Sunscreens Waterproof vs Water-Resistant
It is important to note the difference between waterproof and water-resistant. Water-resistant sunscreens retain their effectiveness for 40 minus in water, while waterproof products will protect you for up to 80 minutes in water. Water lets almost all UV rays from the sun through. You will want to be especially careful to wear a waterproof sun protection product when swimming or exerting physical activity outdoors.
Computing the SPF (Sun Protection Factor)
To find out how long you will be protected by a sunscreen, multiply the SPF by the amount of time you are able to spend in the sun to produce a barely pink coloration (usually 6 to 12 hours later) without using sunscreen protection. For example, if it normally takes 20 minutes of sun exposure for your skin to turn just barely pink 6 to 12 hours later without a sunscreen, and you use a sunscreening product with an SPF of 15, you will be protected for up to five hours.
20 minutes x 15 (SPF) = 300 minutes (5 hours)
You will not gain further protection by reapplying sunscreen after the five hours (in this example). It’s time to get out of the sun!