Final answer:
A patch test or P.D. test for permanent hair color should be performed 48 hours before applying the dye to prevent allergic reactions. The permanent hair color alters the hair structure by removing natural pigment and infusing synthetic dye.
Step-by-step explanation:
Before applying permanent hair color, a patch test, also known as a P.D. test (Preliminary Disposition test) or allergy test, should be performed to ensure the client does not have an allergic reaction to the dye. This test should ideally be done 48 hours before the hair coloring procedure. As the name suggests, permanent hair color changes the structure of hair by removing the natural color and replacing it with synthetic dye. The process involves a chemical reaction where 1,4-diaminobenzene or its derivatives oxidize to form larger color molecules that become trapped inside the hair shaft, leading to long-lasting color.
It is important to note that, while the name 'permanent' might suggest that the new hair color lasts forever, it refers to the change in the hair's structure until such time as the hair is cut, not the persistence of the dye itself, which can fade over time.
Considering the prevalence of hair coloring, with significant numbers of individuals coloring their hair, it's crucial to perform a P.D. test to prevent potential allergic reactions and ensure the safety and comfort of the client.