SPF indicates the amount of UVB rays blocked by a sunscreen. SPF 30 will block 97.4% of UVB rays, while SPF 50 will block 98%. As you can see, the difference is relatively small. Although the numbers increase, your degree of protection isn’t significantly better. In fact, the difference between SPF 30 and 50 is minimal—SPF 50 only blocks an extra 0.6% of UVB radiation.
Percentage of blocked UVB rays
SPF 15* 93%
SPF 30 97,4%
SPF 50 98%
SPF 60 98,3%
*Health Canada recommends a sunscreen with a minimum SPF of 15.
And very high SPFs don’t come without risks. They give people the impression that they are extremely well protected, so they tend to spend more time in the sun. However, the SPF value is not a time-dependent protection, but rather indicates the proportion of blocked sun rays. This is why we have to re-apply sunscreen every two hours, regardless of its SPF, to efficiently protect the skin.