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Fair enough, but your average t-shirt is also somewhere around SPF 10, leaving you almost completely exposed even when you feel fully clothed.

It's like UV has some sort of x-ray goggle-like ability...

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SPF means "sun protection factor". SPF 10 is not "completely exposed" it's actually a very high level of protection, it means that it will take 10x as much sun exposure to lead to the same amount of UV-B reaching your skin, more or less.

SPF 10 is 90% covered, and cotton T-shirts are usually higher than that. You'll find that you can see visible light through a T-shirt at around a 10× reduction too.

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