SPF: what's behind these three letters?
The letters SPF mean “sun protection factor” and indicate the protection the applied product offers your skin.
Generally speaking, the SPF number indicates the time your skin can be exposed to UV radiation without it burning.
How is it calculated?
SPFs depend on skin phototype. Consider the case of skin which takes 10 minutes to turn red (MED: minimal erythema dose) at a certain level of radiation. If an SPF of 30 is applied to it, it can remain in the sun for 300 minutes without burning. In other words, SPF 30 provides 5 hours of protection.
As you can imagine, calculating SPFs depends on many factors including the characteristics of the skin, how it is protected (tanned) at this time of the year and solar radiation, which in turn depends on latitude, weather conditions and hours of impact or exposure.
So, would a high SPF (e.g., 50) be enough to spend a whole day at the beach?
The short answer is no. If we use the above-mentioned MED (10 minutes), an SPF of 50 would give us 500 minutes of exposure, i.e., more than 8 hours. But the calculation isn't as simple as that.
Remember that not all sunscreens are the same or protect the skin equally. We know that SPF products, basically sunscreens, have to be perfectly absorbed in order to provide these protection values. Calculations are very arbitrary and depend on many factors, such as the skin’s absorption capability, thickness and intrinsic characteristics, as well as temperature, environmental conditions and contact with water.
Factors that influence SPF
One of the factors that most influences the choice of SPF is skin phototype, because the less natural skin pigmentation the higher the SPF product you should use.
Remember what skin phototype you have to know which product will be most useful for you and the health of your skin:
- Type I: always burns, never tans.
- Type II: usually burns, tans minimally.
- Type III: sometimes mild burn, tans uniformly.
- Type IV: burns minimally, always tans well.
- Type V: very rarely burns, tans very easily.
- Type VI: Never burns, never tans.
According to the Sun Protection Guide endorsed by the Spanish Health Ministry and Spanish Cancer Association, the following are very useful recommendations depending on skin type and UVI (ultraviolet index):
But to be honest, SPF calculation is tricky. Depending on the product, it could have been obtained using different values and vary from one pharmaceutical lab to another in accordance with the amount of product applied per surface unit or type of light source used, meaning that products are not always comparable.
Another concept often linked to sunscreens is the difference between SPF, which measures protection against UVB rays, and PPD (persistent pigment darkening), the index that measures the ability to produce lasting or immediate tanning and which indicates protection against UVA.
