Benzophenone-4

What it is

Benzophenone-4 is a water-soluble UV filter. Unlike in a sunscreen, in hair products it generally does not serve to protect the skin but to protect the product and the hair colour: it absorbs UV light before it can alter dyes and other light-sensitive constituents. The quantities used are correspondingly small.

For the assessment the distinction within the substance group matters. Benzophenone-4 carries a sulphonic acid group, which makes it water-soluble and charged. That sets it clearly apart from the better-known, fat-soluble benzophenone-3, around which the public debate mostly revolves.

Why

• Clarify the role first: here the substance protects the product and the colour, not the skin. A hair product with a UV filter is not a sunscreen and does not replace one. The quantities used are correspondingly small.
• Why the classification reads “increased attention”: contact and light allergies are documented for benzophenones, and the concentration in the product is unknown. The classification therefore follows the substance group and the missing figure — it expressly does not establish that the substance causes harm in its intended use.
• Not transferable, and this is the key point here: the public debate about hormonal effects overwhelmingly concerns benzophenone-3. That is a different molecule with different solubility and different uptake behaviour. Without a separate examination its findings must not be transferred to benzophenone-4.
• Water solubility and charge argue against appreciable uptake through intact skin. That is an expectation drawn from the substance properties, not a measurement; it supports the classification but does not carry it alone.
• Mind the form of application: in a setting mousse the substance stays in the hair and can touch the scalp. With a spray product inhalation would be added as a separate route — that would have to be considered separately and is not assessed here.
• Origin corrected: until 10.09.2026 the short field named an origin that was natural or synthetic depending on the raw material. A synthetic route is documented — benzophenone-4 is formed by sulphonation of benzophenone-3 with chlorosulphonic acid, and that starting material is in turn built up from resorcinol. No source was found for a natural derivation. The correction changes nothing about the result: origin is factual information and does not enter any assessment.

When the assessment differs

• Documented allergy to benzophenones or known photosensitivity of the skin: the product is then unsuitable for that person.
• Irritated, broken or inflamed scalp skin: uptake through a disrupted skin barrier is higher.
• Spray application: inhalation would be a separate exposure route and is not assessed here.
• Eye contact: practically relevant during application.
• Children: not assessed here.

Open questions

• Concentration in the actual product: not available. As a product and colour protectant the usual levels are low, but that does not replace a figure and is part of the reasoning for the classification.
• Regulatory status: approval and maximum-level rules exist for UV filters. They have not been checked here and are deliberately not quoted second-hand.
• Origin in the short field: changed on 10.09.2026 from “synthetic or naturally derived, depending on the raw material” to “synthetic”, supported by the patent named below. What remains open is which process was used for the actual raw material and how pure it is — neither can be read from the ingredient list.
• Data on the frequency of contact and light allergies to this individual substance: not evaluated here. The classification follows the substance group.

Sources for this assessment

• Patentschrift US 5,072,034 (BASF, erteilt am 10.12.1991), Preparation of 4-alkoxy-2-hydroxybenzophenone-5-sulfonic acids. Die Beispiele 1 bis 3 beschreiben die Herstellung von 2-Hydroxy-4-methoxybenzophenon-5-sulfonsäure — das ist Benzophenone-4 — durch Umsetzung von 2-Hydroxy-4-methoxybenzophenon mit Chlorsulfonsäure; derselbe Text nennt für den Ausgangsstoff eine Friedel-Crafts-Acylierung von Resorcin. Patentliteratur, abgerufen am 10.09.2026.