Are hair dyes and chemical straighteners linked to cancer?

Dyes: what goes in, and how much gets in

Permanent (oxidative) hair dyes are built from aromatic amines⁸, and two major carcinogens were removed from dye formulas in the mid-1970s⁴. 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 risk⁸. 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 products¹. Keratin "smoothing" treatments are a separate type, and the type FDA flags for formaldehyde². Researchers have not pinned down which ingredient matters. They point to parabens, bisphenol A, metals and formaldehyde as candidates¹⁰. 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 it².

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 tattoo³.

What to do

• Dye: the largest cohort linked a higher cumulative dose to a few cancer types, so dyeing less often is a reasonable step⁶.
• Straighteners: use them less often and avoid formaldehyde-releasing products^1,2.
• Salon workers: better ventilation and hygiene measures may reduce risk⁴.

For indoor formaldehyde more broadly, see Formaldehyde.

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 use¹².

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 association¹. NIEHS researchers estimated that 1.64% of never-users would develop uterine cancer by age 70, compared with 4.05% of frequent users¹⁰. 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 everyone⁵.

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 null⁷.

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 significance⁵. Radiolabel studies show little dye reaches the bloodstream⁸, 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 myeloma⁴. 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 limits¹¹.

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.