Night shift work ‘probably’ causes cancer in humans

The available evidence suggests working night shifts “probably” causes cancer in humans, a group of top experts has concluded.

In June 2019, the working group of 27 scientists from 16 countries met at the World Health Organisation’s International Agency for Research on Cancer (IARC) in Lyon, France, to finalise the evaluation of the carcinogenicity of night shift work. The working group included scientists whose work had previously reached divergent conclusions, some sceptical of a link between night shifts and breast cancer and some strongly supportive of an association.

The group assessment will now become the official IARC cancer rating for night shift work, and will be published in volume 124 of the IARC Monographs.

Writing in the Lancet Oncology in July 2019, the authors note: “Overall, the Working Group concluded that a positive association has been observed regarding night shift work and breast cancer; however, given the variability in findings between studies, bias could not be excluded as an explanation with reasonable confidence.”

They also noted studies “provide some evidence that night shiftwork is positively associated with risk of prostate and colorectal cancer; however, because the studies were few in number and the results lacked consistency, chance and bias could not be ruled out.”

The authors conclude: “In sum, the Working Group classified night shift work in Group 2A, ‘probably carcinogenic to humans’, based on limited evidence of cancer in humans, sufficient evidence of cancer in experimental animals, and strong mechanistic evidence in experimental animals.”

In a 24-7 economy, night work could be a growing threat to health. A TUC analysis of official figures last year indicated the number of people working night shifts in the UK has increased by more than 150,000 over the past five years. The union body said the number working nights now stands at more than 3 million workers – or one in nine of the total UK workforce.

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