An international team of 24 scientists convened in Lyon, France, at the International Agency for Research on Cancer (IARC), a branch of WHO, to study the relationship between cancer and shift work and the firefighting and painting occupations.

The research team assessed animal experiments and epidemiologic studies to find that working the night shift for an extended period of time may increase the risk of breast and colon cancer in women and prostate cancer in men.

Dr. Erhard Haus of the HealthPartners Research Foundation chaired one of the study’s subgroups and explained that disruption to regular sleep rhythms may impede production of melatonin, a hormone believed to inhibit the cell damage that causes cancer.

“Shift work that interferes with regular nighttime sleep disrupts circadian rhythms, our body’s natural clock,” Haus said. “This impedes biologic function by suppressing the immune system, reducing melatonin production and may damage genes leading to the production of abnormal cells.”

In recent years, several emerging studies have linked graveyard shift work with increased risk of cancer, but no definite conclusions have been made. The new WHO study conflicts with a recent epidemiological study of Swedish workers that argued other factors besides shift times may responsible for increased cancer risk.

According to HealthPartners Research Foundation, 15 to 20 percent of the U.S. and European working population are employed in shift-work jobs, particularly within the health care, communication, hospitality, leisure and transportation industries.

In addition to their findings in shift work, scientists classified the on-the-job chemical exposure painters experience in their work as carcinogenic. The researchers also determined that the occupational exposure for firefighters, who often work night shifts and are also exposed to chemicals, smoke and dust, was “possibly carcinogenic.”

A report on the IARC study will be published in the Dec. 7, 2007 issue of Lancet Oncology.

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