Canadian Environmental Health Atlas

Understanding Our Environment is Key to Promoting Health and Preventing Disease

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Shift Work: "Probably Carcinogenic"

Shift Work: "Probably Carcinogenic"

Shift Work: “Probably Carcinogenic”
In 2007, in response to evidence from experimental research and large epidemiological studies such as the Nurses’ Health Study, the World Health Organization’s International Agency for Research on Cancer (IARC) identified “shift work that involves circadian disruption” as a health hazard.1 Listed in the group 2A risk category (“probably carcinogenic to humans”), shift work was ranked with ultraviolet light radiation, diesel exhaust fumes, and industrial chemicals. This ranking was just below the group 1 category (“carcinogenic to humans”), which includes asbestos and tobacco smoke. The IARC decision caused debate in the scientific community, with conflicting opinions about the validity and quality of the evidence used to support the decision.

In response to the IARC decision to list shift work as “probably carcinogenic,” the Danish National Board of Industrial Injuries awarded financial compensation to 38 women with breast cancer in 2009. These cases involved women who worked at least one night per week over a 20- to 30-year period and had no other identified risk factors, such as a family history, to explain their development of breast cancer. Thus far, Denmark is the only country to award compensation to women with breast cancer related to a history of shift work.

Future Research
While there is no doubt that shift work is associated with adverse health outcomes, the role of light at night needs to be investigated further so that, if necessary, effective interventions can be developed. As Dr. Ron Saunders from the Institute for Work and Health in Toronto stated in 2010, “Shift work won’t go away. It’s part of our economy, therefore, the next step is to determine what we can do to lessen its potential health effects.”