In June 2019 a working group of 27 scientists from 16 countries met at the International Agency for Research on Cancer (IARC) in Lyon, France to finalise their evaluation of the carcinogenicity of night shift work. This evaluation will be published in volume 124 of the IARC Monographs and an initial summary of their work was published in The Lancet in July 2019.
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The IARC had previously examined night shift work as part of a study on shift work and had determined it to be ‘probably carcinogenic to humans’ (Group 2A). The main reason for this is the disruption to circadian rhythms, which act as a type of biological clock. These rhythms determine the variations over a 24-hour period of a whole range of activities, such as waking/sleeping, blood circulation, urine production and bowel movements, and hormone production levels. Certain cancers, such as breast and prostate cancer, are ‘hormone-dependent’, i.e. their development can be caused or facilitated by disruptions in hormone production.
The updated 2019 evaluation is based on the publication of numerous studies since 2007 that have refined knowledge of the subject. The working group chose the term ‘night shift work’ to better describe the exposure circumstances and to reflect the main evidence base for the human cancer studies.
It is estimated that around 20 % of workers around the world do some form of night work, and it is particularly common in some sectors, such as health care, transport, industrial activities and trade logistics.
The group concluded that night shift work was linked to breast cancer, based in particular on an important study on the health of nurses, the ‘Nurses’ Health Study II’. Other studies revealed a similar association for prostate and colorectal cancer. The classification of night work as ‘probably carcinogenic’ is based on limited evidence of cancer in human studies, sufficient evidence in experimental animal studies and strong mechanistic evidence in experimental animals.
The IARC is an agency of the World Health Organization. One of its main activities is to identify carcinogenic agents and classify them. This work is published in monographs. The IARC bases its findings on published scientific studies, yet most specialist regulatory bodies are prepared to use data that are provided by industry players and that have not been the subject of publications, which would ensure closer monitoring of the methods used. That is why the IARC is regularly attacked by the chemical industry. In particular, its classification of glyphosate as ‘probably carcinogenic to humans’ (2A) led to a campaign by the American Chemistry Council and threats to cut all United States funding for the activities of this international agency. A similar campaign against the IARC was launched at the start of the 2000s when it classified second-hand smoke as ‘carcinogenic to humans’ (Group 1).
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