Hair Relaxer Use and Risk of Uterine Leiomyomata in African-American Women

Lauren A. Wise; Julie R. Palmer; David Reich; Yvette C. Cozier; Lynn Rosenberg

Disclosures

Am J Epidemiol. 2012;175(5):432-440. 

In This Article

Discussion

In this large population of premenopausal US black women, we observed increased risks of uterine leiomyomata in association with ever use of hair relaxers, duration of use, frequency of use, and total number of burns experienced during use. A composite variable for frequency and duration of use showed evidence of a dose-response relation. Positive associations were also observed between frequency of hair relaxer use and risk of uterine leiomyomata among the long-term users (i.e., duration of use ≥ 10 years). We found no association of uterine leiomyomata with age at first use or type of formulation used. Although stronger associations were seen for leaner women and women living in the South, there was no evidence of statistical interaction by body mass index or region. The stronger results for surgically confirmed uterine leiomyomata cases may be explained by the higher rates of uterine leiomyomata-related hysterectomy in the South,[50,51] the region in which the prevalence of hair relaxer use was greatest. Adjustment for genetic admixture (i.e., percentage of European ancestry) among the subset of women with genotyping data showed appreciable attenuation of effect estimates, but genetic admixture did not fully explain the positive associations.

Systematic bias in the reporting of hair relaxer use by case status is unlikely because reports of hair relaxer use were ascertained prior to the diagnosis of uterine leiomyomata. Any random misclassification of hair relaxer use would have diluted associations for the extreme categories of exposure. Although we assessed history of hair relaxer use at a single point in time only (i.e., in 1997), results were similar across the different time periods of study observation. Retention of the baseline cohort was approximately 80% through 2009, thereby minimizing potential for bias due to differential loss to follow-up. Our validation study of uterine leiomyomata indicated high accuracy in reporting, and we controlled for a wide range of potential confounders. Moreover, stronger results were observed in subgroups of women among whom misclassification of uterine leiomyomata is lower (e.g., younger women and women with a recent pelvic ultrasound examination).[4] The large sample size and high incidence of uterine leiomyomata in this population conferred excellent statistical power to detect small increases in risk.

Because lye relaxers are thought to contain harsher chemicals (e.g., sodium hydroxide) than no-lye formulations, we hypothesized that women who more often used lye relaxers would have a higher risk of scalp lesions or burns, thereby increasing dermal absorption of putative endocrine-disrupting chemicals. One potential explanation for the lack of difference in association between lye and no-lye formulations is that both products pose a similar likelihood of scalp lesions and burns. Scalp inflammation and scarring alopecia have been documented anecdotally with both lye and no-lye relaxers.[16] In addition, although most women currently reported using no-lye formulations, most had probably used lye formulations in the past,[16] which could have diluted the association.

The study had no information on the individual brands of chemical relaxers used or on constituents that might be considered estrogen disruptors. Thus, we are unable to provide insight as to the specific mechanisms by which hair relaxers could have influenced risk. It is possible that hair relaxer use is a proxy for exposure to other products containing high levels of endocrine-disrupting chemicals[17] and that the hair relaxers themselves are not the causal agents. For instance, it has been documented that US black women have higher mean levels of exposure to environmental chemicals such as phthalates,[52] bisphenol A,[53] and common insecticides[54] than other ethnic groups. Finally, we observed that the percentage of European ancestry decreased with increasing use of hair relaxers in a subset of women from our cohort, and that adjustment for genetic admixture attenuated the observed associations for hair relaxer use and uterine leiomyomata incidence. Therefore, although our study raises the hypothesis that hair relaxer use is associated with an increased risk of uterine leiomyomata, we cannot rule out the possibility that greater African ancestry, thought to increase genetic predisposition to uterine leiomyomata,[55] explains part or all of the observed association. Future studies are needed to confirm whether hair relaxer use and uterine leiomyomata are indeed associated.

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