While it is a well-known fact that second-hand smoke is harmful, a new study conducted by a University of California researcher suggests that both second-hand and third-hand smoke are as deadly as first-hand smoke.
While first-hand smoke refers to the smoke inhaled by a smoker and second-hand smoke to the exhaled smoke and other substances emanating from the burning cigarette that can get inhaled by others, third-hand smoke is the second-hand smoke that gets left on the surfaces of objects, ages over time and becomes progressively more toxic.
"We studied, on mice, the effects of third-hand smoke on several organ systems under conditions that simulated third-hand smoke exposure of humans," Manuela Martins-Green, a professor of cell biology who led the study, said.
"We found significant damage occurs in the liver and lung. Wounds in these mice took longer to heal. Further, these mice displayed hyperactivity," the researcher said.
The results of the study provide a basis for studies on the toxic effects of third-hand smoke in humans and serve to inform potential regulatory policies aimed at preventing involuntary exposure to third-hand smoke.
Third-hand smoke is a potential health threat to children, spouses of smokers and workers in environments where smoking is, or has been, allowed.
Contamination of the homes of smokers by third-hand smoke is high, both on surfaces and in dust, including children's bedrooms.
Re-emission of nicotine from contaminated indoor surfaces in these households can lead to nicotine exposure levels similar to that of smoking.
Third-hand smoke, which contains strong carcinogens, has been found to persist in houses, apartments and hotel rooms after smokers move out.
The team led by Martins-Green found that the mice exposed to third-hand smoke in the lab showed alterations in multiple organ systems and excreted levels of a tobacco-specific carcinogen similar to those found in children exposed to second-hand smoke (and consequently to third-hand smoke).
In the liver, third-hand smoke was found to increase lipid levels and non-alcoholic fatty liver disease, a precursor to cirrhosis and cancer and a potential contributor to cardiovascular disease.
In the lungs, third-hand smoke was found to simulate excess collagen production and high levels of inflammatory cytokines (small proteins involved in cell signaling), suggesting propensity for fibrosis with implications for inflammation-induced diseases such as chronic obstructive pulmonary disease and asthma.
In wounded skin, healing in mice exposed to third-hand smoke showed many characteristics of the kind of poor healing observed in human smokers who have gone through surgery.
Finally, in behavioral tests the mice exposed to third-hand smoke showed hyperactivity.
"The latter data, combined with emerging associated behavioral problems in children exposed to second- and third-hand smoke suggests that with prolonged exposure, they may be at significant risk for developing more severe neurological disorders," Martins-Green said.
The study is published in PLOS ONE.