Great Read on the "Anatomy of a Sunscreen"

Intentional vs. Incidental Exposure

Products that contain sunscreening ultraviolet (UV) filters intended to provide sunburn protection can be divided into two basic categories. The first is when there is “intentional” exposure to sunlight for extended periods of time. This is typically associated with recreational activities such as swimming, hiking, sports activities, etc., with exposure to sunlight during the period of highest sunburn potential—10 am to 4 pm. These activities often induce sweating and may also include exposure to water for a significant amount of time. Sunscreen products for these activities are designed to resist removal by sweat or water by incorporating polymer substances and emulsifiers that lock the sunscreen filters onto the surface of the skin.

- See more at: http://www.skininc.com/skinscience/ingredients/Anatomy-of-a-Sunscreen-370696051.html#sthash.h5GM733K.dpuf