Breast cancer: an occupational disease.

B reast cancer has just been considered as an occupational disease in Denmark. Recent published studies showed that working on rotating night shifts was associated with a higher incidence of subsequent breast cancer among the female nurses and fl ight crews. 5,6 Many factors have association with the development of breast cancer. Some of these risk factors cannot be changed; those included age, gender, family history of breast cancer, ethnicity and genetic build up, obesity, past history of alcohol use, history of childbirth, use of diethylstilbestrol (DES), and history of exposure to ionized radiation. Some other variables such as socioeconomic status may be modified. Abnormal photic stimuli, and altered rest-activity cycles with advancing age have been linked with some endocrine disturbances. 8 The hypothalamic-pituitary -adrenal axis dysfunction, interrupted diurnal cortisol patterns, carbohydrate metabolism imbalance, elevation of sympathetic nervous system activity and thyroid dysfunction have also been associated with graveyard shift work. 8 After 10 years working at night, the risk of developing breast, prostate and colorectal cancer increases significantly. 9 An increased risk associated with duration of light at night work, quality of sleep, number of times a subject got up during the night and turned on a light were also reported in shift workers. 1 Exposure to light at night suppresses normal nocturnal increase in melatonin— a hormone with antiproliferative effect. Loss of circadian variation in blood corti-sol levels might increase the exposure to a higher level of estrogen in women which in turn can increase the risk of breast and intestinal cancers. 10 Shift work would also cause a higher rate of prostate cancer in men. 9 The wavelengths of light in the blue region of the visible spectrum (e.g., fluorescent light which is commonly used in various environments with artificial light) have the highest potency to decrease in melatonin levels. 11 Permanent night shift workers adopt a new circadian rhythm than those who rotate between day and night shifts. Melato-nin has less reduction among individuals working on permanent night shifts than those who work on rotating night shifts. Usually, those who work night shifts have a lower socioeconomic status and may have less concerns about their health status; they also smoke more often than women who did not work night shifts. 12 Generally, women working night shift were elder, have a higher emotional stress and smoke more often than those who had never worked on graveyard shifts. …