A study led by researchers from Lawrence Berkeley National Laboratory has found for the first time that thirdhand smoke -- the noxious residue（残渣，剩余） that clings to virtually all surfaces long after the secondhand smoke from a cigarette has cleared out -- causes significant genetic damage in human cells. Furthermore, the study also found that chronic exposure is worse than acute exposure, with the chemical compounds in samples exposed to chronic thirdhand smoke existing in higher concentrations and causing more DNA damage than samples exposed to acute thirdhand smoke, suggesting that the residue becomes more harmful over time.

"This is the very first study to find that thirdhand smoke is mutagenic," said Lara Gundel, a Berkeley Lab scientist and co-author of the study. "Tobacco-specific nitrosamines, some of the chemical compounds in thirdhand 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."

Their paper, "Thirdhand smoke causes DNA damage in human cells," was published in the journal Mutagenesis. The lead investigator was Bo Hang, a biochemist in the Life Sciences Division of Berkeley Lab; he worked with an interdisciplinary group, including chemists from Berkeley Lab's Environmental Energy Technologies Division -- Gundel, Hugo Destaillats and Mohamad Sleiman -- as well as scientists from UC San Francisco, UCLA Medical Center and the University of Texas.

Berkeley Lab scientists (from left) Altaf Sarker, Mohamad Sleiman, Lara Gundel, Bo Hang and Hugo Destaillats worked on the thirdhand smoke study.

The researchers used two common in vitro assays, the Comet assay and the long amplicon-qPCR assay, to test for genotoxicity and found that thirdhand 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 secondhand smoke exposure.

"Until this study, the toxicity of thirdhand smoke has not been well understood," Hang said. "Thirdhand smoke has a smaller quantity of chemicals than secondhand smoke, so it's good to have experimental evidence to confirm its genotoxicity（基因毒性）."

It is the first major study of disease-related mechanisms to come out of the California Consortium on the Health Effects of Thirdhand Smoke, which was established two years ago largely as a result of work published in 2010 by Gundel, Destaillats, Sleiman and others. The Consortium is funded by the Tobacco-Related Disease Research Program, which is managed by the University of California and funded by state cigarette taxes.