“When their kids are out of the house, they might smoke. Or they smoke in the car. Or they strap the kid in the car seat in the back and crack the window and smoke, and they think it’s okay because the second-hand smoke isn’t getting to their kids,” Dr. Winickoff continued. “We needed a term to describe these tobacco toxins that aren’t visible.”

Third-hand smoke is what one smells when a smoker gets in an elevator after going outside for a cigarette, he said, or in a hotel room where people were smoking. “Your nose isn’t lying,” he said. “The stuff is so toxic that your brain is telling you: ’Get away.’”

The study reported on attitudes toward smoking in 1,500 households across the United States. It found that the vast majority of both smokers and nonsmokers were aware that second-hand smoke is harmful to children. Some 95 percent of nonsmokers and 84 percent of smokers agreed with the statement that “inhaling smoke from a parent’s cigarette can harm the health of infants and children.”

But far fewer of those surveyed were aware of the risks of third-hand smoke. Since the term is so new, the researchers asked people if they agreed with the statement that “breathing air in a room today where people smoked yesterday can harm the health of infants and children.” Only 65 percent of nonsmokers and 43 percent of smokers agreed with that statement, which researchers interpreted as acknowledgement of the risks of third-hand smoke.

The belief that second-hand smoke harms children’s health was not independently associated with strict smoking bans in homes and cars, the researchers found. On the other hand, the belief that third-hand smoke was harmful greatly increased the likelihood the respondent also would enforce a strict smoking ban at home, Dr. Winickoff said.