Oct. 6, 2004 -- The search for nonsmoking-related causes of bladder cancer has led to the identification of three new chemical compounds associated with increased risk. Though earlier work implicated hair dyes as likely cancer promoters, researchers say it is not yet clear how people become exposed to these compounds.
The compounds are chemical cousins of the known cancer-causing substance 4-ABP, which was shown in a recent study to be a contaminant in many commercial hair dyes. But hair dyes have not been tested for the related compounds, known together as arylamines, a study researcher tells WebMD.
Most experts now believe that about half of all bladder cancers are caused by cigarette smoking, but the evidence for other environmental causes, including hair dyes, remains unproven.
"Exposure to the compounds like the ones we identified in this paper may well account for the remainder of the risk that is not attributable to cigarette smoking," says Paul L. Skipper, PhD of the Massachusetts Institute of Technology. "But I don't want to put the spotlight on [a single potential source] as the problem."
Hair Dyes and Other Possible Sources: 'Scientific Priority'
In earlier work, study co-researcher Manuela Gago-Dominguez, MD, and colleagues of the University of Southern California found 4-ABP levels to be higher in nonsmoking bladder cancer patients than in nonsmokers without cancer.
In this study, they measured levels of nine arylamines in roughly 300 bladder cancer patients and a similar number of healthy volunteers without bladder cancer.
Levels of all nine arylamines were higher in the cancer patients than in the healthy volunteers. High levels of three of the arylamines were all independently linked to an increased risk of bladder cancer.
"Results of (these studies) thus implicate exposure to arylamines as the causal factor responsible for most cases of bladder cancer in humans," the researchers wrote. "Tobacco smoke as a source of these carcinogenic arylamines is already well known. Therefore, identifying the non-smoking-related sources of these carcinogenic arylamines should become a high scientific priority."
Studies on Hair Dyes Ongoing
Although hair dyes are at the top of the list of potential sources, two scientists at the forefront of research on the subject tell WebMD that the evidence favoring a connection is inconclusive at best. The two will meet next week in Baltimore at a workshop sponsored by hair dye industry leaders to discuss the direction of future studies.
Early this year, Yale researcher Tongzhang Zheng, ScD, reported that long-term use of dark shades of permanent hair dye can double the risk of non-Hodgkin's lymphoma. Zheng tells WebMD that his findings suggest but do not prove that hair dyes actually cause cancer.
A big unanswered question, he says, is whether the hair dye formulations used today pose the same risk as formulations used several decades ago.
"Hair dye companies have done a lot over the past 25 years to change these products to address concerns about safety," he says.
Johns Hopkins professor of epidemiology Kathy Helzlsouer, MD, who has reviewed the clinical studies on hair dye use and cancer, says the best clinical evidence suggests no increase in breast cancer and only a small increase in blood cancers in hair dye users. The bladder cancer risk associated with hair dye use is not clear, she says.
One problem with assessing hair dye risk is that bladder cancer is relatively rare among women and women are the primary users of hair dyes. Just 15,000 women in the U.S. are diagnosed with the disease each year, compared with 38,000 men.
Helzlsouer says several studies that are under way may clear up the confusion about the role of hair dyes in cancer.
"Hopefully this question will be settled soon, and we can tell women what the risks are, if any," she says.