Colouring hair at home is routine for many people, and the instructions on the box include a step that is easy to skip: testing the dye on a strand of hair well before the main event. This article explains what that test is for, how long a reaction can take to appear, what the NHS says to do if one happens, and how a dermatologist’s patch test differs from the one on the packet.
Why a routine colour can cause a reaction
According to the NHS, hair dye reactions are fairly common but usually mild. The reaction can be irritation, where the product directly upsets the skin, or an allergy, where the immune system responds to a specific ingredient. The two can look alike on the scalp, which is part of why guidance for both begins in the same place: stop using the product if you feel unwell.
The ingredient most often discussed is PPD (paraphenylenediamine). The NHS says many permanent dyes, and some semi-permanent ones, contain it, and that it can irritate the skin or trigger an allergy. The same page says people may be more at risk of an allergic reaction if they have, or have previously had, a black henna tattoo. The NHS describes the paste used for these tattoos as containing high levels of PPD, which can make a later reaction to PPD products, including hair dye, more likely.
Contact dermatitis more generally is described by the NHS as a type of eczema triggered by contact with a particular substance. It separates irritants, which damage the outer layer of skin and are the more common cause, from allergens, which provoke an immune response and are less common. The NHS lists cosmetics among possible sources of allergic contact dermatitis.
What a reaction can look like
The NHS lists the usual skin reactions to hair dye as:
- stinging or burning
- an itchy rash
- dryness or tightness
- soreness
- blisters
Skin tone changes how a rash appears. On lighter skin it tends to look red. On brown or Black skin, the NHS says, it may look darker brown, purple or grey, or be harder to see. That matters for the strand test and for self-checking afterwards: on darker skin, feel for heat, tightness and itching as well as looking for colour change.
When might a reaction appear?
This is the detail that catches people out. The NHS states that symptoms can take up to 72 hours to appear. A person who rinses out the colour on a Saturday morning and feels fine that afternoon has not necessarily cleared the hurdle.
The same page advises doing a strand test at least 48 hours before you want to use the dye and following the instructions on the packet. The two numbers fit together in a simple piece of illustrative arithmetic:
| Step | Timing |
|---|---|
| Strand test on the packet’s instructions | At least 48 hours before the planned colour |
| Planned colour | Saturday at 10:00 |
| Latest start for a strand test | Thursday at 10:00 |
| Reactions to the colour itself can appear until | Tuesday at 10:00 (72 hours later) |
In practice that means the test belongs in the diary earlier in the week, not on the same afternoon. A test done on Friday evening for a Saturday morning colour falls short of 48 hours and defeats the point.
What the strand test is, and what it is not
The NHS says to test the dye on a strand of hair at least 48 hours before use, to follow the packet instructions, and to avoid continuing with the dye if you feel unwell or react after a patch or strand test. It also advises not to leave dye on for longer than recommended and to wear gloves.
It is worth being clear about its limits. The test is a manufacturer’s step, designed to be done at home with the product you intend to use. It does not tell you which chemical you react to, and it cannot make a product safe for you. If it produces a reaction, that is the result: the NHS advice is not to proceed.
Because allergies can arise at any point, a clear result once is not a lifetime guarantee. Following the packet’s instructions each time you colour, particularly if you change brand or shade, keeps the test meaningful. The brand’s own leaflet remains the authority on exactly how to carry it out.
What to do if your skin reacts
For mild symptoms the NHS gives a straightforward sequence.
- Wash your hair and scalp thoroughly with a mild shampoo straight away if you feel a reaction starting.
- Speak to a pharmacist. For mild skin symptoms, the NHS says a pharmacist can suggest emollient moisturisers and antihistamine tablets or creams to ease itching.
- Contact a GP if symptoms worsen after a reaction to hair dye. The NHS says a GP may prescribe emollients, steroid creams or tablets for inflamed skin, or antihistamines.
- Choose differently next time. The NHS suggests a non-permanent dye if a permanent one has caused a reaction before, and avoiding PPD-containing products altogether if you have reacted to hair dye or a black henna tattoo.
One practical safety note comes from the same page: creams and ointments can dry onto clothing and bedding, which increases fire risk, so avoid naked flames and smoking while using them.
Which signs mean an emergency?
Serious allergic reactions to hair dye are possible but, the NHS says, rare. Anaphylaxis tends to start within minutes of contact with a trigger, according to the NHS page on the condition. The NHS says to call 999 straight away if:
- the lips, mouth, throat or tongue suddenly swell
- breathing becomes very fast or difficult
- the throat feels tight, or swallowing is a struggle
- the skin, tongue or lips turn blue, grey or pale (on darker skin, the palms or soles may show this)
- someone suddenly becomes very confused, drowsy or dizzy
- someone faints and cannot be woken
The NHS adds that a swollen, raised or itchy rash can also be a sign of a serious reaction. For anaphylaxis it advises using an adrenaline auto-injector if one is available, calling 999 and saying you think it is anaphylaxis, and lying down with the legs raised rather than standing or walking, even if you feel better. It also says anaphylaxis needs hospital treatment even if the symptoms ease. Do not drive yourself to A&E; the 999 call handler will advise.
What does a dermatology patch test involve?
A strand test and a clinical patch test share a name and little else. According to the British Association of Dermatologists (BAD), patch testing is carried out in a specialist dermatology department. Patches containing common allergens are placed on the patient’s back and removed after two days. The skin is examined immediately and again after a further two days. A small rash at a patch site is called a positive patch test, and the dermatologist then advises which contact allergens to avoid.
The BAD says patch testing is usually done when the cause of a rash is unknown or when it does not respond to topical treatments. The NHS says a GP may refer someone to a dermatologist for a patch test after a hair dye reaction, because it identifies the specific chemical or ingredient the person reacts to. The NHS also mentions that home patch test kits exist for people who suspect an allergy, but a clinical test is the route to a named cause.
For anyone who has had a significant reaction, that distinction is valuable. Knowing the specific allergen is more useful than guessing, and it can affect which products, including some cosmetics and other personal care items, to avoid.
Working with colour when it is part of the job
The BAD gives hair dye as an example of a personal care product that can cause allergic contact dermatitis, and it uses a hairdresser who is allergic to hair dyes to illustrate a case where a change of job may be needed. That is a reminder that repeated, frequent contact is a different situation from an occasional home colour. Gloves and following product instructions are the NHS’s basic precautions, and anyone who handles colour often and notices recurring skin problems on the hands or face should raise it with a GP.
For readers who want to check what is in a product before buying, the way ingredient lists and warnings are laid out is covered in the guide to reading a cosmetics label. The principle of introducing one new thing at a time, so that a reaction can be traced, is also discussed in the piece on starting retinol slowly. Fragranced products are another common source of skin reactions, and how a fragrance is built explains why one label line can hide many ingredients.
A short routine worth keeping
Pulling the official advice together, a sensible approach to colouring at home looks like this:
- Read the packet and keep it, so you know which product and shade you used.
- Do the strand test at least 48 hours ahead, and treat a reaction as a reason to stop.
- Wear gloves and do not leave the dye on longer than directed.
- Rinse thoroughly with mild shampoo if you feel stinging or burning.
- Keep an eye on your skin for up to three days afterwards.
- Know the emergency signs, and call 999 if they appear.
- If you have reacted before, or have had a black henna tattoo, ask a pharmacist or GP before trying PPD-containing products again.
Hair colour is a low-risk routine for most people, and the measures above are small. They exist because the rare serious reaction is serious, and because the ordinary one, an itchy and sore scalp that arrives days later, is easy to connect to the wrong cause.
This article is general information, not medical advice. If you have had a reaction to hair dye, or are unsure whether a product is suitable for you, speak to a pharmacist, your GP or a dermatologist.
Sources: NHS (Hair dye reactions); NHS (Contact dermatitis); NHS (Anaphylaxis); British Association of Dermatologists (Contact dermatitis patient information)
