Sunscreens or photoprotectors are all those products (creams, lotions, milks, etc.) that are applied to the skin in order to protect it from the harmful effects of ultraviolet A (UVA) and/or ultraviolet B (UVB) rays. This is possible because their composition contains substances called filters, capable of slowing down the action of radiation by absorbing, scattering or reflecting it.
The SPF is the index that measures the protective capacity of a sunscreen against the harmful effects of ultraviolet radiation type B (UVB) on the skin and indicates the number of times that the protector INCREASES the natural defense capacity of the skin against redness prior to the burn.
There is a popular misconception that SPF is related to sun exposure time, for example, usually many people calculate that if they normally burn under the sun in 1 hour, with an SPF 50 sunscreen they will have 50 hours of protection without burning because the following calculation is made: SPF 50 x 1 hour = 50 hours of protection. This is not true unfortunately. The spf is not directly related to the TIME of sun exposure, the spf is related to the NUMBER of times that the natural defense of the skin increases, therefore, it is important to recognize the importance of the replication of the sunscreen every 2 or 3 hours (depending on the skin color) especially when we have friction with clothing, towels or other materials that contribute to the protector being removed from the skin.