Working nights cited as 'probable' cancer risk

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Working nights cited as 'probable' cancer risk
MARTYN MCLAUGHLIN

NIGHT shift workers may face an increased risk of cancer, say researchers.

The World Health Organisation is to classify shift work as a "probable" carcinogen, suggesting it poses a similar risk to agents such as anabolic steroids, ultraviolet radiation and diesel engine exhaust.

The theory could have ramifications for millions of people, especially in the developed world, where one person in five works nights.

It would also mark an about-turn for the scientific community, which has previously discarded such notions.

Richard Stevens, a cancer epidemiologist and professor at the University of Connecticut Health Centre, admitted scientists had traditionally dismissed any link between night work and cancer as "wacky".

Two decades ago, he published a paper that suggested a link between exposure to light at night and the development of breast cancer. At the time he was trying to establish why breast cancer incidence suddenly shot up from the 1930s in industrialised societies, where night work was considered a mark of progress. Most scientists were bewildered by his proposal.

But in recent years, several studies have found that women working nights for many years are indeed more prone to breast cancer, and that animals which have their light-dark schedules switched grow more cancerous tumours and die quicker.

Some research has also shown that men working at night may have an increased rate of prostate cancer.

Because these studies have been carried out mainly among nurses and airline crews, larger studies in more varied populations are needed to confirm or disprove the findings.

The idea that shift work might increase cancer risk is still viewed with scepticism by some, but many doubters will likely be won over when the WHO's cancer arm, the International Agency for Research on Cancer (IARC), publishes the results of its analysis in the December issue of the Lancet Oncology.

The American Cancer Society said it was likely to add shift work to its list of "known and probable carcinogens" when the IARC makes its reclassification. Until now, the society has labelled it an "uncertain, controversial or unproven effect".

Vincent Coglianom, director of the monographs programme at the IARC, which decides on carcinogen classifications, said: "There was enough of a pattern in people who do shift work to recognise that there's an increase in cancer, but we can't rule out other factors."

Scientists suspect shift work disrupts the circadian rhythm, the body's biological clock. The hormone melatonin, which can suppress tumour development, is normally produced at night, so working in artificial light at night may reduce melatonin levels, possibly raising the chance of developing cancer.

Sleep deprivation, which leaves the immune system vulnerable, may also be a factor.

[HunterMan]

I heard starlight gives you cancer