LAS VEGAS--Epidemiological studies have suggested for years that second-hand smoke can as much as double the risk of cancer in nonsmokers. New findings presented here today at the semi-annual meeting of the American Chemical Society now suggest how that might happen. They provide the first direct chemical evidence that common environmental levels of second-hand smoke expose people to detectable levels of a potent carcinogen.
Tobacco is one of the only sources of a chemical called n-nitrosamine (NNK), which has been shown to cause cancer in laboratory animals. In rats, high NNK levels induce adenocarcinoma, a type of cancer that in humans is the most common lung cancer in nonsmokers exposed to second-hand tobacco smoke. And 4 years ago, chemist Stephen Hecht and his colleagues at the University of Minnesota Cancer Center in Minneapolis, found traces of NNK, metabolized into a compound known as NNAL-Gluc, in the urine of nonsmokers exposed to second-hand smoke in the lab. But to detect any NNAL-Gluc, Hecht's team had to expose the volunteers to smoke concentrations higher than those usually encountered--two to three times higher than in the average smoking section of a restaurant, for example.
By improving the sensitivity of their NNAL-Gluc detector 20-fold, Hecht and colleagues have gauged the effect of more ordinary exposure levels of second-hand smoke. The team tested out the improvements by evaluating nine nonsmoking hospital workers caring for live-in patients in the smoking area of a Canadian veterans hospital. On the last day of a workweek, each volunteer provided urine samples, and all had detectable amounts of NNAL-Gluc. Although the concentrations were about 70 times lower than those found in smokers, Hecht says the study, "establishes a link between the epidemiology and the chemistry."
Other cancer experts agree. The study is "concrete evidence" that the volunteers were exposed to a tobacco-specific carcinogen, says Curtis Harris, who heads the laboratory of human carcinogenesis at the National Cancer Institute in Bethesda, Maryland. While any one nonsmoker has a low chance of developing cancer from the levels measured in the study, Harris says that the large numbers of nonsmokers exposed to cigarette smoke makes the risk significant for the population as a whole.