Why do nonsmokers develop lung cancer? The answer is, obviously, uncertain at present. While there may be genetic factors involved,18 a number of environmental factors have also been implicated. In Asian women, the role of exposure to cooking oils has been implicated in a number of studies but is as yet unproven.17,19 Environmental tobacco smoke, or “passive smoking,” has been the subject of intense scrutiny. In most studies, the relative risk for lung cancer has been on the order of 1.2 to 1.3, suggesting a modest increase in lung cancer risk.21Passive smoking, however, is unlikely to explain the high incidence of lung cancer in nonsmokers in this Singapore study, as the mean age at diagnosis for nonsmokers compared to smokers was almost 10 years younger. As there is a clear dose-response relationship between smoking exposure and lung cancer, one would expect lung cancer due to passive smoking to occur at a later age than that for smokers. Other environmental factors implicated in lung cancer in nonsmokers include exposure to radon, arsenic, asbestos, chromium, nickel, and other occupational carcinogens.22 Critical analysis of the currently available literature would suggest that exposure to such carcinogens is probably associated with a low risk of lung cancer and is almost certainly not relevant to the current Singapore study by Toh et al.