In 2002, the FDA issued regulations regarding sunscreen that require companies to eliminate the use of the word “waterproof” as a valid claim. In truth, no sunscreen can be waterproof because it must be reapplied if you have been sweating or immersed in water for a period of time. The only approved terms for use on sunscreens, reflecting studies that prove they have limited ability to stay in place when people are in water or sweating, are “water-resistant” or “very water-resistant.” A product that is water-resistant means the label’s SPF value has been measured after application and 40 minutes of water immersion; it must keep the same SPF value to use the term water-resistant. A very water-resistant product means the SPF value on the label must remain intact after 80 minutes of water immersion.
If you are swimming or sweating, you absolutely should use a sunscreen that’s labeled water resistant or very water-resistant. Water-resistant sunscreens are formulated quite differently from regular sunscreens so pay attention to the label: if it doesn’t say water resistant, don’t use it for exercise or swimming. (Source: Photodermatology, Photoimmunology, and Photomedicine, December 2008, pages 296-300.)
For normal wear, I do not recommend daily application of water-resistant sunscreens. The acrylate-type ingredients that help keep sunscreens on when swimming or sweating also make them somewhat tacky or sticky under makeup. For regular application, when you aren’t exercising outside or taking a dip, a regular sunscreen with SPF 15 and good UVA protection is the best choice.