Workers Health & Safety Centre

Exposure to night shift work when young raises risk of breast cancer

Young night shift worker
Nurses who’ve worked at least 20 years of rotating night-shifts as young adults have a two-fold increase risk of developing breast cancer, according to a recently published study.
Researchers examined 24 years of data from more than 193,000 women enrolled in the Nurses’ Health Study (NHS) and the Nurses’ Health Study II. The excess risk was found in those enrolled in the second study, who were, on average 20 years younger compared to participants in the original study cohort.
In previously published studies looking at the first 10 to 12 years of data from both NHS and NHS II, researchers found an overall increased risk for breast cancer regardless of age in those working rotating night shifts when compared to people who do not work at night.
These same studies offered significant evidence to an expert working group convened by the International Agency for Research on Cancer (IARC) that ultimately designated shift work involving circadian rhythm disruption as “probably carcinogenic to humans” back in 2007 (Group 2A).

Younger population at risk

“An increase in breast cancer risk among women who work night shifts has previously been reported and this new research essentially confirms prior studies,” says Dr. Eva Schernhammer, study co-author and associate epidemiologist at Boston’s Brigham and Women’s Hospital. “However, in light of the age range of the NHS II population, we suspect that the increased risk among this younger population is reflective of the timing of the exposure to shift work, which was accrued primarily during younger ages, and that younger women may be more vulnerable to this risk.”
This vulnerability, suggested the researchers, may relate to timing of exposure to shift work with respect to “breast tissue change—the period between onset of puberty and breast involution due to childbirth or aging.”

Limiting shift work may reduce risk

Additional findings in this follow-up study adds to the risk prevention discussion. Researchers reported a diminished risk for those who performed 30 or more years of shift work when compared with nurses who never worked at night.
“Follow-up for this older group of women occurred primarily after their retirement from shift work,” Schernhammer said. “This is important to point out, as an association between increased risk of breast cancer and shift work is no longer seen among this group. We suspect that is because once shift work ends, the risk also diminishes.”
In short, these finding suggest less exposure, less risk. For young workers, especially pregnant and nursing mothers, this might be an important prevention solution.
This point was addressed by Dr. Matteo Bonzini, an Assistant Professor of Occupational Health in the Faculty of Medicine of the University of Insubria in Italy, speaking in April, 2010 at a Shift Work Symposium hosted by Ontario’s Institute for Work and Health. He offered that the precautionary principle for pregnant women and shift work is “justified”. He explained “women should be advised against working non-traditional work schedules during pregnancy and should always be allowed to change to daytime work.”
Pregnant women and nursing mothers in Quebec and those under federal jurisdiction (Division VII, Part III, Canada Labour Code) have varying degrees of reassignment protection under occupational health and safety law.
Other jurisdictions including Ontario, Manitoba and Saskatchewan, have some protection under human rights provisions. Here in Ontario, for instance, women have a right to request changes to job duties or rules that affect their health when pregnant. Employers must accommodate these requests, unless doing so would cause undue hardship. Although it should be noted, the Human Rights Commission charged with enforcing these provisions says it has yet to deal with this particular issue and cautions, each case is determined on its merits. Hence, this is why workers and their representatives continue to press for more specific health and safety protections for pregnant women and nursing mothers.

Further health impacts

Beyond breast cancer, shift work has also been linked with health impacts ranging from reproductive issues and gastrointestinal disorders to mental health issues and cardiovascular disease. Research suggests night, evening, rotating and irregular shifts are all linked with an elevated risk of workplace incidents resulting in injuries. Shift work can also impact work-life balance.  

Night shift work common in Ontario

According to CAREX Canada estimates, more than 800,000 Ontarians work regular night or rotating shifts. Among these workers are nurses, truck drivers, first responders, miners, gas station attendants, fast food workers and those employed in manufacturing.

These exposures, according to a recently released study authored by the Occupational Cancer Research Centre (OCRC), may be responsible for 180 to 460 new cases of breast cancer each year.

WHSC training and resources can help

For our part, the WHSC offers Hours of Work training and a Shift Work resource line to help workplace parties better understand the risks posed along with workplace strategies aimed at eliminating or reducing the risk posed by work shift.
To learn more about WHSC training:
Call:    1-888-869-7950 and ask to speak to a training services representative

Want to know more about Shift Work from the WHSC?
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