To the Editor:
The recent case-control study by Hansen 1 investigated the linkage between female breast cancer and night work. The author reconstructed the employment histories of 7,035 Danish women with breast cancer back to 1964, and compared them with an equal number of controls. Hansen found an increased risk of breast cancer among female night workers (odds ratio 1.5), but we have some reservations about the study.
The classification of cases and controls into the night worker/non-night worker category was not based on an individual ascertainment of each woman’s work history, but rather on her being in a trade in which at least 60% of women work at night. The use of this 60% cut-off point also excluded female hospital workers (principally nurses), who are the single largest group of female night workers in the population being investigated. 2
The study did control for important confounders such as parity, socio-economic status and age at birth of first and last child. The authors did not have information on individual alcohol consumption.
Hansen states that previous descriptive studies have found a linkage between female breast cancer and night workers such as flight attendants and radio/telegraph operators. Flight attendants, however, are exposed to cosmic radiation, which may be a risk factor for female breast cancer. 3 Radio and telegraph operators are exposed to electromagnetic radiation, which again may be a risk factor for female breast cancer. 4 Thus, the observed increased risk of female breast cancer in these descriptive studies may not be primarily due to night work, but may be mediated by other risk factors.
Tom O’ Connell
1. Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology 2001; 12: 74–76.
2. Hansen EJ. The Distribution of Living Conditions. Main Results from a Welfare Study. Part 1. Theory, Method and Summary. Copenhagen: Teknisk Forlag, 1978.
3. Ballard T, Lagorio S, De Angelis G, Verdecchia A. Cancer incidence and mortality among flight personnel: a meta-analysis. Aviat Space Environ Med 2000; 71: 216–224.
4. Caplan LS, Schoenfeld ER, O’ Leary ES, Leske MC. Breast cancer and electromagnetic fields—a review. Ann Epidemiol 2000;10:1:31–44.