Sun Protection: What is SPF and what does it mean?

Although useful, this measure remains "imperfect". This globally accepted standard primarily reflects the ability of a sunscreen to prevent certain UV rays from "burning" the skin. More specifically, it refers to the amount of solar energy required to produce a slight redness on protected skin, in accordance with the ideal conditions for sunscreen use, compared to that required on skin completely devoid of sun protection. However, it does not take into account the effects of the remaining UV rays, which can also contribute to the premature aging of the skin (photoaging) and the formation of skin cancers (photocarcinogenesis).

It is often read that UVB rays are the cause of sunburns. However, erythemal UV rays are composed of 85% UVB (between 290 and 320 nm) and 15% UVA-II (between 315 and 340 nm).

Contrary to popular belief, the SPF does not represent the duration one can stay in the sun without risk. Indeed, many consumers think that if a person starts to slightly redden after 10 minutes spent outside without sun protection, applying an SPF 50 sunscreen would theoretically allow them to stay in the sun 50 times longer before getting a sunburn. This means they could stay in the sun for 8 hours without getting sunburned (10 minutes * 50 = 500 minutes) and about twice as long as with an SPF 30.

However, this is a misconception. The SPF is related to a received and filtered solar energy dose, not a fixed duration. Several factors can affect the degree of sunscreen protection: the intensity of UV radiation at a given time and place, the phototype, the amount of sunscreen applied, the activities carried out (swimming, etc.), the intake of certain medications, or even the frequency of reapplication. That's why even a product displaying a high SPF must be applied generously and regularly to maintain optimal protection.

In addition to clearly labeling the SPF, sunscreens must also provide a description of the level of protection they offer (low, medium, high, or very high) on the same label.

But then, what does the number visible after the SPF correspond to?

The number following the SPF label indicates the percentage of erythemal UV rays blocked. The higher it is, the greater the percentage of UV light it blocks. Indeed, an SPF 50 means that 1/50 of the erythemal UV rays will reach the skin, assuming that the sunscreen has been correctly applied, or that the user is protected 50 times better than without sunscreen. However, many believe that a high SPF offers twice, or even three times the protection of an SPF 30, thus giving a false sense of security.

In reality, the increase in protection is not linear. An SPF 30 already provides nearly optimal protection when applied correctly, blocking about 97% of erythemal UV rays. An SPF 50, on the other hand, blocks about 98%, and an SPF 100 about 99%. Thus, the difference in protection between an SPF 30 and an SPF 50 is minimal (about 1%) and does not provide substantial protection. However, although the gains in protection are marginal, they can become significant for sensitive skin, light phototypes, or in contexts of frequent or prolonged exposure.

Therefore, it is crucial not to rely solely on the SPF, but also on the product's formulation, particularly the "broad spectrum" label that includes long UVA rays, as well as proper application and regular reapplication to maintain effective sun protection.

It's important to remember that no sunscreen can block 100% of erythemal UV rays, thus providing complete protection from sunburn and the resulting skin damage.

How is the SPF value of a sunscreen determined?

To determine the SPF of a sunscreen, rigorous tests in vivo in the laboratory (ISO 24444) are conducted on a non-tanned skin area of volunteers (e.g., inner part of the arm). The sunscreen is then generously applied (2 mg/cm2 of skin), and then exposed to an artificial source of UV rays (solar simulator) for approximately 20 minutes.

In Europe, the Sun Protection Factor (SPF) is accepted if it is determined based on at least 10 subjects.

After 24 to 48 hours, the exposed areas of the skin are examined, and the fraction of UV radiation that will produce a slight redness with and without sun protection is measured. The SPF value is determined by dividing the amount of UV rays needed to cause a slight sunburn on protected and unprotected skin.

Although it is the current SPF test, several test methods in vitro will soon be implemented: the in vitro "double plate" method (ISO 23675) and the hybrid diffuse reflectance spectroscopy method (HDRS - ISO 23698) are in the final phase of international standardization. They are expected to become the first ISO standards published for SPF tests in vitro.

While laboratory testing is fairly effective in determining the SPF of a sunscreen, it only provides an indication of its effectiveness against erythemal rays and not against UVA rays, which penetrate deeper.