 times an hour.

The new research on parking lots is important because scientists have been trying to figure out the sources of PAHs for years, said Ted Schettler, science director of the Collaborative on Health and the Environment, a group of medical professionals trying to reduce environment-related diseases.

“This parking lot (research) is very interesting because it could be there’s a large contributor out there that people didn’t know about,” said Schettler, who was not involved in the research.

Components of coal tar escape parking lots and driveways and get into the environment, causing stunted growth in creatures that live in streams, scientists have shown. Research also reveals that the chemicals in coal tar kill tadpoles, cause tumors on fish and eliminate whole species of tiny aquatic creatures near the base of the food chain.

One congressman, Rep. Lloyd Doggett, D-Texas, is calling for a nationwide ban of the coal tar pavement sealants, which are applied by big contractors as well as operators with little more than a truck and a spray tank.

Not only was the toxic house dust found in apartment units surrounded by paved parking lots, but USGS researchers also measured contamination in dust from apartment house parking lots and the driveways of a few single family homes. The most dangerous coal tar component — a PAH chemical called benzo[a]pyrene — was found in driveway dust at two suburban single family homes at thousands of times the level that would trigger a cleanup at a toxic-waste site.

The United States has no standard for benzo[a]pyrene in house dust, but Germany has an official guideline of 10 parts of the chemical for every 1 million parts of dust, which it says is necessary “to avoid adverse health effects.” In the USGS tests of apartments near coal-tar lots, a third of the apartments showed levels of the toxic chemical exceeding that standard. Some PAHs, including benzo[a]pyrene, are “highly potent” when it comes to causing cancer, according to the EPA.

The EPA did not provide a representative to discuss the new findings with InvestigateWest, despite repeated requests. Rep. Doggett began asking for EPA action in 2003. In 2009 the agency launched research on coal tar sealants that is expected to be completed this year. In a July letter to Doggett and answers to written questions from InvestigateWest last month, the agency did not offer an explanation for the delay.

While there are no reliable estimates of the total amount of the coal tar sealants applied to pavement nationwide, the industry has said that some 59 million gallons — enough to fill nearly 90 Olympic-sized swimming pools — are applied in Texas each year. In the much-smaller watershed surrounding New York City’s harbor, something like 1.4 million gallons is estimated to be applied annually, according to a 2007 study for the New York Academy of Sciences.

Local governments in Austin, Washington, D.C. and the county that includes Madison, Wis., have banned pavement sealants containing coal tar after findings of PAHs in local waterways. In its place, they rely on the second main type of sealant used in the U.S., which is asphalt based.