Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers.

BACKGROUND Use of hair straighteners and chemical relaxers has been associated with increased incidence of breast, uterine, and ovarian cancers. However, their potential association with non-reproductive cancers remains unknown, despite evidence that some ingredients in these products may be genotoxic. We therefore examined use of hair straighteners/chemical relaxers in relation to the incidence of non-reproductive cancers. MATERIAL AND METHODS We analyzed data from 46,287 cancer-free women from the Sister Study, a U.S.-wide cohort enrolled between 2003-2009 (ages 35-74). Participants reported frequency of hair straightener/chemical relaxer use in the 12 months prior to enrollment. Incident cancers (melanoma, thyroid, lung, non-Hodgkin's lymphoma, leukemia, pancreatic, colorectal, and kidney cancers) were self-reported and confirmed with pathology reports when possible. We used multivariable Cox proportional hazards models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for hair straighteners/chemical relaxer use and incident cancers, adjusting age, race and ethnicity, educational attainment, and smoking status. RESULTS During a median follow-up of 13.1 years, use of hair straighteners/chemical relaxers was associated with a higher incidence of thyroid cancer (n = 225 cases; HR:1.71, 95% CI : 1.01-2.89), non-Hodgkin's lymphoma (n = 313 cases; HR : 1.62, 95% CI : 0.94- 2.80), and pancreatic cancer (n = 138 cases; HR : 2.66, 95% CI: 1.25-5.66). There was little evidence of dose-response with increasing frequency of use. We observed negligible or imprecise associations for the remaining cancer types. CONCLUSIONS AND RELEVANCE Use of hair straighteners/chemical relaxers may be associated with a higher incidence of thyroid cancer, non-Hodgkin's lymphoma, and pancreatic cancer. Further research is needed to confirm these findings.