The magazine Nature has published a study about how effective solar creams are in protecting you from skin disease. A sunscreen factor 50 seems to limit the number of melanomas and to delay its onset, but it is not able to stop it.

A sunscreen can prevent your skin to get scorched by the direct sun, especially if you are not used to such exposure. But it can give a sense of false security.

The most effective protection, according to a review of recent studies carried out in the last years, remains to avoid direct sun on the skin. The behaviour bound to give the highest risk is that of lying in direct sun suddenly and intensively. The kind of exposure you seek when you travel to a sunny destination for a short holiday.

In any case, the application of a quality sunscreen must be renewed every couple of hours. If you feel that your skin might get a sunburn, stop the exposure altogether.