Researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) in California have found a link between thirdhand smoke (THS) exposure and lung cancer risk in mice.
While the detrimental effects of smoking and exposure to secondhand smoke are well-known, less is known about exposure to THS — pollutants left on furniture, walls and carpets, and in dust after someone smokes tobacco. In their study examining a cohort of 24 mice, researchers discovered that early THS exposure is associated with increased incidence and severity of lung cancer in mice.
“We had done previous studies where we looked at early exposure to THS and found short-term exposures would change immunological parameters,” says Antoine Snijders, scientist at Berkeley Lab and lead author on the study, published in Clinical Science. “At that point we didn’t know what the long-term risks were of THS exposure, so that’s what this study was supposed to address.”
Researchers exposed 4-week-old mice to THS for three weeks, observed them for 40 weeks and then examined lung tumor incidence. In the THS-exposed cohort, the scientists found a roughly threefold increase in lung cancer incidence using the A/J mouse model.
“This risk is dependent on genetic background, and depending on the mouse strain, this risk could be higher or lower,” Snijders says.
THS remains hazardous much longer than secondhand smoke. The study found the risk increase was linked to early exposure, paralleling early childhood in humans. Major routes of THS exposure include dermal exposure, inhalation and ingestion.
“We put a material with the THS in the cage, like kids’ exposure,” says Jian-Hua Mao, Senior Scientist at Berkeley Lab.
This allowed researchers to isolate the effects of THS from other environmental factors, which Snijders notes is more difficult in studies involving humans.
“Just because people are not actively smoking in the house or a public place doesn’t mean that there are no risks of previous smoking history in that place.”
— Antoine Snijders, scientist at Berkeley Lab
Furthering THS Research
The findings build on research including a 2010 study in PNAS that found THS causes carcinogen formation when its components interact with nitrous acid in indoor air, as well as a 2014 paper in Environmental Science & Technology that discovered THS could cause up to 60 percent of the harmful effects associated with secondhand and thirdhand exposure to smoke.
Existing research has already led to policy changes, including legislation in California that targeted THS in home-based daycare centers and a similar bill now under consideration in New York.
Mao points to further avenues of research regarding THS.
“Maybe not everybody responds to THS the same ...,” Mao says. “We need to find what kind of additional genetic factors are affected [by] THS.”
Snijders urges pediatricians to advise parents on how to prevent early exposure of their children, including by repainting walls and removing furniture or replacing carpets in apartments and houses in which smoking has taken place.
“I would just like to see increased awareness of the potential dangers of THS,” he says.