Uh-oh. The kids will be home soon. I'd better have my last cigarette so the air will clear out by the time they get here.

That familiar scenario is one that health professionals are growing more concerned about because evidence of ill effects from thirdhand smoke - the residue that clings to carpets, curtains, couches, clothing and even dust particles - is mounting. When you walk into a room several hours after someone has had a cigarette in there and can still smell it, that's thirdhand smoke.

Most recently, researchers in California found that the danger from inhaling thirdhand smoke remains for many hours after the last cigarette has been tamped out.

"Many smokers know secondhand smoke is harmful, so they don't smoke when their kids are present," said Hugo Destaillats, lead author of the study, published online in November in the journal Environmental Sciences & Technology. "But if, for example, they stop smoking at 2 p.m. and the kids come home at 4 p.m., our work shows that up to 60 percent of the harm from inhaling thirdhand smoke remains."

The team at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, where Destaillats is a staff scientist, looked at two locales. One was a room-size chamber at the lab, where a machine smoked six cigarettes in a row, and then levels of particulate matter and 58 volatile organic chemicals were monitored for the next 18 hours. The other was a smoker's home, where scientists collected data eight hours after the last cigarette was smoked.

The researchers determined that the "total integrated harm" rises significantly for the first five hours after a cigarette is smoked. And it continues to rise more gradually for another five hours.

So we're looking at 10 hours, minimum, before the effects begin to level off.

The California scientists have been honing in on thirdhand smoke ever since a 2010 study found that the nicotine in thirdhand smoke reacts with a common indoor air pollutant - nitrous acid, emitted by unvented gas appliances - to form carcinogens.

In a progression of studies that followed, they also discovered that thirdhand smoke can cause genetic damage in human cells.

Their work is funded by the Tobacco-Related Disease Research Program, funded by the University of California.

"Traditionally, people have been looking at secondhand smoke, mostly by measuring directly what comes out of the cigarette. That is important. A very serious exposure," Destaillats said. "But what we are focusing on in our studies is thirdhand exposure, which takes place over longer periods of time, and perhaps inadvertently."

In addition to breathing it in, people can be exposed to thirdhand smoke simply by touching a surface that has some of the toxic chemicals that have settled out of the air. The chemicals get onto your skin and can be absorbed by your body, Destaillats said. Even people who smoke outside can bring chemicals inside to their family members on their clothing.

The ingestion route is especially worrisome in houses with young children, who are always touching things and then putting their hands in their mouths. "House dust can be a fairly long-term source of exposure to chemicals," Destaillats said. "Studies show that ingestion of certain indoor pollutants by toddlers and babies is significant."

He hopes the research will help thwart a common perception that - other than for smokers themselves - the harm from smoking "only takes place if you can see the smoke or smell it strongly." Even several hours later, "it is a serious exposure."

If a smoker decides to stop a few hours ahead of time, "yes, you will be protecting them from 50 to 60 percent of the harm, but not the other 40 to 50 percent."

Norman H. Edelman, senior medical adviser for the American Lung Association, is most concerned about the effects of thirdhand smoke on asthmatics, many of whom are sensitive to it.

"I'm reminded of families who have young children, young adults, who have asthma, and I say, 'For goodness sake, keep all smoking out of your house.'

"A distressed mother says, 'When grandmother comes to visit, she never smokes, but we can smell it on her clothes and hair.' "

"There's no question that thirdhand smoke is real or that it matters," Edelman said.

Beyond asthmatics, is it injurious to the general population? As the latest research shows, "that's evolving," he said.

"This whole area of environmental health is . . . the best way you could characterize it is, the more you look, the more you find."