What is a key precaution for clients before undergoing chemical hair treatments?

Performing a patch test for allergies is a critical precaution prior to undergoing any chemical hair treatment. This test involves applying a small amount of the chemical product to a discreet area of the skin, typically behind the ear or on the inner elbow, to check for any allergic reactions. Given that chemical hair treatments often contain potent ingredients, it's essential to determine if the client has any sensitivities that could lead to adverse reactions, such as itching, redness, or swelling. By conducting this test, cosmetologists can help ensure client safety and prevent any serious complications that could arise from an allergic response during or after the treatment.

Other options, such as not washing the hair for a week, arriving with styled hair, or notifying about product use, while they may have some relevance to the treatment process, do not directly address the crucial concern of allergic reactions, making them less critical than performing a patch test.