Eliminating claims of a sunscreen being waterproof makes sense since ultimately any sunscreen will wash off in water given enough time and activity. So now instead, sunscreens can claim to be water resistant for up to 40 or 80 minutes, if SPF testing demonstrates persistence of the labeled SPF for that amount of time.
But are water resistant sunscreens really water resistant?
If two different sunscreens are both labeled “SPF 30 with Broad Spectrum Protection” and one is labeled water resistant for 40 minutes and the other has no claim to water resistance, then you choose the sunscreen which has the water resistant claim, right? Well, maybe.
Lets splash a little deeper!
If you apply a sunscreen that’s water resistant for 40 minutes and you swim for 20 minutes, do you need to reapply it when you come out of the water? Hmmmmm... Or do you go back in and set your iPhone alarm for 20 minutes, so you’ll know when to come out and reapply the sunscreen, pretending you’ll hear your alarm when it goes off under your umbrella. And that assumes you really will get your full 40 minutes of protection.
Furthermore, I bet that when you came out of the water after the first 20 minutes, you toweled yourself dry, removing some of your sunscreen. And how active were you in the water? Was your level of activity in the water… Meaning splashing water against your skin… Was it greater than the activity in the tests, so you’ll wind up getting less than 40 minutes of protection?
And did you really use one to two ounces to cover your whole body because if you used less then you won’t get the labeled SPF, let alone the promised length of water resistance. Actually, using less means you’ll get the square root of the SPF, so a 30 becomes less than 6.
This is the bottom line. On a practical level, I’m not sure that water resistant labeling is all that helpful. So regardless of the labeling on the sunscreen, to help minimize cosmetic and pre-cancerous sun damage, reapply your sunscreen immediately after swimming or sweating. Only then can you be assured of the labeled, if not optimal, sun protection.