These SPF craze sounds a bit ridiculous once you realize how sunscreens work: sunscreens are just ZnO and/or TiO2 dispersed in oil. They're fundamentally just base white oil paint(in the way donuts are topologically just coffee mugs).
The core operating principle of sunscreens is that, the more your skin is covered in opaque inorganic metal oxides, the less it is exposed to harmful UV lights. There would be a lot of little tricks and additional paints to make it less irritating and less crazy looking to wear, but the point is, sunscreens fundamentally rely on opacity.
I think just knowing that lets one have a lot more of intuition about sunscreens than reading bunch of sales brochures on SPF or PA figures or wondering if the fancy ones are worth it.
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