Shift work may cause cancer, world agency says

WASHINGTON (Reuters) - Shift workers and firefighters have a higher risk of cancer than the general population and such work should be classified as probably or possibly carcinogenic, the International Agency for Research on Cancer said on Friday.

A team of 24 scientists who sifted through the evidence said more studies must confirm the link, but found that shift work that disturbs the body's internal clock appears to have cancer-causing effects, too.

This internal clock regulates circadian rhythms, a complex system that signals cells to produce various hormones at various times.

"Shiftwork that involves circadian disruption is probably carcinogenic to humans," the French-based IARC, the cancer agency of the World Health Organization, said in a statement. "Occupational exposure as a firefighter is possibly carcinogenic to humans," it added.

The statement, published as what the IARC calls a monograph, could affect a significant number of people.

"Nearly 20 percent of the working population in Europe and North America is engaged in shiftwork. Shiftwork is most prevalent in the health-care, industrial, transportation, communications, and hospitality sectors," the IARC said.

But the IARC's Vincent Cogliano said the evidence was not yet clear enough for anyone to take any action.

"I don't know if this is ready for an employer yet because I don't think we understand fully what it is about shift work that might be causing cancer," Cogliano said in a telephone interview.

First, he said, more study was needed. "Then we would like the national health agencies to look at it and see what kind of action is appropriate."

GETTING MORE EVIDENCE

Cogliano said this was the first time the IARC had examined shift work as a possible cause of cancer, and said the agency would return to the issue in perhaps five years, when more research had been done.

The monograph will be published in the December issue of The Lancet Oncology medical journal, but the conclusions are based on years of published research.

In 2001, a team at the Fred Hutchison Cancer Research Center in Seattle found that women who work night shifts may have a 60 percent greater risk of breast cancer.

Several tests in mice show that circadian clock genes are disrupted in tumor cells.

Other studies provide evidence that firefighters, who breathe in smoke, chemicals and dust and who also work shifts, have a higher risk of cancer and heart disease.

The shift work findings may all have to do with the body's response to light.

The brain's pineal gland produces the hormone melatonin after the body is exposed to either sunlight or artificial light and then darkness, and production is disrupted when people are up at night with the lights on.

Melatonin also acts as an antioxidant protecting DNA from the type of damage that leads to cancer and heart disease.

"Melatonin does a lot about regulating the body's cycle. But I don't think we know how to tinker with the melatonin system ... yet," Cogliano said.

"We are always going to have night workers and shift workers. Some jobs must be done around the clock like nurses. We need to know how to reduce the risk."

Other experts have pointed out that shift workers may have other behaviors that raise cancer risk, such as a higher tendency to drink alcohol or to smoke, or get less sleep.

(Editing by Will Dunham and Eric Beech)