Prof. Luca Valgimigli
We have previously discussed the mechanism of action of sunscreens, underlining that chemical sunscreens absorb the energy of sunlight and subsequently re-emit it in the form of heat, possibly without any alteration in the structure of the sunscreen itself.
Does it always go this way?
Unfortunately, it doesn’t and we wish to discuss here a bit more on the photostability of sunscreens. Although this aspect is often overlooked, but it has major consequences on our health.
After having absorbed sunlight energy, chemical sunscreens are in a higher quantum energy state, from which they can undergo one of three processes:
- they can go back to the lower energy state by losing energy in the form of heat (which is often not perceived by our senses), thereby making ready to start over again and absorb more solar energy;
- they can release the excess energy by breaking their structure into fragments, i.e. they degrade and form free radicals or use the energy to react with other molecules (photochemical reactions);
- they can transfer the excess energy to another molecule by “hitting it”, i.e. they behave as photosensitizers.
Ideally, sunscreens should use only the first route; however, not all the sunscreens are identical and some of them, which are less stable than others, after several absorption-emission cycles, may take the second route (we will discuss in a following post of those sunscreens that take the third route and act as photosensitizers).
What happens if a sunscreen degrades?
If the sunscreen degrades, it most commonly produces free radicals and other dangerous species, and If the sunscreen has been absorbed deeply into our skin, those free radicals can attack and damage proteins and DNA, accelerating the photo-aging processes of the skin.In high quality formulations such damages can be prevented by the abundant presence of antioxidants (e.g. vitamin E) in the formula, so to block free radicals before they can cause any damage.
Therefore, in choosing a solar formulation is important to search high quality products after careful reading of their label!
What is the relation between the degradation of sunscreen and protection factor?
There is, however, another point to take into account: as the sunscreen degrades the solar protection factor (SPF) of the product progressively decreases. And antioxidants cannot help in this reguard.The loss effect of SPF during on exposing to sunlight depends on the photostability of the sunscreens: with highly photostable sunscreens the phenomenon has negligible relevance; however, with little photostable sunscreen molecules, which are unfortunately the most common in commercial formulas, the phenomenon is very relevant, as illustrated in the graphs on the left, displaying the comparison between two real formulations: a famous commercial products (don’t ask which one) and BeC sunscreen SPF15 cream.
Many of people think that “waterproof” sunscreen formulas which can resist for several minutes of swimming in seawater would provide a safer protection for the entire day, since the product would not be washed away.
A look at the graphs clearly tells that a waterproof sunscreen formula does not guarantee safer daylong protection.
First of all, we should consider whether the sunscreen contained in the formula is photostable. Moreover, we should consider that, even if the sunscreen is photostable, during a typical day at the seaside, we dry ourselves with a towel, we roll up in the sand which we clean up by rubbing or washing our skin, we sweat in the heat or during physical activity (e.g. beach sports). All such actions end up removing the sunscreen form our skin anyway. Therefore, a high quality sunscreen formula, based on photostable components, is the ideal choice for a safer protection, but we should not forget that it is wise to re-apply the product several times during the day, particularly in the case of children.