HOW
If you’re on top of an 8000 m peak near the equator, at midday, do you need an SPF 10000 sunscreen? No, because they don’t exist, and that’s not how SPF works.
SPF does not indicate how ‘strong’ the sunscreen is. It is not a magic number. Bigger is not necessarily better. SPF is a comparative number that tells you how long it will take for your skin to burn when you use the sunscreen as compared to when you don’t use it. For example, while using an SPF 30 sunscreen, your skin will take 30 times longer to start turning red as compared to if you weren’t using sunscreen.
Most experts recommend SPF 30, and say anything above that is almost the same. It blocks 97% of the sun’s rays. If you’re spending a lot of time in intense heat, you can upgrade to SPF 50, which blocks 98% of the sun’s rays.
The secret to sunscreen use is actually liberal reapplication. Most users don’t reapply as frequently as they should, and they use much less than they should.
Reapply every two hours, and every one hour between 12 to 2 pm. Experts recommend a full tablespoon per application on your face and neck. 30 grams or three tablespoons for your full body.
Don’t let the clouds trick you into thinking you don’t need sunscreen, or you need less of it. UV rays penetrate cloud cover with little difficulty.