Sunscreen protecting factors or the sun protective factor (SPF) is the laboratory measure of its ability to block ultraviolet B (UVB) rays. The relation between sunscreen protecting factors and the amount of UVB rays blocked is not linear. As researches have shown an SPF 15 blocks about 93% of UVB rays, an sunscreen protecting factors 30 blocks about 96%, and an SPF 60 blocks about 98%. And no sunscreen can offer 100% protection from the sun.
In reality sunscreen protecting factors may differ from actual protection that the user is obtained while using a certain sunscreen. It depends on a variety of factors: amount of sunscreen applied; the frequency of re-appliance while in the sun (it should be reapplied at least once in two hours); time of day and season; the environment and percentage of UV reflected (snow, sand, water), user skin type, and others. It is important to know that sunscreen protecting factor is indicating only the protection from UVB rays. But the UVA rays are also harmful.
Scientific researches have demonstrated that some sunscreen products with SPF 30+ offer a lower protection from UVA rays than 15 SPF rated products. Products that offer both, UVA and UVB protection are usually labeled ?broad spectrum?. The UVC rays cannot pass through the ozone layer so there is no need to protect from them. Some sources state that SPF means how much longer you can stay in the sun without getting sunburned. It is not such a good definition as it is not recommended to use sunscreens to elongate sun exposure. Any sun exposure with or without sunscreens is harmful for our skin. Sunscreens are just increasing the protection and cannot eliminate the damage completely.
So, from this point of view, if without any protection your skin will be mildly sunburned in 10 minutes of sun exposure, using a SPF 15 sunscreen will allow you stay 150 minutes in the sun before getting burned. But these numbers are calculated for an average person in average conditions. So, the real protection we get depends on a lot of factors. Different types of skin differently react to sun exposure.
There are 6 types of skin. Type 1 are people with very fair skin, light eyes, usually freckles, and their skin always burns and never tans. Type 2 includes skin that usually burns and tans with difficulty. Skin of 3rd type sometimes burns and sometimes tans. Type 4 minimally burns, always tans. Type 5 rarely burns and tans profusely. And type 6 never burns and tans deeply (dark skin, dark eyes).
We should remember that children skin is far more sensible and must be obligatory protected from the sun. Infants under 6 years old should not be exposed at all to direct sunlight and the use of sunscreens is not recommended. Sunscreen protection factor is decreasing as you are sweating or getting into water. Waterproof sunscreens are more recommended by they still must be reapplied every two hours to be effective. Weather condition, time of day and season, altitude and latitude can also influence sunscreen protection factor. The UV rays are more harmful in sunny days, at great altitudes, at tropics, in hot seasons and in the hottest hours of the day which are between 11 a.m. and 3 p.m.
Still, sunscreen should be always worn either it is a sunny or a cloudy day. If you want to keep your skin healthy and prevent its premature aging choose a SPF 15+ sunscreen and make sunscreen use a habit.