Patch Test
A: A patch test is a test of the skin to make sure that an individual is not sensitive (allergic) to chemicals in a hair service compound. It's important to perform a patch test before ANY chemical service to prevent potentially serious adverse effects.
If a perm has a waving lotion made by mixing two components test each of the components as well as the neutralizer formula and any conditioners included as well. Test both the color formula and developer mixture used for a color service as well.
Using cotton swabs and dab a tiny amount of the chemicals onto the skin (usual locations are behind the ear at the nape of the neck - making note of the order in which the swabs are placed to know which component you may react to - or inside the elbow on either arm) and allow the spots to dry. Wait 24-48 hours and examine the areas. Generally, if there is going to be a reaction, then this will have given ample time for a reaction to be apparent.
The client should be alert for signs of itchiness or redness in these areas. Any sign of reaction to the chemicals intended for use means that you should NOT proceed with the service. Even a tiny reaction could be dramatically intensified when you increase the amount of exposure and the length of exposure to the levels needed by most of the chemical services.
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See also:
How to perform a strand test
Hair porosity test
How to measure hair elasticity