SAN FRANCISCO, March 8 (Xinhua) -- Early thirdhand smoke (THS) exposure increased the incidence and severity of lung cancer in mice, which suggests potential risks to human health, the U.S. Lawrence Berkeley National Laboratory (Berkeley Lab) said Thursday.
THS is a kind of toxic residue that lingers on indoor surfaces and in dust long after a cigarette has been extinguished, which was already regarded as a health hazard nearly 10 years ago.
Scientists at Berkeley Lab have determined in their latest follow-up study that early THS exposure increases the lung cancer risk in mice following their new experiment on a cohort of 24 mice.
The mice were housed with scraps of fabric impregnated with THS from the age of four to seven weeks, receiving a dose of an estimated 77 micrograms per kilogram of body weight per day.
Experiment results showed that these mice were found to have an increased incidence of lung cancer, larger tumors and a greater number of tumors, compared with 19 control mice.
The latest study is follow-up research done by a team led by Antoine Snijders, Jian-Hua Mao and Bo Hang of Berkeley Lab in 2017, which first reported that brief exposure to THS is associated with low body weight and immune changes in juvenile mice.
Field studies in the United States and China have confirmed that the presence of THS in indoor environments is widespread, and traditional cleaning methods are not effective for removing it.
Exposure to THS can occur via inhalation, ingestion or absorption through the skin, according to the study.
"THS causes hidden risks of tobacco smoking. You cannot see it, but you can smell it," said Hang, a staff scientist in the Biological Systems and Engineering Division at Berkeley Lab.
Hang, who studied at Nanjing Medical University in eastern China in the 1980s, told Xinhua on Thursday that young children were most likely to suffer from thirdhand smoke.
"Young kids who crawl and put objects in their mouths are more likely to come in contact with contaminated surfaces, and are therefore the most vulnerable to thirdhand smoke's harmful effects," said the Chinese American scientist.
He also said the study of his team on the harmful effects of secondhand smoking played a vital role in prompting California state to pass legislation in 2014 banning smoking at all home-based baby-sitting centers, which was a historic milestone.
The most significant progress made in the study of THS is that the long-term effects of THS are determined to be linked to human cancers, Mao, a geneticist senior staff scientist at Berkeley Lab, told Xinhua.
The study is the latest in several THS-related Berkeley Lab discoveries, and is part of an ongoing collaboration between Berkeley Lab and the University of California San Francisco School of Medicine to study the impact of early exposure to thirdhand smoke in mice.