Working the nightshift may cause cancer, say WHO researchers

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Women who do night work for long periods face a higher risk of breast cancer compared to counterparts who work only in the daytime, according to a World Health Organisation (WHO) cancer report released on Friday.
In an overview of work-related cancer risk, researchers say that "circadian disruption" is "probably carcinogenic" while painters and firefighters are also exposed to carcinogenic hazard through their work.
The report is issued by a working group of the International Agency for Research on Cancer (IARC), a unit of the World Health Organisation (WHO).
Their assessment, based on a review of published epidemiological studies, will appear in the December issue of The Lancet, the IARC said in a press statement.
IARC said that night work is "probably carcinogenic," a term that means there is limited evidence of cancers in humans but sufficient evidence of cancers in lab animals.
The data in this field is based on incidence of breast cancers among nurses and flight attendants, backed by animal studies which show that constant light, dim light at night or simulated jet lag substantially boost tumour development.
One of the authors, Vincent Cogliano, said that the studies among the working women showed that shiftwork posed a less than twofold increase in risk.
"It is a real risk but one that epidemiologists would describe as a modest one," he told AFP.
Cogliano added that the risk for other professions and for other cancers was unclear but noted that nearly 20 percent of the working population in Europe and North America was engaged in shiftwork of some kind.
One possible cause for the increased risk is that the circadian system is disrupted by exposure to light at night.
This alters sleep activity patterns, suppresses production of the hormone melatonin and disregulates genes that control tumour development.
Among painters, occupational exposure to chemical solvents, pigments, additives and fine dust or asbestos is linked to "small but significant" increases in the risk of lung cancer and bladder cancer, the IARC said.
For firefighters, occupational exposure from chemicals, smoke and particles is considered "possibly carcinogenic," meaning that there is limited evidence of carcinogenicity in humans and less than sufficient evidence of this in lab animals.
The paper is published under the IARC's "Monographs Programme," in which the agency identifies environmental factors that boost the risk of cancer, thus providing policymakers and regulators with scientific evidence to support decisions on health and safety.