There is a great deal of confusion regarding skin tests for hair services.
I will attempt to clarify the issue here.
Skin tests are required for the permanent colouring of hair only. They are not required for true (where there is no mixing) semi-permanent colours or temporary colours. The reason for this is that permanent hair colouring products contain an ingredient called PPD or paraphenylenediamine sometimes called p-phenylenediamine. It is there to facilitate a permanent colour result. The problem is that a small number of people (around 0.1% of the population) have the potential to develop an allergic reaction to PPD and the reaction can be very intense. In order to predict if the subject has the potential to react to PPD a tiny amount of the product to be used should be placed on the skin and left uncovered for 48 hours. This time is required because it can take up to 48 hours for a reaction to occur.
If a reaction ( itching, redness or swelling) around the test area does occur then the colouring procedure cannot be carried out as the risk of allergic reaction is too great.
If a reaction occurs during the application such as intense stinging, irritation, rash or burning sensation on the scalp, the procedure should be stopped and the product removed immediately. If you experience shortness of breath, seek immediate medical attention. Before colouring your hair again, you should consult a doctor for their opinion.
It should be said that allergic reactions are rare, considering that Millions of people all over the world use hair colourants without experiencing any unwanted effects. In the UK, around 100 million hair colourant applications are carried out every year in homes and salons. Hair colourants are one of the most thoroughly studied consumer products on the market and their safety is supported by a wealth of scientific research. In fact, allergic reaction to hair colourants is much rarer than allergic reaction to food.
If someone has the potential to react to PPD, they may not react the first time they encounter PPD. However, the early exposures will prime their immune system which may then over-react when they encounter PPD again. Their immune system essentially mistakes PPD for an invader and retains a memory of it. When that substance is met again, it is the over-reaction of the immune system rather than the ingredient itself that causes the damage.
Not everyone has that potential to develop an allergy. Those who have the potential will, sooner or later, produce an allergic reaction if they continue to be exposed to PPD.
Some products claim to be PPD free but PPD is essential for permanent hair colourants, some colourants use a variant called PTD (para-toluenediamine). PTD can also lead to allergic reactions in people sensitive to either PTD or PPD. For this reason, all hair colourant products that contain PPD, PTD or any other chemically similar dye must include the warning ‘Contains phenylenediamines’ on the pack.
The best advice is to always perform an allergy alert test 48 hours before colouring the hair, exactly as directed on-pack and in the instruction leaflet.