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It has been hypothesized that nighttime bedroom measurements of extremely low frequency electromagnetic fields (ELF EMF) may represent a more accurate reflection of exposure and have greater biologic relevance than previously used 24-/48-hour measurements. Accordingly, the authors extended a pooled analysis of case-control studies on ELF EMF exposure and risk of childhood leukemia to examine nighttime residential exposures. Data from four countries (Canada, Germany, the United Kingdom, and the United States) were included in the analysis, comprising 1,842 children diagnosed with leukemia and 3,099 controls (diagnosis dates ranged from 1988 to 1996). The odds ratios for nighttime ELF EMF exposure for categories of 0.1–<0.2 μT, 0.2–<0.4 μT, and ≥0.4 μT as compared with <0.1 μT were 1.11 (95% confidence interval (CI): 0.91, 1.36), 1.37 (95% CI: 0.99, 1.90), and 1.93 (95% CI: 1.11, 3.35), respectively. The fact that these estimates were similar to those derived using 24-/48-hour geometric mean values (odds ratios of 1.09, 1.20, and 1.98, respectively) indicates that the nighttime component cannot, on its own, account for the pattern observed. These results do not support the hypotheses that nighttime measures are more appropriate; hence, the observed association between ELF EMF and childhood leukemia still lacks a plausible explanation. [ABSTRACT FROM AUTHOR]

Inconsistent epidemiologic findings on cigarette smoking and female breast cancer risk may reflect insufficient assessment of smoking onset and amount relative to reproductive events. To determine the risk of breast cancer associated with smoking during different periods of reproductive life, the authors evaluated 906 incident breast cancer cases in a nationwide cohort of 56,042 female US radiologic technologists (1983–1998) who responded to two questionnaire surveys. After they accounted for age, birth cohort, and established breast cancer risk factors, smoking-related breast cancer risks differed by smoking during three reproductive time periods (p = 0.003), with a statistically significant 3% increase per pack-year of smoking between menarche and first childbirth (relative risk = 1.03, 95% confidence interval: 1.02, 1.05) and no significant association for smoking after first childbirth. Risk also increased with younger age at smoking initiation (p-trend = 0.06), after adjustment for pack-years of smoking before and after first childbirth, indicating an independent effect of age at smoking initiation. The findings from this study suggest that sensitivity of the female breast to tobacco carcinogens is increased during adolescence and early adulthood but decreases after first childbirth, when most breast tissue has terminally differentiated. [ABSTRACT FROM AUTHOR]