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       Hazards, number 150, 2020
IN THE DARK | HSE refuses to see the light on night work and cancer risks  
When a Health and Safety Executive (HSE) backed study concluded night work should lose its official label as a ‘probable’ cause of best cancer, the regulator said the findings were ‘vital’. It was also wrong, explains Hazards editor Rory O’Neill, with a new review by the UN’s cancer agency concluding the association also holds for other cancers.


The Health and Safety Executive-funded Oxford University study published in October 2016 in the prestigious Journal of the National Cancer Institute could not have been clearer. The paper concluded the classification of night work as a cause of breast cancer ‘is no longer justified’.

The decision in 2007 by the World Health Organisation’s International Agency for Research on Cancer (IARC) to rank night work as a ‘probable’ cause of breast cancer in women, it turned out was “necessarily based on limited epidemiological evidence” but had got it wrong.  The good news attracted global coverage, with a Daily Mail headline proclaiming ‘Working night shifts does NOT raise the risk of breast cancer.’

Andrew Curran, HSE’s chief scientific adviser, said: “Breast cancer is the most common cancer in women so it was vital for us to fund work in this area to establish if there is a link to night work.” He added: “This study has shown that night shift work, including long-term shift work, has little or no effect on breast cancer incidence in women.”

Cancer Research UK (CRUK), which co-financed the study, said it hoped the conclusion “reassures women who work night shifts.”

Big problem

It was a big deal. CRUK’s figures show each year in the UK around 50,000 women are diagnosed with breast cancer and around over 11,000 die. And an HSE study in 2012 concluded each year there are 555 deaths and 1,969 new cases of breast cancer in Great Britain attributable to shift work. If any association was real, then the numbers affected would be rising.

Night working is now at the highest level since current records began. A TUC analysis of Labour Force Survey figures for 2019 indicated 3.25 million people – more than 1 in 9 workers – work in Britain’s night-time economy.

The HSE-backed study prompted IARC to revisit the evidence that led it to the give night work a Group 2A ranking as a ‘probable’ cause of cancer in humans. The UN agency convened a ‘Monographs Working Group’ of experts in June 2019, which included the CRUK-funded scientist Ruth Travis, lead author of the study also backed by HSE.

NIGHT FRIGHT  As night work soars to record levels, more workers are at risk of more related cancers. Around 3.25 million people – more than 1 in 9 workers – work in Britain’s night-time economy.

The findings of the reevaluation, published in an online Monograph in June 2020, established IARC had been right all along. The Oxford University study – which Hazards had criticised as ‘bad science’ (Hazards 136) – had been a dangerous diversion.

The IARC expert group again “classified night shift work as ‘probably carcinogenic to humans’ (Group 2A), on the basis of limited evidence of cancer in humans (for cancers of the breast, prostate, colon, and rectum), sufficient evidence of cancer in experimental animals, and strong mechanistic evidence in experimental animals.”

Expert analysis

Eva Schernhammer, a Harvard Medical School researcher who was also part of the IARC expert group reevaluating the ranking and who had been strongly critical of the Oxford study, told Hazards: “In my view it was a strong (stronger than in 2007) 2A classification, given the evidence from between 2007 and 2019, which added a few additional cancers to the list; as well as the likelihood of bias towards the null primarily – but not exclusively – due to exposure misclassification in several studies, which was identified and put into perspective during the review.”

In essence, the IARC review corrected a situation where too much weight was given to studies where the design or execution had led to an under-estimation of cancer risks.
Schernhammer felt the evidence was stronger than presented in the final monograph, as “several of the referees including myself were frustrated by the seeming lack of proper consideration of the circadian timing system/circadian disruption in the mechanistic group, which may have contributed to a class 2A rather than class 1 assessment.”

A Group 1 IARC classification means there is no room for doubt.

Danish Cancer Society researcher Johnni Hansen, who was also on the IARC expert group, shared Schernhammer’s frustration.

“To my (and others) opinion the evidence concerning night work (that involves circadian disruption) has been strengthened by the 2019 IARC evaluation, and confirmed that the 2007 evaluation was not wrong as postulated by some few researchers, who have contributed to confusion about this topic and probably confused the needs for preventive efforts,” he said.

Seeing the light?

The rating reasserted that night work is a probable cause of breast cancer, with the evidence now supporting the same association with breast, prostate, colon and rectal cancers.

MEN AT WORK  Three cancers that affect men – prostate, colon and rectal cancers – have been recognised by IARC as ‘probably’ related to night work.

That, however, was not the whole story. Unions share Hansen’s concern that the doubt cast on the cancer association by the HSE/CRUK backed paper adversely affected efforts to recognise, address and compensate night work related breast cancer.

Both HSE and CRUK had to make good.  Only it turned out neither organisation was willing to budge.

CRUK’s Amy Hirst told Hazards the IARC review involving top cancer scientists from round the world was “interesting,” but added “there haven’t been any major changes to what we know since IARC’s last review. The evidence on the link between cancer and night shift work is still inconsistent and unclear and the most convincing findings linking the two are in animals. More research is needed before we can say for sure whether it affects people’s cancer risk.

“But there are lots of proven ways to reduce your risk of prostate, colorectal and breast cancer. Our advice to night shift workers would be the same for anyone else – focus on the things that we know can give you the greatest chance of lowering your cancer risk.

“It might feel more difficult to prioritise staying healthy when you’re working shifts but stopping smoking, cutting down on alcohol, and keeping a healthy weight would all make a big difference.”

Hansen was dismayed. He described Hirst’s comments as “arrogant and conservative, scientifically not correct, and in contradiction of international expert evaluations,” including both IARC reviews.

In addition to the IARC monograph, he pointed to a 2018 report from the US government’s Office of the Report on Carcinogens (ROC/NTP) which concluded different night work circumstances were either ‘known’ or could be ‘reasonably anticipated’ to cause cancer in humans.

Schernhammer said the CRUK comments were in “direct contravention” of the facts. While the CRUK-backed study noted “the prospective evidence now available shows that classification of night shift work as a probable human (breast) carcinogen is no longer justified,” this was explicitly undone by the IARC expert group’s evaluation which reviewed the totality of the evidence to date and concluded: “In sum, the Working Group classified night shift work in Group 2A, ‘probably carcinogenic to humans’.”

Other long-time workplace cancer and environmental prevention advocates also expressed dismay at CRUK’s line.

Professor Andrew Watterson, who heads the occupational and environmental health research group at the University of Stirling in Scotland, commented: “It is deeply disappointing that bodies like CRUK, who appear to be in long term denial about a whole range of occupational and environmental carcinogens listed by authoritative international agencies, have again produced their almost Pavlovian response – namely that night shift workers should forget about these occupational risks and watch their weight, alcohol consumption and smoking. It verges on the insulting and offensive.”

Helen Lynn, who coordinates the Alliance for Cancer Prevention, responded: “CRUK has not changed its line in decades – standing still while science races ahead leaving them in the dust trail. I wonder what they would say was at stake if they finally acknowledged that a large percentage of cancers that are outside individual control are caused by environmental and occupational risk factors? What will they stand to lose? Their blame the patient approach has led to measurable suffering and death.”

HSE ‘nonsense’

Like CRUK, HSE was unmoved by the evaluation by the global panel of experts.
“HSE’s position on this matter remains unchanged,” HSE told Hazards. “The IARC review highlighted the challenges in measuring the potential effects of night shift work whilst taking into account other cancer risk factors when performing research studies, and that large-scale studies may not necessarily be able to provide the appropriate evidence to clarify if there is a causal link between night shift work and cancer. We will continue to monitor emerging evidence in this area.”

HSE said a review by its Workplace Health Expert Committee (WHEC), which reported in 2018, “concluded the causal relationship is ‘possible/uncertain’, which is broadly consistent with the conclusion of the IARC re-evaluation in 2019.”

Schernhammer described the HSE comments as “pretty stunning” and “difficult for me to reconcile.” She added that HSE’s comments on the research challenges were “such nonsense!”

“It was the study that they funded, that should not have been conducted at the first place given the limited information it allowed to draw – underpowered, too short follow-up, women long out of night work given their age. But clearly there are other large-scale studies which very much were able to 1) take into account other cancer risk factors; and 2) were very much able to provide the appropriate evidence.”

While HSE monitors the evidence, an increasing proportion of the workforce is involved in night work a top global panel of experts says we should be addressing as a probable cause of preventable breast cancers. These workers have UN-endorsed grounds to worry about night work-related prostate, colon and rectal cancers too.

Breast cancer is overwhelmingly a disease of women workers; but prostate is a male cancer and colon and rectal cancer can affect anyone. As HSE sits on the sidelines, the known at-risk group of night workers it is ignoring has just increased by at least 2 million and could equate to several hundred extra work-related cancers each year.


Graveyard shift: Cancer all-clear for night work based on ‘bad science’, warn scientists, Hazards magazine, number 136, December 2016.
IARC Monographs Volume 124: Night Shift Work, June 2020. Volume 124 webpage
Draft RoC Monograph on Night Shift Work and Light at Night, ROC/NTP, 24 August 2018.
Shift work and breast cancer, WHEC, 2018.



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When a Health and Safety Executive (HSE) backed study concluded night work should lose its official label as a ‘probable’ cause of best cancer, the regulator said the findings were ‘vital’. It was also wrong, explains Hazards editor Rory O’Neill, with a new review by the UN’s cancer agency concluding the association also holds for other cancers.

Big problem
Expert analysis
Seeing the light?
HSE ‘nonsense’

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