Parents who smoke often open a window or turn on a fan to clear the air of second-hand smoke, but experts now have identified another smoking-related threat to children’s health that isn’t as easy to get rid of: third-hand smoke.

That’s the term being used to describe the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing, not to mention cushions and carpeting, that lingers long after smoke has cleared from a room. The residue includes heavy metals, carcinogens and even radioactive materials that young children can get on their hands and ingest, especially if they’re crawling or playing on the floor.

Doctors from MassGeneral Hospital for Children in Boston coined the term “third-hand smoke” to describe these chemicals in a new study that focused on the risks they pose to infants and children. The study was published in this month’s issue of the journal Pediatrics.

“Everyone knows that second-hand smoke is bad, but they don’t know about this,” said Dr. Jonathan P. Winickoff, the lead author of the study and an assistant professor of pediatrics at Harvard Medical School.

In a single year, a coal-fired electric plant deposited more than 2.2 million pounds of toxic materials in a holding pond that failed last week, flooding 300 acres in East Tennessee, according to a 2007 inventory filed with the Environmental Protection Agency.

The inventory, disclosed by the Tennessee Valley Authority on Monday at the request of The New York Times, showed that in just one year, the plant’s byproducts included 45,000 pounds of arsenic, 49,000 pounds of lead, 1.4 million pounds of barium, 91,000 pounds of chromium and 140,000 pounds of manganese. Those metals can cause cancer, liver damage and neurological complications, among other health problems.

And the holding pond, at the Kingston Fossil Plant, a T.V.A. plant 40 miles west of Knoxville, contained many decades’ worth of these deposits.

For days, authority officials have maintained that the sludge released in the spill is not toxic, though coal ash has long been known to contain dangerous concentrations of heavy metals. On Monday, a week after the spill, the authority issued a joint statement with the E.P.A. and other agencies recommending that direct contact with the ash be avoided and that pets and children should be kept away from affected areas.

Residents complained that the authority had been slow to issue information about the contents of the ash and the water, soil and sediment samples taken in and around the spill.

“They think that the public is stupid, that they can’t put two and two together,” said Sandy Gupton, a registered nurse who hired an independent firm to test the spring water on her family’s 300-acre farm, now sullied by sludge from the spill. “It took five days for the T.V.A. to respond to us.”