Summarize:

When you get a new product, such as a chemical peel or even just a facial cleanser, it's important to patch test it first, especially if you have sensitive skin. Patch testing just means you use a small amount on part of your skin to see how you react. In other words, you don't want to rub something all over your face or body and break out in hives everywhere. It's best to limit the area at first. You should also skin test other products, such as shampoos, conditioner, and hair dyes. Basically, if you have sensitive skin, you should skin test any product that comes in contact with your skin. Your inner arm is a good place to test because generally that skin is fairly sensitive. In addition, it won't be too noticeable if you do have a reaction. If the product burns or causes an immediate reaction, wash it off as quickly as possible. If it's a product such as lotion, leave it on your skin. If it's a product like a chemical peel that's meant to be rinsed off, rinse it off at the appropriate time. Wait a full day to see if you have a reaction to the product. A reaction could be your skin turning red, welting up, or showing a rash. You could also have scaliness or oozing skin. Another symptom is itchiness. Next, move to skin testing a more sensitive area. This time, pick a place where the product will be used. For instance, if you are using a facial cleanser, try it just under your ear in a small spot. The reason you need to test it again is because it may affect a more sensitive area of skin even if it doesn't affect your arm. Once again, wait for a full day to see if your skin reacts to the product. If it doesn't, you should be fine to use it.
Understand product skin testing. Put a small amount on your inner arm. Wait for 24 hours. Test a more sensitive area. Wait another day.