Updated January, 2020
Chemical Peels are one of the most popular spa treatments available and they can do wonders for the skin in the right situations. As a former medical spa owner, I oversaw our nurses and aestheticians performing a number of different types of peels including glycolic acid, lactic acid, salicylic acid, AHA, and more.
Peels work by removing dead skin and causing the skin to react with the chemicals used to provide the desired result, which might be reducing acne or skin oiliness, reducing fine lines, smoothing out the skin, etc. The percentage of ingredients and the ingredients themselves vary widely by brand and by the aesthetician.
Each person has their own special "formula" they use to mix the chemicals into a combination that will work best for the customer, but this also allows for error and potential damage to the skin. Anytime you place strong chemicals on the skin, there's always a chance things can go wrong, and for people with darker skin, a chance that they will get hyperpigmentation (lighter areas of skin).