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Women who work predominantly at night are 1.5 times more likely to develop primary breast cancer than those who do not, according to a study published in Epidemiology.

Breast cancer is the second leading cause of cancer death in women in the United States, with approximately 200,000 cases diagnosed each year. Although several risk factors have been associated with breast cancer, the cause of the majority of cases of breast cancer remains unknown. Researchers continue to explore the causes and risk factors for this disease in an effort to improve methods for screening and prevention.

Irregular working hours, including night and shift work, have long been considered a health risk. Irregular work schedules are becoming increasingly more common in industrialized societies. Approximately 26% of men and 18% of women in the United States labor force work variable shift schedules. Approximately 20% of the female labor force in Denmark works at night. Researchers from the Danish Cancer Society conducted a case-control study to investigate the relationship between night work and breast cancer.

The researchers identified 7,035 women with primary breast cancer from the database of the Danish Cancer Registry. They obtained information about past employment by record linkage with the nationwide pension fund. They then matched a control to each subject by random selection from the Central Population Registry database. The controls were alive and without cancer.

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The researchers used the employment histories to determine whether women worked predominantly at night. Women were considered to work predominantly at night if they were employed for at least half a year in one or more of the trades in which at least 60% of females typically had nighttime schedules.

After adjustment for childbearing status, alcohol consumption and socioeconomic status, the results indicated that women who worked for at least half a year in any of the trades with predominantly night work were 1.5 times more likely to develop breast cancer. This risk was further increased as the duration of night work increased. The risk of breast cancer among women who worked at night for over 6 years was 1.7 times higher than the risk for daytime workers.

The researchers concluded that women who work predominantly at night are at an increased risk of developing primary breast cancer. While more research is needed to further define the relationship between breast cancer and night work, women who work at night or who are concerned about the risk of breast cancer may wish to speak with their physicians about appropriate screening for this disease, as the best treatment for cancer is preventing its occurrence. (Epidemiology, Vol. 12, No. 1, pp. 74-77, 2001)

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