Shampoo Ingredients to Avoid.
Eight products cross-referenced.

A surfactant derived from coconut oil used as a secondary cleanser and foam booster. It is one of the most prevalent contact allergens in modern shampoo formulations, and among the most frequently cited ingredients in Reddit discussions on scalp and skin reactions to “gentle” or “fragrance-free” products. The sensitivity is well-documented in dermatological literature.

Who reacts: people with eczema, reactive scalp, or who have developed sensitisation after prolonged exposure. The allergy is often not immediate — sensitisation builds with repeated use, which is why reactions appear in products you have used successfully for months.

The current dominant preservative in cosmetics — replacing parabens as consumer preference shifted. Found in a very high percentage of “natural” and “fragrance-free” products because it is effective, stable, and paraben-free. It has a faint rose-like odour that is usually imperceptible at standard concentrations (up to 1%) but can contribute to the mild chemical note some sensitive users detect in shower steam.

Who reacts: people with general preservative sensitivity or contact dermatitis. Also noted by some users with phenol sensitivity — phenoxyethanol is a glycol ether structurally related to phenol. Reactions are typically localised (scalp irritation, itching) rather than systemic.

The primary cleansing agents in most conventional shampoos. SLS is the most aggressive — it strips the scalp’s natural oils efficiently and raises the hair cuticle, causing squeak on rinse. SLES is milder (ethoxylated version). ALS (Ammonium Laureth Sulfate) is used in Faith in Nature — comparable mildness to SLES. The concern for sensitive users is not usually allergy but irritation and barrier disruption with repeated use.

Who reacts: people with dry, eczema-prone, or already-compromised scalp skin. Also the primary cause of squeak on rinse for users with chemically sensitive or dry hair. Sulfate-free shampoos use glucosides or isethionates instead.

Plant-derived ingredients marketed as soothing or natural. They are the primary source of unexpected smell in fragrance-free shampoos — aloe, chamomile, oat extract, and geranium oil all release detectable scent compounds under heat and steam. For users with specific plant allergies (e.g. chamomile is a member of the Asteraceae family and cross-reacts with ragweed allergy), these ingredients can also cause true allergic contact dermatitis.

Who reacts: people with plant or pollen sensitivities (especially ragweed, chrysanthemum cross-reactors), people with balsam of Peru allergy, and people sensitive to heat-activated scent from botanical compounds. Note: Simple lists chamomile oil and geranium oil despite marketing as “no artificial perfume” — these are real botanical essential oils.

Preservatives that work by slowly releasing small amounts of formaldehyde to prevent microbial growth. Formaldehyde is a known sensitiser and listed allergen. Simple contains Sodium Hydroxymethylglycinate — a formaldehyde releaser that appears in some “natural” product formulations and is not always flagged as clearly on marketing materials. DMDM Hydantoin is the most commonly cited by name in online sensitivity discussions.

Who reacts: people with formaldehyde allergy, which is one of the most prevalent contact allergens tested by dermatologists. The reaction can be severe even at low concentrations. Formaldehyde releasers are increasingly absent from European and UK formulations but still appear in some products.

Preservatives that caused a significant contact allergy epidemic in Europe in the 2010s when MI was approved for use in rinse-off cosmetics at higher concentrations. Subsequently restricted. MI/MCI are now tightly regulated in the EU and UK and largely absent from mainstream shampoos, but they persist in some household cleaning products and older formulations. None of the eight reviewed products contain MI or MCI.

Who reacts: people who developed sensitisation during the epidemic of use (2010–2016) may still react at very low concentrations. If you know you are MI-allergic, this is a confirmed concern for the household cleaning space more than current shampoos.

A widely used preservative pair in “natural” and “free-from” formulations — appearing in most of this batch as the replacement for parabens and MI. Sodium benzoate is a salt of benzoic acid and is listed in the EU’s 26 fragrance allergens because of its relationship to Balsam of Peru allergy. For most users it is innocuous; for users with Balsam of Peru sensitivity or salicylate intolerance it can cause reactions.

Who reacts: people with Balsam of Peru allergy (a well-established contact allergen group that includes cinnamon, vanilla, cloves, citrus peel and related compounds). Potassium sorbate is generally very well tolerated but can cause mild irritation in high concentrations.

A foam booster and viscosity builder derived from coconut oil. Listed on California’s Proposition 65 list as a possible carcinogen at high concentrations. It can be contaminated with nitrosamines (carcinogenic byproducts formed when DEA-based ingredients interact with certain other ingredients). The concern is more about long-term repeated use than acute sensitivity, but it is regularly cited in Reddit sensitivity discussions. Only Simple contains it in this batch.

Who reacts: people who have developed sensitisation to DEA-based compounds, or people avoiding Proposition 65 listed ingredients on precautionary grounds. The DEA reaction is also often cited alongside nitrosamine contamination concerns by users with chemical sensitivity.