Most occupations in which women typically engage are not associated with increased breast cancer risk. However, there are some careers that appear to increase risk and perhaps should be avoided by breast cancer survivors and those at high risk for breast cancer. These include jobs involving potentially harmful conditions.
These include long-term night shift work, as well as workplace exposure to radiation, heat, and certain industrial chemicals. The impact of pesticides is less clear, but there is evidence that repeated or concentrated exposures could increase breast cancer risk.

Night shift work and breast cancer risk

Nurses and other women engaging in long-term night shift work have increased risk of breast cancer, according to most, but not all studies. The higher risk associated with night work might be the result of mechanisms influenced by the perception of light by the eye, such as melatonin or circadian synchronization.

Circadian rhythm disruption and breast cancer

Melatonin is a hormone vital to the regulation of circadian rhythms, daily 24-hour cycles that control most physiologic processes. Circadian synchronization depends on an internal clock that is synchronized to light-dark cycles. Circadian rhythms are popularly thought of as being driven solely by the master clock gene in the brain, which receives information directly concerning light-dark conditions through the eyes.
However, specialized timekeeping cells in organs apart from the brain also incorporate clock genes. These genes encode molecular functions with somewhat autonomous timing. Tumor cells can thereby act out of sync with the circadian rhythms established for their organs and this desynchronization has been shown to help drive tumor growth.

Evidence concerning night shift work

The circadian rhythm/melatonin hypothesis is supported by the fact that blind women have a lower risk of breast cancer than sighted women. Blind women with no perception of light whatsoever have an even lower incidence of breast cancer than blind women with some light perception. Exposing the eyes to light (especially blue light wavelengths) when melatonin synthesis normally occurs reduces or eliminates melatonin synthesis. A lack of melatonin has been linked to increased risk of breast cancer.
Occasional night shifts, even over of a period of decades, appears to have little influence on risk of breast cancer. However, working five or six consecutive night shifts over a period of years heightens risk. One study of Norwegian nurses reported no increase in breast cancer risk after long periods during which nurses worked at least three night shifts per month. Higher, but statistically nonsignificant risk of breast cancer was observed in women who worked at least five years with at least four consecutive night shifts. However, significantly increased risks of breast cancer were found in nurses who worked at least five years with six or more consecutive night shifts. Another Scandinavian study reported that nurses with periods of permanent night shift work in addition to periods of rotating night and day shifts experienced the highest breast cancer risk, suggesting that the most disruptive shifts have the largest impact on risk.

Radiation exposure and breast cancer risk

Job-related exposure to ionizing radiation, which has been shown to induce oxidative damage and chronic inflammation, also appears to increase breast cancer risk. Female airline cabin crews, radiologic technologists, and orthopedic surgeons are three groups exposed to radiation that have been reported to have increased risk of breast cancer because of their occupations. However, note that exposure to magnetic fields does not appear to increase breast cancer risk.

Industrial plant, related environments and breast cancer risk

Working in or very near to some types of industrial plants can increase breast cancer risk by means of exposure to various endocrine disruptors and other carcinogens or heat. The following industrial work environments have been studied and found to be associated with increased risk of breast cancer: steel and pulp mills, petroleum refineries, mines, thermal power plants, textile plants, electrical manufacturing plants, and rubber and plastics product manufacturing plants. High temperature environments have also been implicated in the development of male breast cancer. Working in some smaller businesses such as drycleaners using tetrachloroethylene and auto repair shops also can increase breast cancer risk.
Exposure to the following industrial and work-related chemicals has been found to be associated with higher risk of breast cancer:
  • Petroleum-based chemicals such as organic solvents, monoaromatic hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs)
  • Metal degreasing and other metalworking fluids and soldering materials
  • Car and truck exhaust
  • Dyes and inks
  • Acrylic, nylon or rayon textile fibers.
Specific job sectors associated with elevated breast cancer risk include automotive plastics manufacturing, textile manufacturing and dressmaking, printing, food canning, bars and/or gambling, and metalworking. Premenopausal breast cancer risk has been found to be particularly high among workers in food canning and automotive plastics jobs.
On the other hand, being employed or living near electric utility lines (which involves exposure to magnetic fields) does not appear to increase risk of breast cancer.

Pesticide or herbicide exposure and breast cancer risk

Although pesticide and herbicide exposures appear to be an obvious possible source of increased breast cancer risk, most low-level exposure does not in fact appear to increase risk. The majority of agricultural workers do not appear to have increased risk of breast cancer. A variety of studies have examined this issue, with contradictory and inconclusive results:
  • An Australian study reported that women who reported noticing pesticide spray drift from agricultural areas in which they resided had increased risk of breast cancer.
  • A study of farm labor union members in California reported that risk of breast cancer was not associated with work with any specific crops or commodities except mushrooms, where the risk was markedly elevated. Chlordane, malathion, and 2,4-D were associated with increased breast cancer risk.
  • A study that analyzed pesticide exposure and breast cancer mortality in Mississippi reported an association between pesticide exposure and risk of breast cancer mortality in three areas: Greenville, Corinth and Yazoo. The total number of acres planted was positively associated with female breast cancer mortality rate, which differed by race and type of crop planted.
  • Prenatal exposure to currently approved pesticides in Denmark appears to cause earlier breast development in daughters of greenhouse workers. However, the long-term consequences of these findings with respect to breast cancer risk are unknown.
  • A study using a mouse model of breast cancer reported that the organochlorine pesticide β-hexachlorocyclohexane accelerated the appearance and number of tumors when compared to unexposed control mice.
  • However, A Japanese study that measured blood samples for polychlorinated biphenyls (PCBs) and nine pesticide-related organochlorines, reported that β-hexachlorocyclohexane levels were not associated with breast cancer risk. Nor were PCBs or other organochlorine pesticides, including dichlorodiphenyltrichloroethane (DDT), trans-nonachlor, cis-nonachlor, dieldrin, oxychlordane, and mirex.
While, generally speaking, the breast cancer risk associated with exposure might be low, it makes sense to avoid pesticides applied in enclosed spaces such as greenhouses and mushroom farms. Girls and boys should be kept away from fields where pesticides are being applied.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined words will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning this topic. For a more complete list of studies, please click on occupation.