Skin damage caused by “wrong” sunscreens: the photosensitization phenomenon

 creme solari becAs we have seen in the previous post, many sunscreens get damaged by sun radiation as they absorb it. The modest photo-stability of such sunscreens has important consequences on the safety and efficacy of sun-care products based on them. In the previous discussion we have seen that photo-instability of sunscreens causes a reduction of sun protection factor (SPF) with time. There are, however, additional interactions of sunscreens with sunlight that have even worse consequences on their safety. We will discuss them in this article.
Solar radiation contains sufficient energy to damage some molecules (M), including some molecules found in our skin, as exemplified in the equation:

M + light→ fragment-A + fragment-B

Fragments indicated as A and B, in the example above, are often free radicals that can attack other molecules, damaging or modifying them. For instance, if such a reaction occurred in our skin, damage could occur to structural proteins like collagen and elastin, contributing to the formation of wrinkles and the onsetting of photo-aging. Furthermore, it could start chain-reactions leading to erythema, other inflammatory states and even genetic mutations (skin cancer). Products containing sunscreens are meant to protect skin from all such damages, and normally they do, by decreasing the amount of UV radiation that hits our skin, i.e. acting as “filters”. But CAUTION !!! Not all the sunscreens are friends of our skin, and actually some might cause bigger damage than that they are expected to prevent. How?
Let’s go back to our previous example. In order for M to react with light, it is necessary that molecule M is at least able to absorb the light at quantum level, i.e. it is necessary the energy carried by light photons hitting M corresponds exactly to the energy gap between quantum levels in the molecule. Often this is not the case and molecule M would be perfectly “safe” if it was not for the presence of other molecules called photosensitizers.

benzofenone

Benzophenone and main benzophenone derivatives used as sunscreen in sun-care cosmetic products as well as in the protection of manufacts. The common base structure is drawn in blue

F + light F*  

then…

F* + M F + fragment-A + fragment-B

This process is well known in photochemistry and benzophenone is among the photosensitizers of broader use in industrial processes to induce photochemical reactions. Benzophenone is also the lead structure for many and, unfortunately, very popular sunscreens, widely used in sun-care cosmetics to give sun protection factor. Most common examples are depicted in the figure above. Sunscreens like benzophenone-3 and benzophenone-4 are structurally related to benzophenone and are potent photosensitizers. In case M is a biomolecule in our skin, such as collagen, elastin, an enzyme or DNA, this can be damages by exposure to sunlight in the presence of photosensitizers (like benzophenone derivatives) much more it would happen in their absence. In other words, certain sunscreens can amplify the damage to our skin caused by sunlight.

Many benzophenone derivatives are available today and 12 of them are of common use.  Those more commonly used in the manufacture of sun-care cosmetics and other goods, are listed in the following:

Benzophenone-1: 2,4-Dihydroxybenzophenone
Benzoophenone-2: 2,2′,4,4′-Tetrahydroxybenzophenone
Benzophenone-3 (or oxybenzone): 2-Hydroxy-4-methoxybenzophenone
Benzophenone-4 (or sulisbenzone): 2-Hydroxy-4-methoxy-benzophenone-5-sulphonic acid
Benzophenone-5 (the sodium salt of sulisbenzone): Benzenesulfonic acid, 5-benzoyl-4-hydroxy-2-methoxy-, monosodium salt
Benzophenone-8 (or dioxybenzone): 2-Hydroxy-4-methoxyphenyl)-(2-hydroxyphenyl)methanone
Benzophenone-10 (or mexenone): 2-hydroxy-4-methoxy-4′-methyl-benzophenone
Benzophenone-11: it is a mixture of benzophenone 2 e 6.

For these reasons BeC does not use benzophenone derivatives in sun-care products !!! BeC uses only physical filters in low protection products and, in high protection products, we use a combination of physical filters and new generation chemical filters, which are highly photostable. Here you can find more info on BeC sun-care products.

Therefore, when we chose a sun-care product, it is very important we pay attention to the label and read the composition: don’t look at the SPF value only!

We often hear that we should not expose to sunlight after having used perfumes or other products, as they can give photosensitization problems. The typical recommendation is to use only sunscreen products, but caution should be paid when we choose the sunscreen product, as even sunscreens can cause the same problems. Therefore, even if you have no particular problem of sensitivity to sunlight and think that any product will do the job, think again and don’t overlook the importance of choosing high quality sunscreens, so to make sure that the problems you don’t have will not be caused by the wrong product.