Hydroxyethyl-p-Phenylenediamine Sulfate

What it is

This substance was developed as an alternative to p-phenylenediamine. It belongs to the same substance class and does the same job, but carries two additional hydroxyethyl groups on the molecule — and that is precisely what changes its behaviour on the skin.

In practice it appears where “PPD-free” is advertised. Whether that is an all-clear depends on the question being asked. For a person with a known PPD allergy it can be an option. As a general statement about a colour's allergy potential, “PPD-free” is of no use.

Why

• At first the figures look good. In a study of 216 people with suspected hair-dye allergy, 40 of them — just under twenty per cent — reacted to PPD, but only two, that is 0.9 per cent, to this substance at the same test concentration. At twice the concentration it was five, or 2.3 per cent.
• The first catch is cross-reactions. Among the 43 people who reacted to PPD, twelve per cent also showed a reaction to this substance — in the same range as toluene-2,5-diamine at fifteen per cent. Someone who does not tolerate PPD may tolerate this colour; it cannot be relied on.
• The second catch is the more important one, and it appears in the same paper. In animal testing the substance is classed as a strong sensitiser, and allergies to it are probably rare only because it is rarely used. The low observed rate may therefore be measuring its prevalence rather than its tolerability. That is a fallacy which the advertising claim “PPD-free” systematically carries with it.
• That is why the classification here reads “increased attention” and not “low risk”. The authors expressly state that the risk of first-time sensitisation could only be assessed in a large controlled study under regular conditions of use. No such study exists.
• For the salon the distinction is usable in practice. “PPD-free” answers the question of a person who is already affected, and does so with reservations. It does not answer the question of whether a colour is gentler for someone without a known allergy.

When the assessment differs

• Known PPD allergy: this substance may be an option, but only after a patch test. Twelve per cent of the PPD-allergic reacted here too in the study.
• A previous black henna tattoo: anyone sensitised to PPD that way belongs to the group with a raised risk of cross-reaction.
• Salon staff: daily contact while mixing and applying. In animal testing the substance counts as a strong sensitiser; occupational exposure is something different from an appointment every six weeks.
• Irritated or broken scalp skin: through a disturbed barrier more of the substance reaches the skin.
• Anyone reading “PPD-free” as a general all-clear: the claim describes what is absent, not what replaced it — the same pattern as with “ammonia-free”.

Open questions

• Concentration in the specific product: not available.
• The evidence rests essentially on a single study of 216 people. For a substance advertised as an alternative, that is little.
• Risk of first-time sensitisation under regular conditions of use: unresolved. The authors of the study used state this expressly.
• Relationship between the finding from animal testing and the low figures in humans: not resolved. The two stand side by side without either explaining the other.
• Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Sources for this assessment

• Frosch P. J., Kügler K., Geier J.: Patch testing with hydroxyethyl-p-phenylenediamine sulfate — cross-reactivity with p-phenylenediamine. Contact Dermatitis, Band 65, Heft 2, 2011, Seiten 96–100. Untersuchung an 216 Personen; hält fest, dass der Stoff selbst als Sensibilisator bekannt ist und dass das Risiko einer erstmaligen Sensibilisierung nur in einer großen kontrollierten Untersuchung beurteilt werden könnte. Begutachtete Fachzeitschrift, abgerufen am 11.09.2026.

Related substances

Applies to: Dye precursors and couplers

• 2,4-Diaminophenoxyethanol HCl — Oxidative dye precursor
• 2,7-Naphthalenediol — Coupler for oxidative hair colour
• 2-Methylresorcinol — Coupler for oxidative hair colour
• 4-Chlororesorcinol — Coupler for oxidative hair colour

Further substances from the same group for which a long-form assessment already exists.