Lung cancer risk estimates for exposure to environmental tobacco smoke remain controversial, a major unresolved issue being misclassification of smokers. We studied misclassification rates in two large cohorts using information on smoking obtained several years apart. Cohort I included Swedish twins born between 1886 and 1925 who answered questionnaires in 1961 and again in 1967 or 1970. Cohort II was a random stratified population sample of individuals born between 1894 and 1945 who responded to postal smoking surveys in 1963 and 1969. We considered those who stated that they had never smoked in the second questionnaire, but who reported smoking or former smoking in the first questionnaire, to be misclassified. In cohort I, 4.9% of male and 4.5% of female ever-smokers were misclassified, corresponding to 11.1% and 1.3% of reported never-smokers, respectively. Cohort II yielded similar results. A follow-up through 1992 of cohort I showed a relative risk for lung cancer among misclassified men of 1.9 [95% confidence interval (CI) = 0.4-9.1], as compared with 4.5 (95% CI = 2.0-9.9) and 13.3 (95% CI = 6.5-27.0) for former and current smokers, respectively. No case occurred among misclassified women. Although misclassification of smokers exists, our results indicate that it mainly concerns light smokers or long-time ex-smokers, who have only a very moderately elevated risk of lung cancer. It therefore appears unlikely that confounding by smoking explains the increased risk for lung cancer related to environmental tobacco smoke exposure.

Background: Genetic factors play a role or roles in the etiology of peptic ulcer disease and the acquisition of Helicobacter pylori infection.

The rate of Helicobacter pylori infection is inversely related to socioeconomic status, and childhood is thought to be the major acquisition period. The authors investigated the importance of childhood environment in the acquisition of H. pylori infection. A cross-sectional study was conducted of monozygotic and dizygotic twins who were reared apart or reared together. Three hundred twins from a subregistry of the Swedish Twin Registry were studied. H. pylori status was evaluated by using an enzyme-linked immunosorbent assay for anti-H. pylori immunoglobulin G. Socioeconomic status during childhood was assessed on the basis of the density of the living conditions and the economic situation of the family that reared the twins. Current socioeconomic status was estimated by using a scale that combined income and education. Dietary elements that were studied included fat and fiber intake and ascorbic acid consumption. The density of the childhood home was consistently found to be significantly associated with the acquisition of H. pylori infection (p = 0.04). Among monozygotic twins reared apart and discordant for H. pylori status, affected twins were raised in homes under poorer socioeconomic conditions than those of their unaffected co-twins (p = 0.02). Additionally, infected twins consumed more ascorbic acid than their unaffected co-twins (p = 0.04). The finding of an effect of socioeconomic status during childhood on the acquisition of H. pylori among monozygotic twins who were reared apart and had an identical genetic makeup but not a common environment confirms the hypothesis that childhood acquisition of H. pylori infection is linked to hygiene practices.