As we all know, smokers continue to smell of smoke long after they've finished their last cigarette. But it's not just a smell; this “third-hand smoke” has many of the harmful substances in cigarette smoke, and it lingers on clothes, bodies, and surfaces. Now we've confirmed that smokers can bring third-hand smoke into smoke-free environments with them.

A paper in Science Advances this week reports finding third-hand smoke in a cinema that has been smoke-free for 15 years. Every time a new audience filed into the cinema, chemist Roger Sheu and his colleagues detected a sharp spike in tobacco-related pollutants. They estimate that the audiences were exposed to the equivalent of between one and 10 cigarettes’ worth of second-hand smoke.

Lingering contaminants

Regulations preventing indoor smoking have done a great job of limiting exposure to cigarette smoke, write Sheu and his colleagues, but third-hand smoke is a “major exposure pathway” to the hazardous substances found in tobacco. With worldwide smoking rates still above 20 percent and public health expert agreement that there is no safe level of exposure to tobacco smoke, this represents a “major risk for nonsmokers,” they say.

Third-hand smoke arises from the hazardous compounds in tobacco settling on surfaces—including smokers’ clothes and bodies, as well as surfaces like furniture—and then mingling with dust or other particles. People are exposed to these chemicals by breathing in the dust or evaporated gases or by touching the contaminated surfaces.

Contamination from third-hand smoke has been found in smokers’ clothing, breath, and homes. But can it be carried into a non-smoking environment? Researchers had thought it was likely, but Sheu and his colleagues wanted to gather evidence to confirm and to see how bad the exposure was.

They chose a cinema in Mainz, Germany, that was well-ventilated and tracked the tobacco-related compounds in the air during multiple screenings of a few different movies. Smokers would have had to finish their cigarettes outside and then take a few minutes to make their way into the cinema, making third-hand smoke the only probable source of these compounds.

Bringing the outside in

Sheu and his colleagues found that the cinema had relatively high concentrations of 35 different compounds that come from tobacco smoke. These concentrations spiked when new audiences entered the cinema at the start of a screening. The spikes were much worse for adult-oriented action films than for screenings of a family film, suggesting that certain audience demographics were bringing in more third-hand smoke.

During the course of each movie, the third-hand smoke compounds started to decrease, partly because these compounds tend to “off-gas” less over time, and partly because of the cinema’s ventilation. But there were more spikes at the end of some films, when audiences left the theater—possibly caused by clothes shifting as people stood up or by people breathing slightly faster as they started moving again, suggest Sheu and his colleagues. People’s exposure to the pollutants would have differed depending on where in the cinema they were sitting—for those sitting next to a smoker, exposure would have been relatively high.

Other public spaces could have far higher rates of contamination. A large, well-ventilated cinema could actually be on the lower end of exposure, while public transit, classrooms, and offices all have people frequently moving out of small spaces that aren’t necessarily well-ventilated. And for parents who diligently smoke outside their homes, the probability of carrying contaminants back inside with them should be worrying.

Health impacts

It’s difficult to work out exactly how much health risk comes from third-hand smoke, but previous research has found that it contributes to somewhere between five and 60 percent of the total disease burden created by combined exposure to second-hand smoke and third-hand smoke. Children are particularly vulnerable.

Trying to figure out just how bad third-hand smoke exposure is will take a lot more work. Sheu and his team point to the need for more controlled research on exactly how smokers emit pollutants, answering questions like how much comes from their clothes compared to their bodies and how contamination changes over time after they finish smoking. Work on vaping will also be important to understand whether e-cigarettes could be introducing a new and different kind of third-hand smoke.

Science Advances, 2020. DOI: 10.1126/sciadv.aay4109 (About DOIs).