Thank you for your valuable question. The Sun Protection Factor (SPF) in sunscreens refers to the product’s ability to screen or block out the sun’s harmful rays. An easy way to decide which SPF is best for you is this: if you use a sunscreen with an SPF15, you can be in the sun 15 times longer than you can without sunscreen before burning. The SPF number on sunscreens only reflects the product’s ability to screen UVB rays and currently there is no FDA-approved rating system that measures UVA protection levels. So look for a good broad-spectrum sunscreen with an SPF of at least 15 which contains avobenzone, titanium dioxide or zinc oxide to ensure you are protected against UVA and UVB radiation.
Sunscreen should be applied to all sun-exposed areas liberally enough to form a film when initially applied. It takes 20-30 minutes for sunscreen to be absorbed by the skin, so it should be applied at least a half an hour before going out in the sun. It is recommended to reapply sunscreen liberally after 2-4 hours in the sun. You should also always reapply your sunscreen after swimming or excessive sweating. How well the sunscreen stays on the skin after swimming, bathing or perspiring is just as important as the SPF level. A product labelled ‘water-resistant’ maintains its SPF level after 40 minutes of water exposure. A product labelled ‘waterproof’ maintains its SPF level following 80 minutes of exposure to water.
I hope this helps you make an informed decision with your next sunscreen purchase to maintain your healthy skin!