How to patch test a new hair product

A reaction on your scalp is harder to manage than a reaction on your forearm. The scalp is difficult to see, awkward to treat, and covered in hair. A reaction there can mean days of itching, and in the case of hair dye, considerably worse.

The important misconception to clear: botanical and natural products are not exempt. Plant extracts and essential oils are among the more common causes of contact reactions, precisely because they contain many biologically active compounds.

1. 01Choose a discreet, thin-skinned area: the inner forearm, or behind the ear near the hairline. Behind the ear is closer to scalp skin.
2. 02Clean and dry the area.
3. 03Apply a small amount of the product, undiluted and exactly as you would use it, over an area about the size of a coin.
4. 04Leave it uncovered and undisturbed. Do not wash the area.
5. 05Check at one hour, at 24 hours, and again at 48 hours.
6. 06If nothing happens by 48 hours, it is reasonable to proceed.

For a leave-on product, leaving it on is the correct test. For a rinse-off product such as shampoo, you can apply it, leave a few minutes and rinse, then watch the same site over 48 hours.

Stop and wash it off

• Redness that persists rather than fading in minutes.
• Itching, stinging or burning.
• Small bumps, blisters or a raised rash.
• Swelling.
• Skin that feels hot or tight.

Probably fine

• Brief mild tingling that settles within a few minutes.
• A slight sheen or residue from the product itself.
• Faint pinkness immediately after rubbing it in that fades quickly.

Permanent hair dye containing PPD, para-phenylenediamine, can cause severe allergic reactions including significant facial swelling. Manufacturers recommend a patch test 48 hours before every application, not just the first, and that advice exists because reactions genuinely happen to long-term users.

Black henna deserves a separate warning. It is not henna. It typically contains high concentrations of PPD applied directly to skin, and it has caused serious reactions and lasting scarring. Avoid it.

• You have eczema, psoriasis or generally reactive skin.
• You have reacted to a cosmetic or plant product before.
• You have a known plant or nut allergy, since carrier oils are frequently nut derived.
• You are pregnant, when it is reasonable to be more cautious generally.
• The product is for a child.
• The scalp is currently broken, sunburnt or irritated. Wait until it has settled.

If you have a significant reaction to anything, stop using it, keep the packaging, and see a doctor. The ingredient list is useful information for working out what caused it, and worth keeping rather than discarding.