Houses where smokers have lived remain polluted with tobacco smoke for at least six months after the smoker has quit and may continue to pose a threat to nonsmokers who live there, a new study found.

The report, in Tobacco Control, found that small particles from burning tobacco penetrate multiple surfaces — carpets, upholstery, pillows, blankets, clothes, even wallpaper and ceiling tiles — and remain long after smoking has stopped.

Researchers studied 65 smokers who were quitting. Periodically over six months, they measured levels of nicotine and other tobacco-specific compounds on household surfaces, in dust and in the urine of nonsmokers living in the same household.

There were significant short-term reductions in nicotine on surfaces and in dust, which then leveled off and remained steady, but still detectable, at the end of the study period.

Even after six months, urinary cotinine levels — a measure of exposure to tobacco — were still detectable in the nonsmokers.

“We tend to see smoke in the air and then it’s out of sight, out of mind,” said the lead author, Georg E. Matt, a professor of psychology at San Diego State University. “But it leaves compounds in indoor environments that can do harm to our bodies, especially children, and sometimes we cannot see or smell it. No level of exposure to tobacco is safe.”