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To recap, so far both sides have one point each, with inorganic sunscreens being less likely to trigger a negative reaction, and organic ones being more cosmetically elegant. In this post, we will analyze the two families of sunscreens in terms of the following characteristics:
- Photostability: How much of a sunscreen (active) remains effective after it’s been exposed to sunlight for a sustained length of time. What about in combination with other sunscreens?
- Permeability: How much of a sunscreen comes into contact with living tissue, or in other words penetrates past the stratum corneum, after it’s been applied to the skin.
- Photoreactivity: What short-term negative reactions occur after a sunscreen is exposed to sunlight for a sustained length of time and penetrates past the stratum corneum.
***In this series, I will refer to inorganic sunscreens as iOSs (not Apple, mind you) and organic sunscreens as OSs. While there are many individual compounds in each group—particularly OSs, for the purposes of this post, I will only refer to individual iOSs and OSs as parts of their respective groups. Complete and comparative ingredient profiles will not be seen in this series. Furthermore, I will attempt to discuss technologies that are more relevant and reflect the sunscreen technologies and tendencies of the current market. For example, while para-aminobenzoic acid (PABA) has many documented negativities, it will be ignored in our discussion, due to the fact that it is hardly used nowadays in sunscreen formulations.
Most of us know that OSs provide protection by absorbing UV light, and then transforming that light energy (photons) into some other form of energy such as heat. Now, this “transformation” of energy has two consequences for OSs: reversible or irreversible structural reconfigurations. Reversible reconfiguration means that a particular OS will cycle between various photoisomerizable forms such as the trans- and cis- structures, in addition to intermolecular hydrogen transfer, depending on the OS. As they simultaneously convert back and forth, they will reach chemical equilibrium, thereby achieving photostability (1). Irreversible reconfiguration means that a particular OS will break down into complex byproducts, thereby rending it ineffective and incapable of functioning as source of UV protection.
Now these two pathways are not mutually exclusive in respect to any particular organic UV filter. Both occur in every OS. Therefore, the photostability of an OS depends on how much more often it undergoes reversible, rather than irreversible structural reconfiguration. Herein lies the problem: as with most topics in skin care, the spectrum of the photostability (of OSs) varies considerably. Most OSs, such as avobenzone, are inherently unstable and degrade (at different rates) upon UV exposure. To combat this issue, most manufacturers combine various OSs together to “ensure” photostability. Unfortunately, these combinations also vary quite drastically in terms of photostability, particularly when exposed to radiation in the UVA range (2).
To make things even more complicated, OSs have been shown to be largely affected by the vehicular bases and solvents used; the formulary techniques employed such as encapsulation via lipid nanoparticles, which consequently can alter the amount of protection by more than two-fold or 200% (3); and the presence of other OSs and iOSs. For example, multiple studies have confirmed that the combination of octinoxate and avobenzone increases the rate of photodegradation faster than either compound does so alone (4). It was further shown that this occurs regardless of what other UV filters are present.
As you can see, the photostability of OSs is quite heterogeneous! While the FDA does have regulatory oversight over whether or not these UV filters are effective, there exists no comprehensive testing when it comes to how STABLE these compounds are in various combinations (with other UV filters and ingredients such as antioxidants); formulations (with other solvents and carrier fluids); and with different preservative systems!
So the most obvious question is that: Why would an average consumer want to deal with all this INSANENESS, when the other group of sunscreens embodies the alluring concept of simple, effective, and stable protection against UV light? Or do they…?
Like as before, most of us know that iOSs function by reflecting and scattering (UV) light, thereby protecting the skin via physical rather than chemical mechanisms. It is also known that iOSs are inherently photostable, because like a mirror, they simply rest on the skin inertly, and “bounce” light away; there is no change in state or configuration. So I guess it’s pretty obvious that OSs are the loser in this round, right?
Before we jump the gun, let’s take a closer look at the literature. Before the advent of nanoparticles, it was completely true that iOSs were remarkably photostable; they easily outlasted the onslaught of UV light. Their sheer size and “strength” alone allowed them to stand like mountains: unshakable pillars that stood resolute; unyielding to the petty and whining whims (winds) of UV light. Okay, all literary BS aside, these “mountains” were unfortunately quite visible on the skin, as discussed last week.
So along came nanotechnology, which allowed iOSs to be reduced to nano-sized particles, while being almost invisible on the skin. However, at such minute sizes, these inorganic filters no longer have the same chemical and physical properties when compared to their larger counterparts. With the change in size, their behaviors in fluid dynamics (meaning that the iOSs will be more largely affected by the vehicle), and photostability are no longer the same. If the particles are too small, they lose some of their scattering effect and therefore, do not give as good protection as larger particles (5). Note that study (5) also shows that this occurs more so with titanium dioxide (TiO2) nanoparticles, than zinc oxide (ZnO) nanoparticles.
But what does this mean, that iOSs too are undependable? Well, the good news is that, while TiO2 and ZnO nanoparticles’ photostability has drawn some concern in terms of efficacy, with the latter more so than the former, many iOSs don’t use nanoparticles. They use micro-sized particles, which are also quite transparent but are definitely too large to penetrate into the skin. The bad news is that, the consequences of nanoparticles’ ability to absorb UV light are related more to their photoreactivity and permeability, which will be discussed next, rather than their photostability. So don’t sigh in relief just yet.
For now, it can be said that while not completely photostable, iOSs are generally more dependable than OSs, because not all iOSs utilize nanotechnology.
iOSs = 2; OSs = 1
The skin’s topmost layer, the stratum corneum (SC), consists of non-nucleated corneocytes that act as a barrier to prevent external contamination. It is this layer that prevents many dangerous chemicals from being absorbed systematically. However, it’s not infallible. In this section, we will discuss what sunscreens get past the SC, and how much of them get past.
As OSs are mostly lipid-soluble, they more easily penetrate past the SC lipid bilayers and come into contact with the living tissue of the lower epidermis. While oxybenzone appears to be the biggest offender (6), other OSs have been implicated, albeit less frequently depending in-part on the vehicle used (7).
iOSs on the other hand, are too large (even in their nano-sized incarnations) to significantly penetrate the stratum corneum of intact skin (8). However, some studies have shown that damaged skin (such as sunburned or inflamed skin) can allow significant penetration (9), so the risk factor of nano-sized iOSs cannot be overlooked.
However, for intact skin (which I hope most of us have), iOSs appears to be less systematically absorbed than OSs.
iOSs = 3; OSs = 1
In this section, we will talk about what happens when the irreversible reconfigurations (as discussed above) occur.
The photoreactivity of OSs typically refers to their ability to directly generate free radicals. Multiple studies including this one (10), indicate that OSs are capable of generating reactive oxygen species (ROS) after being exposed to UV light. Despite that most of this only occurs when the OSs are in contact with nucleated epidermal keratinocytes (meaning they traversed the stratum corneum), because it was shown in the previous section that some OSs do get past the stratum corneum (due to ease of solubility, and choice of vehicular base), this ability cannot be ignored. Other studies have naturally shown that this generation of reactive oxygen species can induce lipid peroxidation (lipid peroxidation is a well-known indicator of significant ROS activity) (11), depending in-part on the OSs used. Furthermore, the photosensitizing properties of organic UV filters in conjunction with this ability to generate free radicals, can also reduce the antioxidant activity of enzymes such as superoxide dismutase (12), if they come into contact with viable skin cells. So a sunscreen’s photoreactivity goes hand-in-hand with its permeability. Please keep in mind that using any sunscreen is still better than using none at all.
Now, several studies have surprisingly shown that iOSs are not completely inert, with TiO2 being able to absorb up to 70% of UV light in aqueous solutions (13), making them capable of catalyzing or indirectly inducing oxidative damage. And while coated forms are more stable, even they are not completely so (14). How is that possible you ask? It’s because there is such a wide array of factors to consider when it comes to nanoparticles, that coating can’t guarantee a lack of photoreactivity. These factors include: particle size and morphology (sphere, tube, rod, wire), dispersion patterns (aggregation, agglomeration), use of coatings (thickness, type, purity), and doping (contamination by other metals). All of these can affect the photoreactivity profile of iOSs; it can be just as infuriatingly confusing as that of OSs. Thankfully however, in combination with their permeability profile, it appears that iOSs are marginally less photoreactive than OSs.
While iOSs appear to once again slip in for the win, the most important concept to learn from this section is that, because both groups of sunscreens are capable of generating free radicals, the use of antioxidants with sunscreens is an absolute must (15)!
iOSs =4; OSs = 1
While the iOSs have pulled ahead by three points (4-1), keep in mind that just because iOSs win a category, doesn’t mean it has no fault in that given category. The battle isn’t over quite yet. All three of the characteristics discussed today (photostability, permeability, photoreactivity) were in preparation for next week’s post of toxicity, which will be about what long-term effects OSs and iOSs may have on the body. So check back next week (and the two week after that)! Actually, just subscribe to FutureDerm! Haha!
About the author: John Su is an established skin care expert and aspiring dermatologist. He also runs a blog, The Triple Helix Liaison, dedicated to providing unbiased, meaningful, and insightful information about skin care. For his full bio, please visit our About Page.