"There are two types of sunscreens: chemical and physical. Physical sunscreens behave more like clothing, blocking the suns rays without adding controversial ingredients to the skin (this is the largest organ of the body and absorbs anything you put on it). Chemical sunscreens essentially absorb the rays and stop them from going into the skin — the higher the SPF, the higher the chemical level in the product.
"Physical or mineral sunscreens contain natural ingredients that create a barrier for UV rays. Physical sunscreens can actually last longer than chemical ones. It's not the chemical ones go bad, but rather that the chemicals lose their potency after time, making the product ineffective. Because chemical sunscreens fall under the pharmaceutical umbrella and not the cosmetic one, they tend to have expiration dates instead of the time-after-opening symbol, so look at each individual product to be sure." — Silliker
