Hair dye allergy explained: patch tests, black henna and why switching brands isn't enough

Quick answer: Most hair dye allergies are caused by PPD (paraphenylenediamine), an ingredient in many permanent and darker shades. You can become allergic at any time, even after years of colouring with no problems, and a reaction can take up to 48 hours or more to show. That is why salons ask for a skin test at least 48 hours before every colour appointment. Black henna temporary tattoos often contain high levels of PPD and can make you allergic. If you have reacted once, switching brands is not enough, because many dyes share the same or related ingredients. See a doctor, who can refer you for proper allergy testing.

It often starts small. An itchy hairline the evening after a colour, ears that feel hot, eyelids that look a little puffy the next morning. Many people put it down to a strong product or a sensitive day and book their next appointment as normal. Then the next reaction comes faster, and it is bigger.

Hair dye allergy is not common, but it is well understood, and a little knowledge goes a long way. Here is how it works, why your stylist or brow artist always asks for that patch test, and what to do if your skin ever reacts.

What is PPD and why does it cause reactions?

PPD is the ingredient behind long lasting, natural looking colour in many permanent hair dyes. On its own it is colourless; it develops into colour when mixed with the developer. DermNet, the dermatology resource from New Zealand, explains that it is the partly developed form that can trigger an allergy, and that darker shades tend to contain more of it. A related ingredient, PTD (para toluenediamine), is used in many dyes too and can also cause reactions.

The good news is that most people never react. In the largest European study of its kind, researchers patch tested 2,739 randomly selected adults in five countries and found 0.8% were allergic to PPD, even though around half of everyone interviewed had coloured their hair at some point.

How sensitisation works

An allergy to hair dye does not usually happen the first time. Your immune system first has to learn to recognise the chemical, which is called sensitisation. That can happen after one exposure or after many. Once you are sensitised, your body treats the chemical as a threat the next time it meets it.

This is why the classic story is someone who has coloured their hair for 10 or 20 years and then suddenly reacts. Anaphylaxis UK explains that each further exposure can bring symptoms on sooner and make them more severe. DermNet adds that once you are sensitised, the allergy is likely to be lifelong.

Hair professionals handle dye every day, so they are at greater risk. A review published in Contact Dermatitis found PPD allergy in 28.6% of hairdressers who were patch tested, compared with 4.3% of other patients tested for skin allergies. It is one reason your stylist wears gloves for every colour.

Reactions can take up to 48 hours or longer

Unlike a food allergy, which tends to show within minutes, hair dye allergy is usually delayed. Anaphylaxis UK says symptoms can appear up to 48 hours after colouring, and the NHS says they can take up to 72 hours. So feeling fine as you leave the salon does not mean you are in the clear. If something appears over the next few days, it is worth taking seriously.

Why salons insist on a patch test

Dye manufacturers tell professionals to carry out a skin test, often called an allergy alert test, at least 48 hours before every colour service. A small amount of the product goes behind your ear or on your inner elbow, and you check it over the following two days. Anaphylaxis UK is clear that the 48 hour wait should not be cut short, and that the test should be repeated every time, because allergies can develop and product formulas can change.

In the United States, the FDA advises a patch test every time before dyeing your hair. It is the same message from regulators and manufacturers around the world, even though the exact rules on salons differ from country to country.

A patch test is not a guarantee. The FDA and DermNet both note that a reaction can still happen after a clear test. But it is the best early warning there is, and a professional who asks for one is looking after you.

Black henna holiday tattoos

Real henna is a plant dye that stains the skin orange to reddish brown. So called black henna is not henna at all. The FDA explains that the black colour often comes from hair dye containing PPD, sometimes applied neat, and that PPD is not permitted in products put straight onto the skin.

Painting a strong dose of PPD onto skin and leaving it for hours is an easy way to become sensitised. The European study above found that people who had a black henna tattoo were more than five times as likely to be allergic to PPD: 3.2%, compared with 0.6% of those who had not. A 2018 British Skin Foundation survey found three quarters of people did not know black henna contains PPD, or that it could make a future hair dye reaction more likely.

If you or your children are offered a temporary tattoo on holiday, the simplest choice is to say no thanks to anything black. If you have had one in the past, tell your stylist or brow artist before your patch test.

Brow and lash tints

Brow and lash tints can contain the same family of dyes, so the same patch test rules apply. Rules on these products vary around the world. In the EU and UK, eyelash tints containing PPD are for professional use only and must carry warnings, including that black henna tattoos may increase the risk of allergy. In the United States, the FDA does not permit coal tar hair dyes, the group that includes PPD, to be used on eyebrows or eyelashes, and warns that using hair dye near the eyes can cause serious injury.

Brow henna is popular, and many products are plant based. The FDA notes that even brown henna products may contain added ingredients, so ask your artist what is in theirs, and have a patch test first. Our henna brows guide and lash lift guide explain what to expect from a professional appointment.

Why switching brands is not enough

It is natural to think a reaction was down to one product. But Anaphylaxis UK says that after a reaction, changing shade or brand will not protect you, because so many dyes use the same ingredients. In the EU and UK, dyes containing PPD or related chemicals carry a label saying "contains phenylenediamines", and "natural" or "PPD free" on the box does not mean allergy free.

Alternatives are not straightforward either. DermNet reports that around half of people allergic to PPD can tolerate PTD based dyes, which means half cannot, and about 10% also react to semi permanent colours. A dermatologist can patch test you properly and help you find what is safe for you. It is also worth telling your doctor or pharmacist, as Anaphylaxis UK notes that people allergic to PPD can react to some related medicines, including benzocaine, a numbing ingredient in some sore throat and mouth ulcer remedies, and sulfonamide antibiotics.

The same idea applies in nail salons, where acrylates are the usual culprit. Our blog on the truth about nail allergies explains more.

Signs of a reaction and what to do

Mild reactions usually show where the dye touched your skin: itching, redness, dryness or a rash along the hairline, ears, neck or eyelids. On brown or black skin, the NHS notes a rash may look darker brown, purple or grey, or be harder to see. More serious reactions cause marked swelling of the scalp and face, and eyelids can swell shut.

If you notice a reaction starting, the NHS advises washing your hair and scalp straight away with a mild shampoo. A pharmacist can suggest antihistamines and soothing creams for mild symptoms. See a doctor if symptoms get worse or do not settle; they may refer you to a dermatologist for patch testing.

Call your local emergency number straight away (999 in the UK, 911 in the US and Canada, 000 in Australia, 112 across the EU) if you or someone else has:

• sudden swelling of the lips, mouth, tongue or throat
• trouble breathing, wheezing or a tight throat
• difficulty swallowing
• skin, lips or tongue turning blue, grey or pale
• sudden confusion, drowsiness, dizziness or fainting

Severe reactions like this are rare, but they need urgent help.

FAQs

Can you suddenly become allergic to hair dye?

Yes. Sensitisation can build up over time, so you can react after many years of colouring with no problems. That is why a patch test is needed before every colour.

How long after dyeing hair can a reaction start?

Usually within 48 hours, though the NHS says it can take up to 72 hours. Reactions after a first sensitisation tend to come on faster.

Is there a hair dye that is safe for PPD allergy?

There is no single safe swap. Some people tolerate certain PTD or semi permanent dyes, but many do not. Ask a dermatologist to test you before trying anything new.

Can black henna cause a hair dye allergy?

Yes. Black henna tattoos often contain high levels of PPD and can sensitise your skin, making a later reaction to hair dye more likely.

Do I need a patch test for a brow or lash tint?

Yes. Tints can contain similar dyes, so your therapist should patch test you before your first appointment and again if anything changes.

Colour with confidence

A patch test takes a few minutes and gives you peace of mind for every appointment after it. Choose a professional who asks for one, explains the products they use and listens if you have reacted before. Every profile on our directory has verified qualifications and insurance, so you can find a verified professional near you. Browse our henna brows guide before your next booking.