Cancer All-Clear For Night Shift Work Based On Bad Science

By Rory O’Neill, Hazards Magazine editor

A recent Oxford University study concluding that night shift work should no longer be classified as a cause of breast cancer was based on ‘bad science’, top researchers warn.

The large scale ‘meta-analysis using data from 1.4 million women, published online on October 6th, 2016 in the Journal of the National Cancer Institute (JNCI), concluded that “night shift work (including long-term shift work) has little or no effect on breast cancer incidence.” It added, the International Agency for Research on Cancer’s classification of night work as a ‘probable’ cause of breast cancer in women “is no longer justified

But three of the most respected epidemiologists on night shift work and breast cancer, whose work over decades included several purpose-designed studies on night work and cancer risks, say they disagree” with that conclusion, noting a succession of methodological flaws.

Harvard Medical School epidemiologist Eva Schernhammer told Hazards Magazine that given the Oxford study’’s “bad science”, it was “not surprising” that the researchers found no effect. In a detailed criticism of the paper (published online Dec. 15, 2016), she said that the study’s many shortcomings “preclude it from the conclusion of no association between night shift work and breast cancer risk.” Johnni Hansen, a researcher with the Danish Cancer Society, was equally unimpressed. “”They base their conclusion on a poor study, but even worse is that their conclusion may hinder preventive initiatives for night workers,”” he said.

Methodological Flaws

Hansen said that the main cohorts in the Oxford study, which was financed by the Medical Research Council, the Health and Safety Executive, and Cancer Research UK, were “”worryingly old” with many over retirement age. The risk of women developing breast cancer appears to wane in the years after night work ends, so studying retired workers with no recent exposures “misses the point and the cancers,” said Schernhammer. The higher breast cancer risk is seen in women with long exposures,­ at least 15 years, ­ early in their careers. The researchers should have recognized the possibility of such ‘truncation bias’ (i.e., when the passage of time “attenuates or fully eliminates” previously observed risks), added Hansen.


PROBLEM The Oxford University study giving night work a cancer all-clear included many retired workers who’d not worked nights for years. But you need to look at younger workers with a recent long-term history of night work if you want to spot raised breast cancer rates.

Classifying participants in terms of whether they ‘never’ or ‘ever’ worked nights, also meant that a single night shift could have placed a worker in the ‘exposed’ group even though this level of exposure was obviously minimal. “You tend to need longer exposures to show an effect on risk,” said Schernhammer.

Richard Stevens, of the University of Connecticut medical school, who has written influential papers on the topic with both Schernhammer and Hansen, also had concerns that the JNCI meta-analysis ““excluded case-control studies, of which there are many, for no good reason.”” According to Stevens, case-control studies that explore the biological mechanisms behind a possible association give valuable insight into “why and where you might look for an association.” Understanding this process is important.

Stevens, Schernhammer and Hansen, together with Scott Davis, a professor of epidemiology in the University of Washington’s School of Public Health, are the stand-out epidemiologists on night work and breast cancer. Yet not one of them was asked to review the paper. “”We are the four epidemiologists who have been working for by far the longest on the epidemiology of night work and breast cancer,”” said Stevens. ““Any of the four of us would have quickly noticed the severe flaws of the paper and pointed them out to the editors.””

Stevens said it was “absurd” that the night work association with breast cancer was being dismissed on the back of a “troubling” paper by “a distinguished group of experienced researchers who should have known better.” ““Why was the paper written in the first place?”” he asked bluntly. “And why was it published in a high visibility journal like JNCI?”

The Oxford study’’s lead author, Ruth Travis, declined an invitation from Hazards Magazine to address the detailed criticisms of the study.

The International Agency for Research on Cancer is standing by its ranking of night work as a ‘probable’ cause of female breast cancer.

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