There are two categories of UV light — UVA and UVB — that we consider concerning sunscreen. UVB causes sunburn, and UVA has more long-term damaging effects on the skin, like premature aging.
SPF, or Sun Protection Factor, is a measure of how well a sunscreen will protect skin from UVB rays, the kind of radiation that causes sunburn, damages skin, and can contribute to skin cancer.
I highly recommend you to use sunscreen with “broad-spectrum” (which means it covers both UVA and UVB rays) with at least SPF 30. Why so?
Let me give you an example: SPF 15 only screens out about 93% of UVB light, while an SPF 30 filters out 97%. Those with higher SPF don’t offer significantly greater protection and benefit. For example, SPF 50 blocks 98% of UVB light (just a difference of 1% more).
Sunscreens should be applied 30 minutes before sun exposure to allow the ingredients to bind to the skin thoroughly. Please know that any sunscreen will be effective only if it’s reapplied at least every two hourly.