--- topic: "Hair Dye and Straighteners" title: "Hair Dye and Chemical Straighteners" order: 1 description: "What large studies show about hair dye and cancer, why chemical straighteners were linked to uterine cancer in 2022, and how to lower your exposure." question: "Are hair dyes and chemical straighteners linked to cancer?" answer: "Hair dyes are unlikely to raise cancer risk much, but frequent chemical straightener use is linked to higher uterine cancer risk in two cohorts." actions: - "If you use chemical straighteners, stretch the time between treatments. The uterine cancer signal was strongest in women who used them more than four times in a year [1]." - "Check smoothing or keratin product labels for formaldehyde, formalin or methylene glycol, and skip home use. FDA discourages buying these products for home use [2]." - "Skip temporary \"black henna\" tattoos. They are an important cause of allergy to PPD, a hair-dye ingredient [3]." cautions: - "Hairdressers, barbers and other salon workers have a higher cancer risk than the general population, so gloves and good ventilation matter. Pooled bladder cancer risk was about 30% higher [4]." - "With breast cancer in the family, ask your clinician about hair products: among sisters of affected women, permanent dye went with 45% higher risk in Black women [5]." - "Itching, swelling or blistering after dyeing can signal a dye allergy: stop the product and see a clinician; about 6% of surveyed Europeans avoided hair colorants over skin problems [3]." confidence: 3 confidenceNote: "Large cohorts and meta-analyses of hair dye are mostly reassuring; the straightener–uterine cancer link rests on two cohorts with differing strengths of result, so it is plausible but unsettled." evidenceSummary: rctCount: 0 observationalCount: 5 metaAnalysisCount: 2 totalParticipants: "200,000+" replicated: true newestStudyYear: 2023 references: - id: "1" pmid: "36245087" title: "Use of Straighteners and Other Hair Products and Incident Uterine Cancer" authors: "Chang CJ, O'Brien KM, Keil AP, et al." year: 2022 journal: "J Natl Cancer Inst" - id: "2" url: "https://www.fda.gov/consumers/consumer-updates/formaldehyde-hair-smoothing-products-what-you-should-know" title: "Formaldehyde in Hair Smoothing Products: What You Should Know" authors: "U.S. Food and Drug Administration" year: 2021 journal: "FDA Consumer Update" - id: "3" pmid: "26802237" title: "Prevalence of Contact Allergy to p-Phenylenediamine in the European General Population" authors: "Diepgen TL, Naldi L, Bruze M, et al." year: 2016 journal: "J Invest Dermatol" - id: "4" pmid: "19755396" title: "Risk of cancer among hairdressers and related workers: a meta-analysis" authors: "Takkouche B, Regueira-Méndez C, Montes-Martínez A" year: 2009 journal: "Int J Epidemiol" - id: "5" pmid: "31797377" title: "Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women" authors: "Eberle CE, Sandler DP, Taylor KW, et al." year: 2020 journal: "Int J Cancer" - id: "6" pmid: "32878860" title: "Personal use of permanent hair dyes and cancer risk and mortality in US women: prospective cohort study" authors: "Zhang Y, Birmann BM, Han J, et al." year: 2020 journal: "BMJ" - id: "7" pmid: "37821068" title: "Hair relaxer use and risk of uterine cancer in the Black Women's Health Study" authors: "Bertrand KA, Delp L, Coogan PF, et al." year: 2023 journal: "Environ Res" - id: "8" pmid: "15019177" title: "Toxicity and human health risk of hair dyes" authors: "Nohynek GJ, Fautz R, Benech-Kieffer F, et al." year: 2004 journal: "Food Chem Toxicol" - id: "9" url: "https://www.cancer.gov/about-cancer/causes-prevention/risk/myths/hair-dyes-fact-sheet" title: "Hair Dyes, Other Hair Products, and Cancer Risk" authors: "National Cancer Institute" year: 2022 journal: "NCI" - id: "10" url: "https://www.niehs.nih.gov/news/factor/2022/11/papers/uterine-cancer" title: "Hair straightening chemicals associated with higher uterine cancer risk" authors: "National Institute of Environmental Health Sciences" year: 2022 journal: "NIEHS Environmental Factor" - id: "11" pmid: "22035353" title: "Characterization of formaldehyde exposure resulting from the use of four professional hair straightening products" authors: "Pierce JS, Abelmann A, Spicer LJ, et al." year: 2011 journal: "J Occup Environ Hyg" - id: "12" pmid: "15914752" title: "Personal use of hair dyes and risk of cancer: a meta-analysis" authors: "Takkouche B, Etminan M, Montes-Martínez A" year: 2005 journal: "JAMA" --- Dyeing your hair does not appear to raise cancer risk much: a 36-year study of 117,200 women found no rise in solid cancers [6]. Chemical straighteners are the newer worry: in one large US study, women using them more than four times a year had about 2.5 times the rate of uterine cancer [1], though a second study found a weaker link [7]. Hairdressers, who handle these products daily, have higher cancer rates [4]. ## How it works ### Dyes: what goes in, and how much gets in Permanent (oxidative) hair dyes are built from aromatic amines [8], and two major carcinogens were removed from dye formulas in the mid-1970s [4]. A review by authors with cosmetics-industry ties found that less than 1% of the applied dose entered the body in studies with radiolabelled dyes, and its risk assessments saw negligible cancer risk [8]. The International Agency for Research on Cancer (IARC) judged some chemicals that professional hairdressers meet as "probably carcinogenic to humans", and personal hair dye use as "not classifiable" [9]. ### Straighteners: a mixed bag of chemicals Studies of "straighteners" lump together straighteners, relaxers and pressing products [1]. Keratin "smoothing" treatments are a separate type, and the type FDA flags for formaldehyde [2]. Researchers have not pinned down which ingredient matters. They point to parabens, bisphenol A, metals and formaldehyde as candidates [10]. In a salon simulation, keratin smoothing products released formaldehyde when heated, including some labelled "formaldehyde-free" [11]. FDA says a label naming formaldehyde, formalin or methylene glycol means the product contains formaldehyde or will release it [2]. ### Black henna tattoos and dye allergy PPD (para-phenylenediamine) is a common hair-dye ingredient. In a European general-population study, PPD allergy was found in 0.8% of people, but in 3.2% of those who had ever had a black henna tattoo [3]. ### What to do - **Dye:** the largest cohort linked a higher cumulative dose to a few cancer types, so dyeing less often is a reasonable step [6]. - **Straighteners:** use them less often and avoid formaldehyde-releasing products [1][2]. - **Salon workers:** better ventilation and hygiene measures may reduce risk [4]. For indoor formaldehyde more broadly, see [Formaldehyde](/formaldehyde/indoor-toxin-and-exposure-reduction). ## The evidence in detail ### Personal hair dye use The strongest study is the Nurses' Health Study, a prospective cohort of 117,200 US women followed for 36 years. Ever users of permanent dye had no higher risk of solid cancers (hazard ratio 0.98; 95% CI 0.96 to 1.01; 20,805 cases), blood cancers (1.00; 0.91 to 1.10) or cancer death (0.96; 0.91 to 1.02). There were small signals: basal cell carcinoma (1.05; 1.02 to 1.08), and a higher cumulative dose went with more hormone-receptor-negative breast cancer and ovarian cancer. Hodgkin lymphoma was raised only in women with naturally dark hair (70 cases in all, 24 in dark-haired women) [6]. A JAMA meta-analysis of 79 studies found pooled relative risks of 1.06 (0.95 to 1.18) for breast cancer and 1.01 (0.89 to 1.14) for bladder cancer. Blood cancers were slightly higher at 1.15 (1.05 to 1.27) across 40 studies, with no effect of permanent dye or heavy use [12]. ### Straighteners and uterine cancer The Sister Study followed 33,947 women aged 35 to 74 who had a uterus, enrolled in 2003 to 2009. Over an average of 10.9 years, 378 uterine cancers occurred. Women who reported any straightener use in the previous 12 months had an adjusted hazard ratio of 1.80 (1.12 to 2.88). Frequent use, meaning more than four times in the year, had 2.55 (1.46 to 4.45; trend P = .002). Dyes and perms showed no association [1]. NIEHS researchers estimated that 1.64% of never-users would develop uterine cancer by age 70, compared with 4.05% of frequent users [10]. The relative increase is large; the absolute risk stays modest. Limits matter. Use was self-reported once, at enrollment, for the previous year only, and the question grouped all straightening, relaxing and pressing products, so it cannot say which chemicals are responsible. All participants had a sister with breast cancer, so results may not carry over to everyone [5]. A second cohort gives a mixed answer. In the Black Women's Health Study, 44,798 Black women with a uterus were followed from 1997 to 2019, with 347 uterine cancers. Compared with never or light use, heavy relaxer use (15 or more years and at least five times a year) had a hazard ratio of 1.18 (0.81 to 1.71), not significant. Among postmenopausal women the ratios were 1.60 (1.01 to 2.53) for moderate use and 1.64 (1.01 to 2.64) for heavy use. In premenopausal women the results were null [7]. ### Conflicting and mixed results The Sister Study linked permanent dye to breast cancer in Black women (hazard ratio 1.45; 1.10 to 1.90) but not clearly in White women (1.07; 0.99 to 1.16). It followed 46,709 women for an average of 8.3 years, with 2,794 breast cancers. Straightener use showed 1.18 (0.99 to 1.41), which just missed significance [5]. Radiolabel studies show little dye reaches the bloodstream [8], while cohort results suggest subgroup effects that exposure studies do not explain. Both can be true: effects may depend on product type and how often it is used. For workers the picture is clearer. A meta-analysis of 247 studies found that hairdressers had pooled relative risks of 1.27 for lung cancer, 1.30 (1.20 to 1.42) for bladder cancer, 1.52 for laryngeal cancer and 1.62 for multiple myeloma [4]. In a simulation of four professional keratin products, mean airborne formaldehyde reached up to 3.47 ppm during blow-drying. Even "formaldehyde-free" products could produce levels that meet or exceed occupational limits [11]. ### Bottom line Confidence 3 of 5. For personal dye use, large cohorts and meta-analyses are mostly reassuring. For straighteners, one well-run cohort signals a meaningful relative risk and a second finds it mainly in postmenopausal women, so the link is plausible but unsettled. Reducing frequency and avoiding formaldehyde-releasing products is a sensible, low-cost response.