SPF, Showoff Perfect Faces

According to the ISO 24444 standard, SPF is a ratio: the amount of energy required (power and duration) to cause a sunburn on skin protected with sunscreen is compared to that of unprotected skin exposed to the same emitting power.
So, if a person measured how long they could stay out in the sun without protection before getting sunburned, they would, in theory, only need to multiply that time by the SPF of the sunscreen applied to find out how long they could stay out in the sun without getting burned. For example, a person who is expected to get sunburned after 10 minutes of exposure should, in theory, have 150 minutes of sun protection with an SPF15 sunscreen.
As a reminder, this is only theoretical because to be constantly protected during the exposure time, sunscreen should be re-applied every two hours in a standardized quantity (often higher than the compliance of the standard consumer).

SPF values measure protection against sunburn, the inflammation caused by UVB rays. SPF does not indicate any UVA protection, which explains why some regulations require a minimum UVB/UVA ratio of 3 :1 on all products.

According to this table, while in theory, an SPF 100 product would allow you to be exposed for twice as long as an SPF 50 product, we only note one percentage point of additional UVB filtration. One might assume that’s a lot of extra filters for such minimal improvement. But, when one remembers the variable use of a sunscreen product from one consumer to the next and how different this method is from the ISO 24444 one, one should not buy a too high SPF and overindulge in sun exposure. The authorities have therefore recommended limiting this value to a maximum of 60, known as SPF 50+.

Another factor of variation is the type of sun exposure.

• UV rays are more intense around the equator and at high altitudes because the atmosphere is thinner.
• According to the angle of the sun to the earth, the intensity of UV rays varies according to the season and the hours of the day.
• The exposure is more intense on reflective surfaces such as snow, water, and sand.
• Light skin will absorb UV rays easier than darker skins because they are less loaded with melanin, the skin’s natural pigment that helps protect against UV rays.

As for UVA, the protocol used for SPF is not adaptable, as it would require too much energy to trigger a similar biological phenomenon with a 100% UVA energy source. Therefore, a method based on pigment response (Meirowsky pigmentation) was developed to account for the immediate brown pigmentation on the skin (due to the oxidation of melanin pigments) caused by the entry of UVA into the dermis. According to the ISO 24442 standard, the minimum pigmentogenic dose (“tanning”) on protected skin by a sun cream is measured in comparison to unprotected skin.It is a reproducible method that accounts for the photostability (good or bad) of the UV filters but only applies to dark skins (which can tan) and favors short UVA.
In vivo methods are therefore quite restrictive, as they require an expensive panel of different phototypes, and are not always reproducible between institutions. In vitro methods, on the other hand, are faster and cheaper, allowing for an initial estimate of the desired SPF. 

What about in vitro measurements? Are they simplier?
Find out these answers by reading the full article and more in the last issue of addiactive! Link below