Chemicals in a cancer-causing substance used to seal pavement, parking lots and driveways across the U.S. are showing up at alarming levels in dust in American homes, prompting concerns about the potential health effects of long-term exposure, a new study shows.

The substance is coal tar sealant, a waste product of steel manufacturing that is used to protect pavement and asphalt against cracking and water damage, and to impart a nice dark sheen. It is applied most heavily east of the Rockies but is used in all 50 states.

But scientists with the U.S. Geological Survey say the sealant — one of two types commonly used in the U.S. — doesn’t stay put. It slowly wears off and is tracked into homes on the shoes of residents.

The USGS study, which found high levels of chemicals used in the sealant in house dust, marks the first time researchers have raised alarms about potential health effects for humans — especially young children — from the parking-lot coatings.

Taken with previous studies indicating that the chemicals contaminate waterways, where they have been shown to harm insects and tadpoles, it raises serious questions about the advisability of using coal tar as a sealant, the scientists say.

“This is the kind of thing where, when you give a presentation, people’s eyes get big — even scientists,” said Barbara Mahler, a USGS hydrologist who directed the latest research.

The scientists’ published their research Jan. 11 in the science journal Environmental Science & Technology. The research focused on a class of chemicals known as polycyclic aromatic hydrocarbons, or PAHs, which are a significant component of coal tar. The research examined both parking lot dust and dust tracked into homes.

A known carcinogen

Coal tar is known to cause cancer in humans. That finding dates to the 1770s, when chimney sweeps in London were found to have high levels of scrotal cancer. Late the next century, it was associated with skin cancers among creosote workers. PAHs themselves are listed by the U.S. Environmental Protection Agency as a probable human carcinogen, based on laboratory studies in which they caused cancer in animals.

Emerging evidence also suggests that babies exposed to PAHs while in the womb may be more prone to asthma and other ailments, and may have lowered IQs.

The new USGS study compared house dust from 23 ground-floor apartments in Austin — 11 with coal tar-sealed parking lots and 12 coated with other substances, or not sealed at all. The study found that dust in the apartments next to the coal-tar-sealed lots had PAH pollution levels 25 times higher, on average, than the other lots.