Magnetic Fields, Night Shift Work and the Risk of Breast Cancer among Female Textile Workers in Shanghai, China

This study was conducted to investigate possible associations between workplace exposures to extremely low frequency magnetic fields and rotating shift work, two factors that result in melatonin suppression, and the risks of female breast cancer in a large cohort of women employed in the Shanghai textile industry utilizing a case-cohort study design. The cohort included 267,400 textile workers who were originally recruited for a randomized trial of the effect of breast self examination on breast cancer mortality. All women in the cohort were followed for the development of breast cancer through July 2000, and diagnoses were verified by review of pathology reports or slide review. In total, 1709 breast cancer cases were identified. A reference subcohort of 3153 breast cancer free women randomly selected from the cohort using an age stratification scheme for a series of occupational case-cohort studies and 1627 controls selected for two previous nested case-control studies of breast cancer constituted the comparison group of 4780 women for the present study.;Work history information was collected from all study subjects. A job-exposure matrix for magnetic fields was developed based on over 1000 full-shift measurements that were collected using EMDEX meters from volunteer workers in 57 of 502 factories where the study subjects had worked. Detailed historical shift work information was collected from 502 factories where the study subjects had worked. Hazards ratios and 95% confidence intervals were calculated using Cox proportional hazards modeling adapted for the case-cohort design to determine risks of associated exposures with breast cancer.;We did not observe statistically significant association between cumulative exposure to magnetic fields and overall risk of breast cancer. There were isolated excess risks of small to moderate magnitude observed among women who were less than 50 years old at diagnosis, but no dose-response trend was observed. No associations were observed when exposures were lagged. We did not observe consistent associations between the risk of breast cancer and years of night shift work, or with number of nights worked during the entire employment period. Again, no associations were observed when exposures were lagged. Risk of breast cancer was not appreciably altered in women who were jointly explosed to both high levels of magnetic fields and long durations of shift work.;This study does not support the hypothesis that magnetic field exposure increases the risk of breast cancer. Our findings add to the growing epidemiologic evidence that exposure to magnetic fields has no appreciable influence on the risk of breast cancer in women, regardless menopausal status.;The findings from this study provide no evidence to support the hypothesis that night shift work increases breast cancer risk. The positive association between shift work and breast cancer observed in western populations, but the absence of an association observed in this and another study conducted in a Chinese population suggest that the effect of shift work on breast cancer risk may be different in Asian and Caucasian women.;This is the most comprehensive investigation of two workplace exposures, magnetic fields and shift work, in relation to risk of breast cancer among textile workers. This study provides reassurance to women who are exposed to high levels of magnetic fields, and/or who perform rotating shift work for a prolonged period of time, that these exposures are unlikely to appreciably increase their risk of breast cancer.