Thirdhand"smoke"isn't actually smoke at all.It's the residue of nicotine and other chemicals in tobacco,some of which are toxic,that remain long after active smoking is over.

　　Some of these chemicals stick to surfaces,and others attach to dust particles.Still others often penetrate deep into wallboard,drapes and upholstery.As the compounds linger,they may react with oxidants or other particles in the room's atmosphere.The chemical reactions can create potentially harmful byproducts that can become airborne.

　　Science has known about this type of environmental pollution for years,sparking the creation of smoking and non-smoking rooms at hotels,restaurants and the like.

　　But thirdhand smoke has also been found in environments which were not known to be contaminated by smokers,which led researchers to ask how that could happen.

　　To find out,Gentner and some of his PhD students set up an experiment in a movie theater that had not allowed smoking for more than 15 years.They supplied fresh air into the theater,making sure that no smoking or other contaminants entered the space.

　　Sophisticated equipment measured airborne particles before and after moviegoers arrived.Right away,they saw a huge spike in levels of hazardous chemicals.

　　As people came into the theater,the concentrations went up,and then decreased over time,Gentner said.

　　"But they didn't completely disappear after the audience left,"he added."In many cases,the persistent contamination was observable the following day in the unoccupied theater."

　　The testing continued over a four-day period across different movie genres.Perhaps not surprisingly,levels of nicotine and other chemicals were lower in the G-rated movies geared toward kids.

　　"Despite cases where we had audiences of over 200 people for some of the G-rated movies,the pollutants were much larger for R-rated films,even with smaller audiences,"Gentner said."Those movies would likely draw older audiences that might be more likely to smoke."

　　Despite declines in smoking in some developed nations,there are still over a billion smokers worldwide,according to the World Health Organization.

　　"That billion smokers contribute to about 880,000 deaths from secondhand smoke,"said Dr.Jagat Narula,a cardiologist at Mount Sinai Morningside in New York City who researches the health impact of smoking."I call it a form of murder."

　　Learning about the Yale study's findings on the level of off-gassing from human bodies and clothing was not surprising,said Narula,who was not involved in the study.

　　"But it is disturbing,"he said."Numerous reports have shown that there is no safe level of exposure to secondhand smoke.

　　"If future research replicates these findings on thirdhand smoke,it means that smokers could potentially still do harm even if the act of smoking took place in a different space."