The Sun Protection Factor, or Sun Protection Factor, is a number that defines the level of protection offered by a sunscreen from the sun's harmful radiation.

Basically, an SPF rating tells you how long you can stay in the sun without burning with your sunscreen on, compared to not wearing sunscreen. For example, if you burn after the first 15 minutes without wearing and apply a sunscreen with a protection factor of 10, it will take 10 times longer to burn in the sun, that is 2.5 hours.

The SPF number is determined through in-house experiments that expose people to a spectrum of light intended to essentially replicate the midday sun, when the sun's rays are at their strongest. Choose a sunscreen that says "broad spectrum" or "full spectrum" on the label and has an SPF of at least 30, so you're protected from both UVA and UVB rays. SPF15 blocks 93% of UVB rays, SPF30 blocks 96.7%, SPF50 blocks 98% and SPF100 blocks 99%.

The protection index is chosen based on the percentage of melanin present in your skin, that is, how light or dark your skin is, whether you have sensitive skin or not, whether you exercise and sweat, because sweat and water can deplete the protection faster.

What we must point out, however, is that the estimation of these times is approximate, because you can get a burn sooner depending on the season, geographical location and weather conditions. For example, the intensity of ultraviolet radiation increases with altitude by 15%-20% per 1000m. So the higher you go, the higher the protection index you need.