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

Abstract

Use of hair straighteners and chemical relaxers has been associated with increased incidence of breast, ovarian, and uterine 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 and chemical relaxers in relation to the incidence of non-reproductive cancers. We analyzed data from 46 287 cancer-free women aged 35-74 years from the Sister Study, a US-wide cohort enrolled between 2003 and 2009. Participants reported frequency of hair straightener and chemical relaxer use in the 12 months prior to enrollment. Incident cancers (melanoma, thyroid, lung, non-Hodgkin 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 straightener and chemical relaxer use and incident cancers, adjusting for age, race and ethnicity, educational attainment, and smoking status. During a median follow-up of 13.1 years, use of hair straighteners and chemical relaxers was associated with a higher incidence of thyroid cancer (n = 225 cases; HR = 1.71, 95% CI = 1.01 to 2.89), non-Hodgkin lymphoma (n = 313 cases; HR = 1.62, 95% CI = 0.94 to 2.80), and pancreatic cancer (n = 138 cases; HR = 2.66, 95% CI = 1.25 to 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. Use of hair straighteners and chemical relaxers may be associated with a higher incidence of thyroid cancer, non-Hodgkin lymphoma, and pancreatic cancer. Further research is needed to confirm these findings.