This article was originally posted in the Montreal Gazette.
There has been a lot of media coverage lately about chemical hair straighteners and uterine cancer, with some headlines claiming that these hair products increased the risk by 80 per cent while some claimed the risk more than doubled. While these numbers seem shocking and worrisome, we need to remember an important mathematical point. Differences can seem relatively large when expressed as percentages while being actually quite small in the absolute sense.
Headlines like this are always worrisome, but the fact that this story emerged right after the recall of some dry shampoos raised a lot of questions about the safety of hair products. It is worth pointing out that the two issues were unrelated. The recall had to do with a contaminant found in the affected products. There was no implication the shampoos themselves were dangerous.
Still, the timing probably cemented the possibility of risk in the minds of many. People still often ask about the safety of hair dyes because of the mistaken belief that they are labelled as carcinogens by the World Health Organization. In fact, the WHO designated hair dyes as Category 3 or “unclassifiable,” given the lack of evidence with regard to their potential harm from personal use. It’s also important to point out that the question was regarding bladder cancer, not uterine cancer, which further limits its applicability to this case.
But even on its own, the current analysis linking hair straighteners to uterine cancer is worrisome. The study was an analysis of the Sister Study, which was first designed to look at risk factors of breast cancer. Researchers used questionnaires to record how often the women in the study used various hair products and then recorded who among them developed cancer over the next few years. Their analysis, after excluding women with a history of uterine cancer, found the 80-per-cent increased risk that was widely reported. But what is interesting is how many of the products they studied show no increased risk at all. Hair dyes, for example, showed no increased risk of uterine cancer. Bleach similarly had no association. Neither did body waves or perms, and women who got highlights actually had a slightly lower risk. The only signal for increased risk was in women who used straighteners.
However, even this association came with some caveats. The signal for increased risk only manifested itself in women who used straighteners, relaxers or pressing products more than four times per year, not those who only used them occasionally. Also, that risk was mitigated in women who exercised regularly. This could suggest that the reason we saw more uterine cancer was because of baseline differences between the groups of women involved and that much of the risk of uterine cancer could be mitigated by general lifestyle factors.
But one of the most important caveats relates to absolute risk. On the relative scale, a near doubling of cancer risk sounds worrisome. But the risk of uterine cancer throughout this study was fairly low. Women who never used straighteners had a roughly one per cent chance of developing uterine cancer over the study period (about six years on average), while those who only used the products occasionally had a risk slightly below one per cent, whereas frequent users had a risk of 1.65 per cent at the six year mark. When you extrapolate out, the researchers predicted that number for frequent users could rise to 2.41 per cent by age 70.
So, while the relative risk of a two-fold increase seems dangerous, the absolute risk of around a one percentage point difference is probably less so. Put that way, the risk posed by chemical hair straighteners is probably pretty small, especially for women who only use them occasionally. Of course, there’s even less risk for those who prefer the hair they were born with.