The number next to SPF is a measure of how well the sunscreen will protect your skin against UVB rays, which cause sunburning and skin cancer.
E.g. if your skin burns in the sun after 1 minute, SPF 15 will multiply 1 minute by 15; so under normal circumstances SPF 15 will not let your skin burn till 15 minutes.
Whether skin burns in 1 minute or 20 minutes depends largely on the complexion; lighter skin burns quickly.
Laboratory tests are conducted on human volunteers using specialised equipment to determine the time for skin burn without and with application of SPFs.
Another difference in different SPFs is percentage of UVB rays they block.
E.g. SPF 15 blocks 93% UVB rays; SPF 30 blocks 97% and SPF 50 blocks 98%.
After SPF 50, the difference is very small and hence a lot of countries insist that anything above SPF 50 is marked as SPF 50+ and no number is given.
The sunscreens are made of two kinds of substances.
The first kind absorbs the rays, changes them into heat and releases them from skin as infrared rays; these kinds are called chemical sunscreens and they get mixed with the skin.
The second kind has material such as zinc oxide or titanium dioxide that reflect the rays; these kinds are called physical sunscreens and they form a whitish layer on top of skin.
While there is no measure for protection against UVA rays, there are ingredients such as Avobenzone and Octocrylene that protect against UVA as well.
Sunscreens that provide protection against UVA and UVB usually have ‘Broad Spectrum Protection’ written on them (these are the ones you should buy).
While UVB causes sun-burning and skin cancer, UVA causes skin ageing and pigmentation.
