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.

Photo-degradation of sunscreens and skin damaging

soleWe 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. Possibly… does it always go this way? Unfortunately it doesn’t. For this reason we wish to discuss here a bit more on the photostability of sunscreens. Although this aspect is often overlooked (guess why?), 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:

  1.    they can go back to the lower energy state by loosing 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;
  2.     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);
  3.     they can transfer the excess energy to another molecule by “hitting it”, i.e. they behave as photo-sensitizers.

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).
If the sunscreen degrades, it most commonly produces free radicals and other dangerous species. 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, it is very important to always choose high quality products after careful reading of their label.
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.

photo-degradation_EN

Variation of the UV-Vis absorption spectra of two examples of sunscreen formulation, after exposure of the formulation to the same “dose” of sunlight (UV irradiation). The upper graph refers to a formulation with NON photostable sunscreens, which evidently shows a reduction of the spectrum after UV exposure, meaning a marked decrease in the SPF. The lower graph, instead, shows the behavior of a (BeC) formulation with photostable sunscreens: only negligible variation of the spectrum is displayed after irradiation implying no loss of SPF and more durable performance.

The fact that a sunscreen formula looses its SPF on exposing to sunlight depends on the phostability 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 us 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 this is not the case: 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!!! Secondly, 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.

UV filters in sunscreens: things we should know on their nature and functioning

Sunscreens or UV filters are natural or synthetic compounds that are included in cosmetic formulations to protect the skin from damages caused by solar exposure. In the case of specific sunscreen formulas (to be used for sunbathing), they are responsible for the Solar Protection Factor, SPF, whose value indicates the degree of protection the formula will guarantee to our skin, thereby avoiding erythema and other damages like photo-aging. What do UV filters exactly do and how do they act? The principle is very simple: UV filters reduce the amount of solar radiation reaching the surface on which they are applied, for instance skin surface. The difference among the many UV filters lies in the mechanism by which they achieve this goal and they can be grouped in two categories.

Physical and chemical filter - effecto on skin

Mechanism of action of physical and chemical filters on the skin

Physical filters reflect (back) a portion of solar radiation, letting only a limited fraction of light cross them and reach the skin. Therefore, they do not interact with solar radiation and they are not altered by it at all. Chemical filters, instead, absorb a portion of solar radiation, and use it to achieve a higher quantum energy state. Immediately after, such excess energy is released to the environment in the form of heat, thereby making the filter ready to absorb solar energy again. The different mechanisms of action are summarized in the picture on the left. In both cases, the amount of solar energy allowed to pass – which is not reflected nor absorbed – depends on the amount of filters applied on the skin, which determines the solar protection factor.
Both types of filters offer advantages and disadvantages: physical filters have the advantage of being completely stable and not being damaged by solar radiation. Furthermore, compounds like Zinc oxide are totally inert and are very safe for skin. Their disadvantage lies in the difficulty to reach high SPF values without causing the so-called “white effect”. In order to reduce this effect, it is possible to use them in micronized form, which is much more precious (BeC uses this form in all sunscreen formulas). However, even micronized forms do not allow reaching high protection without becoming “visible” on the skin. Therefore, in high protection formulas they are usually combined with chemical filters. Chemicals filters, infact, have the advantage of being very effective and of offering excellent performance while being completely invisible. They disperse very well in the formulation affording a more homogenous protection (corresponding to a more even tan!); however, since they absorb the solar energy, they are subjected to the risk of photo-degradation, or they could undergo photochemical reactions.
At this point, let’s better clarify a very important aspect. Often, the expression “chemical filter” is confused with “synthetic filter”, i.e. artificial, while it is generally believed that physical filters are natural. This is not necessarily true! Physical filters are normally inorganic, i.e. minerals, and normally they are natural, but they can as well be synthetic, i.e. artificially produced.

Oryzanol

gamma-oryzanol: a natural “chemical” filter extracted from rice

On the other hand, the most common chemical filters are often synthetic, i.e. man made; however, there exist also examples of natural chemical filters. Among the most impportant ones, there is certainly gamma-oryzanol, which is extracted from rice bran (see picture on the left), and other examples are cinnamic acid, typical of cinnamon but found also in Brassica vegetables (cabbage, broccoli, etc) and in shea butter, carotenoids(e.g. from carrots, tomatoes and several red-orange fruits), Vitamin E and many more. These compounds have also the advantage of possessing additional properties (e.g. antioxidant, soothing, calming, anti-age); unfortunately, however, they are nor as effective as synthetic chemical filters. BeC uses natural chemical filters when high SPF values are not requested (e.g. in Huile SolE’), and combines natural and synthetic chemical filters when high protection is aimed at.

The take home message is: don’t be fooled by the words! The adjectives Chemical or Physical, for filters, indicate their mechanism of action not their origin. The origin can either be natural or synthetic, but the most important aspect is their stability toward solar radiation, which is fundamental for our safety under sunlight.