Lotions, deodorants, acne treatments and other products can cause skin irritation if you have an allergic reaction to the chemicals, or if the chemicals break down into potentially hazardous substances. The first potential culprit is the active ingredients in a cosmetic product. Some antiperspirants, for example, can cause an allergic reaction, making you itch or, even worse, break out in a rash. Cosmetics can contain strong active ingredients, like alpha-hydroxy acids, that can irritate or even damage the skin if they are not paired with proper use and protective products such as sunscreen. Test each new cosmetic product carefully, and use it as directed. Discontinue using any product if you have skin irritations or other adverse reactions.
Other potential culprits are additives, such as colors and fragrances and preservatives. For some people, these can also cause allergic reactions. Fortunately, manufacturers have responded to people with allergies and sensitive skin by putting out fragrance-free cosmetics and other products that are free of additives and, sometimes, preservatives.
Though some cosmetics contain preservatives to give them a longer shelf life, they can change composition over time or when exposed to heat or bacteria. For example, foundation makeup with sunscreen is limited to the shelf-life of the sunscreen (about two to three years), can lose its sunblock quality if exposed to high temperatures, and can spread bacteria if you use your finger or a reusable applicator to scoop it from the bottle. To prevent skin irritation, acne and other harmful effects, only use a cosmetic product for its manufacturer-recommended shelf-life (some have expiration dates printed on them), and follow instructions on the label for proper storage. Also, keep makeup applicators clean with soap and hot water to remove bacteria.