Sunscreen is no.1 product to prevent photo aging and skin cancer (UV is the top reason of skin cancer). You should wear sunscreen all year round, even on cloudy days, because up to 80 percent of the sun’s UV rays can pass through clouds (American Academy of Dermatologists). There are two types of sun rays: UVB and UVA. UVB rays cause immediate damage to your skin. They are responsible for tanning or burning your skin. UVA rays are responsible for aging your skin and are able to reach your skin through clouds, smog or glass.
WHY TO REAPPLY
Modern chemical sunscreens do not absorb (soak into the skin). Instead, they adsorb (stick to the surface of the skin). The reason why it’s important to reapply is because 1) some sunscreens are not photostable, meaning they lose their ability to protect from UV rays after a while, and 2) you may sweat or rub your sunscreen away at some point.
HOW OFTEN TO REAPPLY
You only have to reapply after 2 hours of sun exposure. So if you’re indoors most of the day you don’t have to reapply at all. If you apply it in the morning and are indoors most of the day or getting to work or class, you don’t need to reapply. If you are going to be outside a lot (gardening, hiking, beach trip, cycling, running errands, etc.) then you should be reapplying. Use spray sunscreen to reapply over makeup.
Sunscreen is vital to protecting your skin from photo aging and skin cancer. Your lifestyle ultimately dictates how often to reapply sunscreen. Put on sunscreen only in the morning if you spend most of the day indoors. Reapply sunscreen every 2 hours, when you hike, garden or work outside. For convenience, use 2-in-1 products, like moisturizer+sunscreen.