SPF is a measure of UVB protection (does NOT indicate UVA protection) that refers to how many times longer a person could be in the sun before getting burnt compared to being outside without sunscreen. Suppose that a particular person would normally burn after 15 minutes on an average sunny day. If this person applied a sufficient amount of SPF 30, then she would theoretically develop a burn after 7.5 hours (30 times longer than with no sun screen). However, this does NOT mean that she is protected for 7.5 hours, as damage to the DNA in the skin occurs well before the skin reddens. The SPF rating is NOT intended to be used to calculate how long someone can or should be in the sun and if misinterpreted can give a person a false sense of security.
How much protection are you getting with each SPF?
SPF 5: 80% of UV radiation is screened
SPF 15: 93% of UV radiation is screened
SPF 30: 97% of UV radiation is screened
SPF 50: 98% of UV radiation is screened
SPF 100: 99% of UV radiation is screened
The numbers above demonstrate that there is not a very meaningful difference between the higher SPFs. The bottom line is that no matter the SPF, sunscreen must be applied at least every 2 hours of sun exposure to adequately protect the skin. You don’t need to get burned to experience the harmful effects of UV radiation.