How to Patch-Test a Hair Dye Before You Colour
Permanent hair dye is a common cause of severe scalp and face reactions. A proper allergy alert test before every colour can catch trouble early. Here is how to do one.
Why bother
Permanent and many semi-permanent hair dyes contain reactive colour chemicals, most often para-phenylenediamine (PPD), which is one of the more common causes of significant cosmetic allergy. A reaction can range from an itchy, weepy scalp to dramatic swelling of the face, eyelids and neck. Because the allergy is delayed, problems often show up a day or two after colouring rather than in the salon chair. An allergy alert test gives the dye a chance to reveal itself on a small patch of skin before you put it all over your scalp.
The method
- Read your kit’s instructions first, because the exact method and timing vary between brands.
- Mix a tiny amount of the dye as directed and dab it behind the ear or on the inner elbow, on clean, dry skin.
- Leave it undisturbed for 48 hours, without washing or covering the spot, since reactions can take 12 to 72 hours to appear.
- Read the result honestly. Any redness, itching, swelling or blistering means do not use the product. A clear test is reassuring, though not an absolute guarantee.
A note on safety
An allergy alert test is not the same as the medical patch testing a dermatologist does to confirm a specific allergen. If you have ever had a strong reaction to hair dye or to a black henna tattoo, do not keep experimenting at home. The British Association of Dermatologists and your GP can arrange proper testing and advise on whether any colour is safe for you. Never colour your hair if a test site reacts, or if you feel tingling or swelling starting.
Make it easier
Before you even test, it helps to know exactly what a box contains. A free tool like INCI Decoder lets you paste the ingredient list and see whether PPD or its relatives are present, which is a good broad screen, though it reads the same for everyone rather than against your own history. Once you know your triggers, a personal-list app like ClearaScan lets you save PPD once and scan a product to flag it, and it also keeps a Reaction Journal, a shared Care Circle so others can scan for you, and a Trusted Products list for items you have cleared, and it is currently in early access. (Disclosure: our editor co-founded ClearaScan, and we are not paid to mention the others.)
The Sensitive Skin Ingredient Decoder
A free one-page guide to reading a skincare label fast: the irritants to watch, the fragrance loopholes, and what "fragrance-free" and "hypoallergenic" really mean.
Common questions
Do I really need to test every time, even with my usual dye?
Yes. Manufacturers advise an allergy alert test before each application, because sensitivity to hair dye can develop suddenly after years of using the same product without trouble.
What ingredient usually causes hair dye reactions?
The most common culprit is para-phenylenediamine (PPD), a dye precursor in most permanent colours. Related chemicals like PTD can cause similar reactions, and people sensitised to PPD often react to several brands.
Are black henna tattoos linked to this?
Yes. So-called black henna temporary tattoos often contain high levels of PPD and can sensitise people, making a later reaction to ordinary hair dye more likely and more severe. Genuine plain henna is a different product.