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12

Around sunrise and sunset, the sun is much less intense. You would get around 5 times less intensity in the first or last hour of sunlight than in the middle of the day. Here is a graph of this effect (It's from a paper, though the paper itself is behind a paywall), and another one which also shows the effect of latitude. Therefore, while you can’t say ...


12

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

As found here: "With every 1000 m in altitude, UV levels increase by approximately 10 per cent." Percentages are tricky to work with, so here is a worked-out example. Suppose you start out at sea level (0m), and you climb all the way up to Mt. Everest's summit (8848m). Suppose also that at sealevel, you normally need to apply sun block factor 15. Then, ...


7

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 ...


5

Your mileage may vary, so know thyself. Start conservative, it is easier to back-off on protection than it is to treat sunburns. SPF 50 is not too high and you may want an even higher SPF when starting. Sunscreen that contains titanium dioxide or zinc oxide is great because they protect against both UVA and UVB. Since they are also inorganic compounds ...


3

A skin is a skin, so there's no difference if you take off your T-Shirt or your pants first time in the season. The less the body part is used to the sun, the more protection it needs. Well, the only difference is the skin on the penis, which is usually naturally a bit darker, so it already has more sun protection (which doesn't mean you can't get sunburns ...


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. ...


2

I can't help but think that this question may trolling, but here goes anyway. There is a difference between skin and skin if one includes the glans as it is mucous membrane: anything marked "for external use only" might cause unexpected irritation. Don's humorous remark about capsaicin is a good example that I'm sure more than a few men are accidentally ...



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