The IARC has labeled these occupations as "probably carcinogenic to humans," a classification the agency reserves for those exposures backed by fairly strong evidence. In 1993, the IARC found that hairdressers and barbers were probably exposed to cancer-causing substances, but at that time, evidence of an increased cancer risk in this population was "inadequate." This week's report, published in the Lancet Oncology, is based on a review of epidemiological studies published since that time.

Some of the products used by hairdressers and barbers--such as dyes, pigments, rubber chemicals, and curing agents—have been found to cause tumors in rats in laboratory studies or have been known to cause bladder cancer in humans. In some studies, increased risk has been associated with permanent dyes and use of darker-colored hair dyes.

IARC researchers found a small, but consistent, risk of bladder cancer among male hairdressers and barbers. However, according to the American Cancer Society's Michael Thun, MD, vice president, epidemiology and surveillance research, "it was unclear whether that was caused by past exposure to chemicals that are no longer used or continuing exposure to ingredients in contemporary products." Some coloring agents were discontinued in the 1970s because lab studies revealed they had cancer-causing properties.

"IARC classifications are particularly important for regulatory agencies that have direct responsibility for workplace safety," says Dr. Thun. "Workers can reduce skin exposure to these products by wearing gloves."

Some studies have looked at whether personal use of hair dyes is associated with an increased risk of bladder cancer, leukemia, multiple myeloma, Hodgkin disease, non-Hodgkin lymphoma, lung cancer, breast cancer, oral cancer, and cervical cancer, but according to the IARC, the evidence for a link thus far is "inadequate."

The IARC's findings on this topic will be published as Volume 99 of the IARC Monographs. For more information on this topic, see the American Cancer Society documents, Known and Probable Carcinogens and Hair Dyes.