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Night Shift Work Now Implicated In Causing A Higher Risk Of Breast Cancer

According to an analysis of surveys in five countries done by researchers at UdeM and other institutions, women who work the night shift run a higher greater risk of developing breast cancer.

That is the headline of an article published on the UdeMNouvelles website reporting on the latest research into the connection between night shift work and the incidents of breast cancer amongst night workers and concludes that
women who work at night, especially during pre-menopause, may be at greater risk of developing breast cancer.

New analysis of surveys of 13,000 women from five countries; Australia, Canada, France, Germany and Spain, conducted by a team of researchers that includes Anne Grundy, a research associate at Université de Montréal’s Department of Social and Preventive Medicine, part of the School of Public Health, resulted in these findings.

The surveys looked at nearly 6,100 women who had been diagnosed with breast cancer and approximately 7,000 who had no diagnosis. Participants answered self-administered questionnaires or telephone interviews about their occupation and about risk factors for breast cancer.

The average age of the women varied between 55 and 59, depending on the country, and the proportion working nights also varied widely, from 6 per cent in Germany to 17 per cent in Australia. In Canada, the proportion was 16.7 per cent.

12- to 80-per-cent higher risk

Published in the April issue of the European Journal of Epidemiology, the study found that the risk of developing ER-positive breast cancer (that is, of tumours associated with estrogen receptors) and/or HER2 breast cancer increases with the number of hours worked per night, as well as the number of years spent on the night shift.

However, the risk seems to diminish two years after going off the night shift.

The article quotes Anne Grundy as saying:

“Women who work at least three hours between midnight and 5 a.m. run a 12 per cent greater risk of developing breast cancer than women who have never worked at night,” and adding:

“Among pre-menopausal women, the risk associated with working at night increases to 26 per cent.”

The findings of the analysis shows that Night workers who work shifts longer than 10 hours have a 36-per-cent increased risk of breast cancer, again compared to women who have never worked nights. The risk is as high as 80 per cent among women who work night shifts in excess of 10 hours for more than three nights per week.

Anne Grundy commented:

“Women who were still working nights at the time of the study had a breast cancer risk that was 26-per-cent higher than those who had stopped working at night at least two years previously.”

Risk associated with a melatonin deficiency?


The analysis of the five surveys did not specifically consider respondents’ job type (night or otherwise), although the Canadian survey, done in 2013, did compare women working in healthcare and other sectors.

“The risk associated with breast cancer and night work varied little among respondents, regardless of job type,” said Grundy. “Although we are not fully certain, we believe that this risk could be related to the hypothesis that night work disrupts circadian rhythm and inhibits the secretion of melatonin, which may protect against cancer.”

Anne Grundy now hopes to explore the effect of shift work on women's risk of breast cancer. “We need to go further in our research so that labour policies ultimately take into account this risk for women, and so that companies take preventive action and adjust work schedules.” she said.

The findings must now be taken seriously by employers, especially in the call centre industry where shift working up until 10pm and later is common. Previously employers have ignored warnings from Trade Unions and researchers that there i s a risk to the health of their workforce and greater costs for the business if remedial action isn't taken to minimise the risk to their workers.

Warehouses such as those run by Amazon and Sports Direct are also on the list of employers who run their operations based on shift work and often have 24 hour shift working as the basis of their business model.

In the NHS too with it's need for 24 hour nursing and doctors on call, and ambulance crew. The list of industries and services and public bodies which employ workers throughout the night will continue to grow as the world of work changes to more increasingly round the clock, 365 days per year attendance requirements.

Meanwhile, the abstract of the reserarch document states:

Night shift work has been suspected to increase breast cancer risk but epidemiological studies have been inconsistent due to heterogeneous assessment of exposure to night work.

To overcome this limitation, we pooled data of five population-based case–control studies from Australia, Canada, France, Germany, and Spain into a single harmonized dataset using a common definition of night work including 6093 breast cancer cases and 6933 population controls.

The odds ratio for breast cancer in women who ever worked at night for at least 3 h between midnight and 5 a.m. as compared to never night workers was 1.12 (95% CI 1.00–1.25). Among pre-menopausal women, this odds ratio was 1.26 [1.06–1.51], increasing to 1.36 [1.07–1.74] for night shifts ≥ 10 h, 1.80 [1.20–2.71] for work ≥ 3 nights/week, and 2.55 [1.03–6.30] for both duration of night work ≥ 10 years and exposure intensity ≥ 3 nights/week. Breast cancer risk in pre-menopausal women was higher in current or recent night workers (OR = 1.41 [1.06–1.88]) than in those who had stopped night work more than 2 years ago.

Breast cancer in post-menopausal women was not associated with night work whatever the exposure metric.

The increase in risk was restricted to ER+ tumors, particularly those who were both ER+ and HER2+ . These results support the hypothesis that night shift work increases the risk of breast cancer in pre-menopausal women, particularly those with high intensity and long duration of exposure. Risk difference between pre- and post-menopausal women deserves further scrutiny.

Source:  University of Montreal / European Journal of Epidemiology

See also: E-Library Database, category: Cancer (work related)

 


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