Danish night shift workers with breast cancer awarded compensationBMJ 2009; 338 doi: https://doi.org/10.1136/bmj.b1152 (Published 18 March 2009) Cite this as: BMJ 2009;338:b1152
Women in Denmark who developed breast cancer after many years of working night shifts have received compensation despite only limited research supporting the link. The ruling could have implications for compensation claims elsewhere in the world.
Out of 78 cases notified to the national board of industrial injuries in Denmark, 38 have received compensation through their employers’ insurance schemes. All of the women had worked night shift patterns for at least 20 years and were otherwise at low risk, for example, they had low alcohol consumption and no family history of breast cancer.
The Danish decision was based on a ruling by the International Agency for Research on Cancer in December 2007 that “shiftwork that involves circadian disruption is probably carcinogenic to humans.” The agency, which is part of the World Health Organization, classed shift work as a group 2A cancer risk. Category 1 risks are known carcinogens, such as asbestos.
An expert working group from the agency, consisting of 24 scientists from 10 countries, came to their conclusion after they found that six out of eight epidemiological studies noted a modestly increased risk of breast cancer in employees working night shifts compared with those who worked days (Lancet Oncology 2007:8:1965-6, doi:10.1016/S1470-2045(07)70373-X). However, the group notes, “These studies are limited by potential confounding and inconsistent definitions of shift-work, with several focused on a single profession.”
The working group also noted that the incidence of breast cancer was modestly increased in most cohorts of female flight attendants who also experience circadian disruption by often crossing time zones. But the working group concluded that “limitations of studies in these flight attendants include the potential for detection bias, proxy measures of exposure, and potential uncontrolled confounding by reproductive factors and cosmic radiation.” The agency’s decision was also based on animal studies, which have shown constant light, dim light at night, or simulated chronic jet lag can substantially increase the development of tumours.
Johnni Hansen, senior researcher at the Danish Cancer Society, who carried out some of the original research reviewed by the agency, said that more research is needed. He is involved in three more studies that are due to report this year, which he says should give more details about the relationship between night shift work and cancer.
Dr Hansen told the BMJ, “We know that the circadian rhythms become disturbed when someone is exposed to light at night. This means less melatonin is produced. And melatonin appears to protect against breast cancer.”
A spokesperson for the Health and Safety Executive (HSE) in the UK said, “At the moment the evidence of a risk is not so compelling that HSE feels preventive action should be taken in workplaces, and in any case, it is difficult to know what that action should be in the light of current knowledge about how any risk might be caused. The HSE believes, based on the current evidence, the principal risk from shift work is fatigue, which can contribute to human error, accidents, and injuries.”
However, the HSE has recently commissioned a study to look at shift working patterns in relation to cancer and other chronic conditions in men and women using data from two existing large cohort studies; the Million Women Study and EPIC-Oxford (European Prospective Investigation into Cancer and Nutrition). This will report in 2011.
Cancer charities in the United Kingdom were cautious about the implications of the Danish precedent. Kat Arney, senior science information officer for Cancer Research UK, said, “At the moment we don’t know how other lifestyle factors, such as taking hormone replacement therapy, obesity, having fewer children, and alcohol consumption, interact with shift work to increase a woman’s risk of breast cancer. Nor do we know how many years of shift work might have a significant impact on risk. Another problem is that most studies have only looked at specific occupations rather than shift workers in general.”
Sarah Cant, policy manager at Breakthrough Breast Cancer, said, “Further research is needed to determine why shift work might affect breast cancer risk. Whatever their working hours, all women can help reduce their risk of developing the disease by maintaining a healthy weight, taking regular exercise, and limiting the amount of alcohol they drink.”
In the Netherlands the trade union federation FNV has called on all women who have worked night shifts for more than 10 years and have breast cancer to come forward to fight a legal test case.
Cite this as: BMJ 2009;338:b1152