Secondhand smoke in vehicles exposes nonsmokers to toxic compounds

Secondhand smoke in the confined space of a motor vehicle resulted in exposure to several tobacco smoke toxins, and may ultimately increase risk for cancer and for respiratory and cardiovascular diseases in nonsmokers, according to a study published in Cancer Epidemiology, Biomarkers & Prevention, a journal of the American Association for Cancer Research.

"Secondhand smoke in vehicles resulted in substantial exposure to tobacco smoke toxins and increased risk for cancer," said Neal Benowitz, MD, chief of the Division of Clinical Pharmacology and Experimental Therapeutics at the University of California, San Francisco. "Some states have laws banning smoking in cars with children and this study supports the idea that there should be a total ban on smoking while a child is in the car."

Exposure to these toxins, or volatile organic compounds (VOCs), during even a brief period of time resulted in an intake level that far exceeded Environmental Protection Agency (EPA)-listed acceptable levels.

Benowitz and colleagues exposed 14 nonsmokers individually in the backseat of a car to one hour of secondhand smoke from a smoker in the driver's seat of the vehicle. The smoker smoked three cigarettes at 20-minute intervals. The car was stationary and the windows were kept open by 10 cm. The researchers measured the levels of nine VOCs before secondhand smoke exposure and zero to eight hours after exposure.

"We found significant increases in the levels of several VOCs, the most important ones being 1,3-butadiene, benzene, and acrylonitrile," Benowitz said. "1,3-butadiene and benzene are known human carcinogens and acrylonitrile is a probable human carcinogen."

After one hour of exposure, nonsmokers had a 2.1-fold increase in 1,3-butadiene, a 1.6-fold increase in benzene, and a 1.7-fold increase in acrylonitrile.

The researchers then used these measurements to estimate increased exposure for a nonsmoker exposed to secondhand smoke one hour a day, five days a week for 51 years. At this level of exposure, the researchers estimated that there would be an excess of 16 to 28 cancers per million adults.

"The acceptable risk from the EPA is one excess cancer per million people," Benowitz said. "Just looking at these three VOCs, and not including exposure to other VOCs, we can say that even moderate secondhand smoke exposure far exceeds an acceptable cancer risk."

This study was supported by the Flight Attendants Medical Research Institute, a U.S. Public Health Service grant, and the National Institutes of Health.

Benowitz served on smoking cessation advisory boards for Pfizer, has been an occasional consultant to McNeil and GlaxoSmithKline PLC, and has served as a paid expert witness in litigation against tobacco companies.