A. Although you can keep leftover sunscreen from one season to the next, like many things (see: lunch meat, socks), it won’t stay fresh forever. The Food and Drug Administration requires that all sunscreens maintain their optimal strength for at least three years, but you should also check the printed expiration date on the bottom or the side of the product.
Also, be aware of how the product looks and smells. “Most commonly, a foul odor indicates that the preservative has failed,” says Zoe Draelos, a dermatologist in High Point, North Carolina. If the product has gone bad, it will no longer be effective at shielding UV rays and may even cause skin irritation. When in doubt, toss the sunscreen and buy a new one.
One more thing to note: A bottle of sunscreen is usually not enough to last an entire season. Every time you go out in the sun, you should apply one ounce of sunscreen (equivalent to the amount in a shot glass) to exposed areas of your body for burn-proof protection. ―Sarah Smith
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