Over the past two days, we’ve elucidated the differences between various classes of hydroxy acids (HAs) and established their mechanisms of action. But we have yet to explain and rationalize away some of the misplaced stereotypes of HAs and their entailing effects. A clear understanding of these misconceptions is crucial when finding and recommending products for yourself and others.
Misconception #1: Salicylic Acid (SA) is a Beta Hydroxy Acid (BHA).
I’m sure many of you have noticed in Parts I and II, that I made a distinction between SA and BHAs. Like I said in Part I, SA is characterized by its functional groups being attached to an aromatic benzene ring, rather than a linear carbon chain like those seen in BHAs. This means that SA is not only structurally different from BHAs, but also physiologically so.
But how did this misconception arise? Most likely, SA was marketed as a BHA when actual BHA products were introduced to the industry, in order to establish a sense of validity. After all, most people wouldn’t recognize the subtle technicalities of chemical nomenclature. In this case, the confusion revolves around the fact that the carbons of aromatic (think cyclic) compounds like SA are given Arabic numerals, rather than the Greek letters given to those of non-aromatic (think linear) compounds like BHAs.
These days, while SA being categorized as a BHA is a misnomer, the misconception is so widespread that even dermatologists refer to SA as a BHA. For them, it’s SA’s many therapeutic benefits that are important, rather than semantic “correctness.”
As far as SA products go, we like Obagi Clenziderm M.D. Daily Care Cream Cleanser, which contains 2% salicylic acid.
Misconception #2: Only the concentration of an HA product matters.
While it’s easy to oversimplify skin care products and say that a 10% HA cream is more potent than an 8% HA one, several other factors contribute to the efficacy of an HA product: the pKa of the acid, the pH of the vehicle, and the vehicle itself.
pKa is the acid dissociation constant (Ka) on a logarithmic scale. Ka is a number that indicates the strength of an acid in solution (vehicle), which is determined by how much an acid dissociates. This further indicates how much of the free and salt forms of the acid develop. Only the free form of the acid causes exfoliation and is physiologically significant. Note that pKa is used in place of Ka because the latter cannot be easily compared to the pH of a solution. But pKa in itself is meaningless without Ka.
The pH of a solution goes hand-in-hand with the pKa of an acid, because the pH of a solution is irrelevant without knowledge of the pKa of the acid present. Keeping in mind that only the free form of the acid is active, here’s how pH and pKa interact:
- When the pH = pKa, that signifies that equal amounts of the free and salt forms of the acid are present in solution (50/50).
- When the pH > pKa, that signifies that more of the salt form, and less of the free form of the acid are present in solution.
- When the pH < pKa, that signifies that less of the salt form, and more of the free form of the acid are present in solution.
Depending on what you’re looking for in an HA product, one of these scenarios may be more desirable than another.
Finally, the characteristics of a vehicle are the most difficult to enumerate and measure because so many factors contribute to how well they work with any given HA. Generally, the best rule of thumb to remember when determining efficacy is that the less viscous a vehicle is, the more effective the HA will be and vice versa: the more viscous a vehicle is, the less effective the HA will be. Therefore, liquid and gel HA products tend to be more potent than lotion or cream ones.
Misconception #3: All HAs make the skin more sensitive to UV light and consequently, to carcinogenesis.
This idea comes from the logic that, because HAs induce exfoliation and thin the stratum corneum, they will allow UVR to penetrate more deeply, given that there will be less layers of dead cells to halt that UVR. Unfortunately, like most things, it is more complicated than that.
Alpha Hydroxy Acids (AHAs), particularly glycolic acid (GA) (being the smallest and therefore, most deeply penetrating HA), have been shown to indirectly photosensitize the skin and make it more prone to pigmentation. However, the thinning of the stratum corneum cannot completely account for the reasons behind such photosensitization. In fact, studies reveal that it’s more about the fact that GA smoothes the skin, which alters its ability to scatter and absorb UVR, rather than stratum corneum thickness.
That same study (2), which used cyclobutane pyrimidine dimers (CPDs), erythema, and sunburn cells (keratinocytes that have undergone apoptosis) as biomarkers for UVR-induced damage, also tested SA in addition to GA. It found that SA does not photosensitize the skin. In fact, both the vehicle and salicylic acid groups resulted in LOWER amounts of CPDs than the untreated group, which suggest that SA may actually have photoprotective characteristics. Several others studies also suggest this concept. However, the exact mechanism isn’t fully understood. Some studies indicate that it’s because SA, being a salicylate, may act as a sunscreen via direct UVR absorption. Remember that several commonly used chemical sunscreens like homosalate, are actually part of the salicylate family of compounds. Other studies indicate that the anti-inflammatory properties of SA via inhibition of the mitogen activated protein-kinases (MAPK) cascade and that of the cyclooxygenase pathway, allow less erythema (a biomarker of UVR-induced damage) to present itself.
Ultimately, AHAs do slightly make the skin more sensitive, while SA does not; it may even be photoprotective. But does that mean that over time, AHAs like GA promote carcinogenesis, and SA does the opposite? Fortunately, a study that involved a 10% GA solution, with a pH of 3.5 applied for 40 weeks, did not show any elevation in the photocarcinogenesis induced by UVR. So really, the photosensitizing effects of AHAs are very, VERY slight. But regardless, you should be wearing sunscreen anyways! That same study, also suggested a photoprotective effect of topically-applied SA, which follows the trend we saw with the other studies. Granted 4% of SA was used in the study, which is higher than what’s typically seen in OTC products. But still, it’s a positive indication.
With all of that in mind, perhaps it’s clearer now why I recommend people to use SA products during the day, and AHA ones at nighttime.
I hope you all learned something from this!
Contributing author: John Su
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