Be careful of studies that give you false results because the authors may not be using statistical theory correctly. A group of researchers at the University of Southern California reported that women who use permanent hair dyes once a month for one yeardouble their risk for bladder cancer and that women who get their hair dyed once a week for 15 years triple their chances for getting bladder cancer.

Hair dyes contain arylamines that cause cancer in animals, but these chemicals do so in high concentrations, not in doses that you get in a beauty parlor where small amounts applied to the scalp are absorbed poorly through the skin. Customers do not breathe in much arylamines in their visits to a beauty parlor.

Let's analyze the statistics. There are 50,000 cases of bladder cancers in the United States each year. Men are four times more likely to get bladder cancer than women. So there are 10,000 cases of bladder cancer per year in 140 million women. That's one in 14,000 women. Doubling the incidence of bladder cancer in women to one in 7 thousand is not statistically significant because two times almost zero is still almost zero. If you do these figures for tripling the incidence of bladder cancer in 15 years, the chances of bladder cancer coming from hair dye is so close to zero that the researchers reached the wrong conclusions from their study. Their study shows that there is no increased risk for bladder cancer in women who get their hair dyed once a month. See why you can't believe every study you hear and why your newspaper reporter should have taken math when he was in college.

International Journal of Cancer, February 2001

Reported 2/15/01; checked 9/3/05

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