Workers Health & Safety Centre

Night shift work-breast cancer link demands action, says research

Female nurses working the night shift at a hospital
Evidence of the link between night shift work and cancer has been mounting for over a decade, but a new Canadian cancer burden study finds we can’t wait for more definite proof.  
“Shift work is common here in Canada and throughout the industrialized world and has emerged as the most prevalent suspected occupational cause of breast cancer,” says Manisha Pahwa, study co-author and research associate at Toronto-based Occupational Cancer Research Centre (OCRC).
Pahwa adds, “Additional research on ways to mitigate the harmful health effects of night shift work is necessary. Though, the strength of current research evidence on a range of health outcomes supports the need for workplace prevention interventions now.”    
The new study, entitled “The Impact of night shift work on breast cancer: Results from the Burden of Occupational Cancer in Canada Study”, found an estimated two to 5.2 per cent of the newly diagnosed cases of breast cancer in women in 2011 were attributable to night shift work. Using data from the Canadian Cancer Society, this means between 470 and 1,200 breast cancers diagnosed in 2011 were likely due to night shift work.   
Researchers assumed a latency period of between 10 and 50 years between night shift work and diagnosis of breast cancer. To account for this, they analyzed data based on the assumption working night shifts between 1961 and 2000 may have contributed to newly diagnosed breast cancers in 2011.

Health care workers most impacted

According to CAREX Canada estimates, approximately 1.8 million Canadians (and of these, 800,000 Ontarians) work regular night or rotating shifts. This latest study suggests the largest burden of breast cancer attributable to night shift work is experienced by nurses and others working in health-related occupations. Those employed in food services would be the next most common, followed by sales and service workers. 
These Canadian estimates are similar to the results of occupational breast cancer burden studies in both the United Kingdom and the United States.
An expert working group convened by the International Agency for Research on Cancer (IARC) first classified shift work involving circadian rhythm disruption as “probably carcinogenic to humans” in 2007 (Group 2A). In their findings, IARC cited studies of nurses and female flight attendants who developed excess levels of breast cancer from working nights over long durations. IARC’s re-evaluation of night shift work in 2019, which incorporated studies conducted since 2007, arrived at the same conclusion.
Beyond breast cancer, night shift work may also be linked with colorectal and prostate cancer. Associations have also been observed between night shift work and certain reproductive issues, gastrointestinal disorders, mental health issues, and cardiovascular disease. Research also suggests night, evening, rotating and irregular shifts are all linked with an elevated risk of workplace incidents resulting in injuries

Prevention and  training

Eliminating exposure to occupational hazards, including night shift work, should always be the initial consideration when developing and implementing a workplace-specific prevention strategy. Though night shift work in many sectors, including health care along with food and accommodation, is often unavoidable.
Most efforts focus on administrative controls relating to shift scheduling and rotation. Examples include restricting the number of evening or night shifts, using forward shift rotation (i.e. from day to afternoon to night) and avoiding early starts to day shifts (e.g. not before 6:00am). Research evidence also suggests workplace-specific shift system design is more effective if done in a “participatory way” involving workers, worker representatives and supervisors.
For our part, the Workers Health & Safety Centre (WHSC) offers Hours of Work training and a Shift Work resource line to help workplace parties better understand this wide-spread hazard along with workplace strategies aimed at eliminating or mitigating potential health impacts.
To learn more:
Call:   1-888-869-7950 and ask to speak to a training services representative