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13

All sunscreens are physical barriers, designed to absorb or reflect UV radiation. They rub off over time, and they will rub off faster when exposed to water/moisture, most commonly from sweating or swimming. Friction (e.g. from towel-drying) also removes sunscreen. Some chemical sunscreens do break down when exposed to sunlight and require the addition of ...


10

As it is stated in this Wikipedia article, the sun protection factor (SPF) roughly describes how the time that your skin is able to protect itself from sunburn is elongated. To take the Wikipedia example: if a person develops a sunburn in 10 minutes when not wearing a sunblock, the same person will prevent sunburn for 150 minutes if he/she wears a ...


8

I'd like to add to the existing answer that while sunscreen effectiveness does decrease as it is absorbed into the body, wears off, or is washed off, high SPF sunscreens will still block most sun after the two hours. The FDA recommendations for reapplication every two hours are basically to provide maximum protection, because the level of protection does ...


2

I'm going to assume that you are employing the SODIS method of water sterilization. To sum up the details of the process, this method is where you fill plastic pop bottles, (PET), up with rather clear water 3/4 full, shake them up, and let them sit in the bright sun for 5+ hours. The mechanism this works by is by utilizing the UV radiation in sunlight. ...



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