Last reviewed 16 June 2020

A prestigious agency of the World Health Organization (WHO) has, once again, come out and classified night shift work as “probably” carcinogenic to humans, after a great deal of heated debate on the subject in recent years.

In 2007, the WHO agency — the International Agency for Research on Cancer (IARC) — originally classified the disruption of the body clock in shift work as a probable cause of cancer.

However, subsequently, this classification was challenged in 2016 after an Oxford University study, published in the Journal of the National Cancer Institute, and co-financed by the Health and Safety Executive (HSE) and Cancer Research UK (CRUK), concluded that “night shift work, including long-term night shift work, has little or no effect on breast cancer incidence”.

The 2016 study attracted headlines around the world, with its findings welcomed by both HSE and CRUK, implying as it did that IARC’s ranking of night work as a probable cause of breast cancer in women was no longer justified.

The study did however attract criticism from some cancer experts as well as concerns from trade unions regarding the swift about-turn in the science.

The controversy led IARC to convene a “Monographs Working Group” in June 2019, and a year later, in June 2020, the findings of IARC’s re-evaluation have been published online.

The findings essentially reject the HSE/CRUK study’s conclusions.

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

In response, a TUC source expressed concerns 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.”

The IARC Monographs Volume 124: Shift Work can be accessed via the IARC’s website.