Sunscreen isn't food (thankfully), so in our minds, it doesn't make sense that it expires. Eggs expire, not sunscreen. And that's where you're wrong. In fact, sunscreen does expire. Grab your sunscreen bottle from all the beach stuff, and you'll note there is probably a date stamped on to the side or top or bottom—somewhere. That date is the expiration date of your sunscreen, and it's important to heed that expiration date and use the sunscreen before that date or throw out the sunscreen if the expiration date has passed. You may see the expiration date, but you may also be asking why. Why does sunscreen have an expiration date? The answer is quite simple.
Sour cream expires because the ingredients in the sour cream grow too old. Fruit molds (hence expiring) if it is left out too long, because it passes the time where it could be useful. Milk expires because it passes its time period for being healthy. Sunscreen is the same way and follows the same simple expiration principles.
The ingredients in sunscreen can grow old, losing the effectiveness, thus, expiring. Once the ingredients are expired, it's useless—the sunscreen won't work anymore, won't protect you from the sun. If the ingredients aren't good anymore, then it is useless to you; sunscreen is supposed to shield and protect you from the harmful effects of ultraviolet radiation, the radiation from the sun that causes burns and even cancer. That is what sunscreen is supposed to do, and if the ingredients expire, then the sunblock won't work, won't protect you like it's supposed to.
If you were unaware of the sunscreen expiration date reality, go to the cupboard or closet where you keep your sunscreen and take an inventory of all your sunscreen. Check the dates on the bottles, and if there is no date, try to think back to when you bought it. Did you buy it last summer or five summers ago? If there isn't a date on the sunscreen bottle when you buy it, it doesn't mean that the sunscreen won't expire; rather, you can estimate that the sunscreen will be good for about three years. After three years, throw it out and buy some new sunscreen. One way to tell if sunscreen is expired is if it is particularly runny; runny sunscreen often indicates that the sunscreen doesn't work anymore, isn't up to perform like it used to. If you want your sunscreen to be effective, then you need to make sure that it's within its expiration date, and then you can enjoy the protection of sunblock all the time!