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The finding is significant for children. Parents or elders who smoke in the family fail to realise that opening the window while smoking, for fresh air to blow away the smoke, isn't good enough. Infants are therefore at high risk of tobacco-related health problems when they inhale, ingest or touch substances containing third-hand smoke.
The study led by researchers from Lawrence Berkeley National Laboratory in California has also found that chronic exposure is worse than acute exposure, with the chemical compounds in samples exposed to chronic third-hand smoke existing in higher concentrations and causing more DNA damage than samples exposed to acute third-hand smoke, suggesting that the residue becomes more harmful over time.
"This is the very first study to find that third-hand smoke is mutagenic," said Lara Gundel, a Berkeley Lab scientist. "Tobacco-specific nitrosamines, some of the chemical compounds in third-hand smoke, are among the most potent carcinogens there are. They stay on surfaces, and when those surfaces are clothing or carpets, the danger to children is especially serious."
The findings have been published in the journal 'Mutagenesis'.
Third-hand smoke is particularly insidious because it is extremely difficult to eradicate. Studies have found that it can still be detected in dust and surfaces of apartments more than two months after smokers moved out.
Common cleaning methods such as vacuuming, wiping and ventilation have not proven effective in lowering nicotine contamination.
The researchers used two common in vitro assays, the Comet assay and the long amplicon-qPCR assay, to test for genotoxicity and found that third-hand smoke can cause both DNA strand breaks and oxidative DNA damage, which can lead to gene mutation.
Genotoxicity is associated with the development of diseases and is a critical mechanism responsible for many types of cancer caused by smoking and second-hand smoke exposure.
"Until this study, the toxicity of third-hand smoke has not been well understood," lead investigator Bo Hang said. "Third-hand smoke has a smaller quantity of chemicals than second-hand smoke, so it's good to have experimental evidence to confirm its genotoxicity."
"You can do some things to reduce the odours, but it's very difficult to really clean it completely," the team said. "The best solution is to substitute materials, such as change the carpet, repaint. Third-hand smoke could become more harmful over time."
To generate the samples, the researchers put paper strips in smoking chambers. The acute samples, generated at Berkeley Lab, were exposed to five cigarettes smoked in about 20 minutes, and the chronic samples, generated at University of California, San Francisco, were exposed to cigarette smoke for 258 hours over 196 days.
During that time, the chamber was also ventilated for about 35 hours.
The researchers found that the concentrations of more than half of the compounds studied were higher in the chronic samples than in the acute. They also found higher levels of DNA damage caused by the chronic samples.
"The cumulative effect of third-hand smoke is quite significant," Gundel said. "The findings suggest the materials could be getting more toxic with time."
Hang and coworkers exposed the human cells by first extracting the compounds from the paper with a culture medium and then using the medium to culture the human cells for 24 hours. The concentrations of the compounds were carefully measured.
"They are close to real-life concentrations, and in fact are on the lower side of what someone might be exposed to," Hang said.
Residual nicotine can react with ozone and nitrous acid — both common indoor air pollutants — to form hazardous agents. When nicotine in third-hand smoke reacts with nitrous acid it undergoes a chemical transformation and forms carcinogenic tobacco-specific nitrosamines, such as NNA, NNK and NNN.
Nicotine can react with ozone to form ultrafine particles, which can carry harmful chemicals and pass through human tissue.
Humans can be exposed to third-hand smoke through inhalation, ingestion or skin contact.
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