Hair care is one of the most overlooked vectors for chemical exposure in personal care routines. Products are applied to the scalp — where absorption rates are higher than most body skin — often heated (which increases absorption and off-gassing), and used daily or near-daily for decades.
The hair care industry is also heavily influenced by the cosmetic industry's and preservative conventions, meaning many of the same problematic ingredients found in skincare appear in shampoos and conditioners alongside additional concerns specific to hair products.
◈Key Facts
The scalp absorption problem
The scalp has a high density of hair follicles, which provide direct channels into deeper skin layers and vasculature. It's also rich in sebaceous glands that produce oil. Combined, these factors mean the scalp absorbs topically applied chemicals more readily than most other skin sites.
For leave-on products — dry shampoo, scalp serums, hair oils, styling products — the exposure period is also long. Something applied in the morning stays on until you next wash your hair.
Ingredients of particular concern
Sodium lauryl sulphate (SLS) and sodium laureth sulphate (SLES)
These sulphate surfactants are the primary foaming agents in most conventional shampoos. SLS is a well-documented irritant that strips the scalp's natural barrier. SLES is less harsh but can be contaminated with 1,4-dioxane during manufacturing. For people with sensitive scalps, hormonal disruption concerns, or scalp conditions, sulphate-free formulations are generally better.
Parabens and formaldehyde releasers
The same preservative concerns from skincare apply here. DMDM hydantoin, imidazolidinyl urea, and quaternium-15 are still found in many shampoos and conditioners. Check ingredients lists.
Silicones in leave-on products
Cyclopentasiloxane (D5) and cyclotetrasiloxane (D4) are cyclic silicones used in many hair serums and heat protectants. D4 is classified as an endocrine disruptor in the EU and is banned from rinse-off products there above a certain concentration. D5 is a persistent environmental contaminant. Both are still freely used in many leave-on hair products globally.
Chemical hair straighteners and relaxers
This is where the evidence is most serious. A 2022 NIH study published in the Journal of the National Cancer Institute found that women who used chemical hair straighteners frequently had significantly higher rates of uterine cancer compared to non-users. The association was particularly strong among Black women, who are disproportionately heavy users of these products.
Chemical relaxers contain potent alkalis (sodium hydroxide, guanidine) alongside various endocrine-disrupting chemicals. The association with uterine cancer is now sufficiently strong that the FDA proposed a ban on formaldehyde in hair straightening products in 2023, though the regulatory process is ongoing.
Aerosol hair products and benzene
Multiple aerosol dry shampoos, hairsprays, and texturisers were recalled from 2021 onwards after testing found benzene contamination — a known carcinogen — in products from multiple brands. This was an industry-wide manufacturing problem rather than an isolated case. Aerosol hair products in general are worth approaching with caution.
Hair dye
Frequent use of permanent hair dye has been associated with bladder cancer risk in occupational studies of hairdressers and in some consumer epidemiological studies, though the evidence in general consumers is less conclusive than in occupational exposure.
The chemicals of most concern are p-phenylenediamine (PPD) and related aromatic amines — the reactive compounds that create permanent colour. PPD is also a potent sensitiser — meaning each exposure can increase sensitivity to subsequent exposures, leading to increasingly severe reactions.
Semi-permanent and temporary dyes have a different chemical profile and somewhat different risk considerations. Henna is genuinely lower-risk, though black henna used in temporary tattoos often contains high levels of PPD and is not the same as natural henna.