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

Abstract and Introduction

Abstract

Hair relaxers are used by millions of black women, possibly exposing them to various chemicals through scalp lesions and burns. In the Black Women's Health Study, the authors assessed hair relaxer use in relation to uterine leiomyomata incidence. In 1997, participants reported on hair relaxer use (age at first use, frequency, duration, number of burns, and type of formulation). From 1997 to 2009, 23,580 premenopausal women were followed for incident uterine leiomyomata. Multivariable Cox regression was used to estimate incidence rate ratios and 95% confidence intervals. During 199,991 person-years, 7,146 cases of uterine leiomyomata were reported as confirmed by ultrasound (n = 4,630) or surgery (n = 2,516). The incidence rate ratio comparing ever with never use of relaxers was 1.17 (95% confidence interval (CI): 1.06, 1.30). Positive trends were observed for frequency of use (P trend < 0.001), duration of use (P trend = 0.015), and number of burns (P trend < 0.001). Among long-term users (≥10 years), the incidence rate ratios for frequency of use categories 3–4, 5–6, and ≥7 versus 1–2 times/year were 1.04 (95% CI: 0.92, 1.19), 1.12 (95% CI: 0.99, 1.27), and 1.15 (95% CI: 1.01, 1.31), respectively (P trend = 0.002). Risk was unrelated to age at first use or type of formulation. These findings raise the hypothesis that hair relaxer use increases uterine leiomyomata risk.

Introduction

Uterine leiomyomata are tumors of the myometrium that are responsive to estrogens and progesterone. Although benign, uterine leiomyomata are associated with significant gynecologic morbidity and are the leading indication for hysterectomy in the United States.[1,2] The incidence of uterine leiomyomata is 2–3 times higher in US black women than in US white women,[3,4] and the lifetime risk of uterine leiomyomata—based on ultrasound evidence of preclinical and clinical disease—is estimated to be as high as 80% in US black women.[4] The black-white disparity in uterine leiomyomata risk is not explained by established risk factors.[3,5]

Hair relaxers (straighteners) have been used by millions of US black women, often for long periods of time.[6] Hair relaxers can cause burns and lesions in the scalp, facilitating entry of hair relaxer constituents into the body.[7–14] The main ingredient of "lye" relaxers is sodium hydroxide; no-lye relaxers contain calcium hydroxide and guanidine carbonate, and "thio" relaxers contain thioglycolic acid salts.[15] No-lye relaxers are advertised to cause fewer scalp lesions and burns than lye relaxers, but there is little evidence to support this claim.[16] Products may also contain hormonally active compounds,[17] such as phthalates, which are not required to be listed separately as ingredients and are often reported under the term "fragrances" or "perfume".[18] Cosmetic products are not subject to premarket approval by the Food and Drug Administration, and a complete list of ingredients is not mandatory,[19] making it unclear what types of chemicals they contain. However, because the vast majority of hair relaxers list "fragrance" as an ingredient, and 100% of popular fragrances tested in a 2002 study were found to contain phthalates,[18,20] most hair relaxers likely contain these chemicals. In addition, some hair relaxer products directly list phthalates as one of their chemical ingredients.[21]

Phthalates from cosmetic products can be absorbed by the skin or inhaled,[22–25] and they have been shown to have estrogenic effects in cell models and experimental animals.[26] Although the role of phthalates in humans is less clear,[26] cross-sectional data from the Third National Health and Nutrition Examination Survey (NHANES) show that self-reported history of uterine leiomyomata was positively associated with urinary levels of monobutyl phthalate and inversely associated with urinary levels of mono(2-ethylhexyl) phthalate.[27] In contrast, a case-control study found significantly higher levels of total urinary monoethylhexyl phthalate in uterine leiomyomata cases than controls.[28] Additional studies investigating the association of putative endocrine-disrupting chemicals (e.g., diethylstilbestrol, polychlorinated biphenyls, and dioxin) with uterine leiomyomata risk have also found positive associations,[29–31] but others have shown inverse[32] and null[33–35] associations.

In this hypothesis-generating study, we prospectively evaluated the association between hair relaxer use and uterine leiomyomata risk in a large population of premenopausal black women. We also explored whether the percentage of European ancestry (measured by a panel of European and African ancestry informative polymorphisms), which we hypothesized would be associated with both hair relaxer use and uterine leiomyomata, was a potential confounder of the relation. To our knowledge, there are no previous studies of the relation between chemical hair relaxers and risk of uterine leiomyomata.

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