Hydroxy Acids Part III: Common Misconceptions of Hydroxy Acids

Skin Care

About the author:  John Su is a Weekly Contributing Writer to FutureDerm.com.  John 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.

Over the past two weeks, 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).

Obagi Clenziderm Cleanser contains 2% salicyclic acid.

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.”

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.

Don’t be scared off by the math: John explains it here.

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.  

Dr. Dennis Gross Alpha Beta Peel
Dr. Dennis Gross Alpha Beta Peel contains glycolic acid and salicyclic acid. Which makes the skin more photosensitive – and which is photoprotective? Read on to find out…

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 (1). 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 (2), 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 (3). 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 (4).

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 (5). So really, the photosensitizing effects of AHAs are very, VERY slight. But regardless, you should be wearing sunscreen anyways! That same study (5), 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! Stay tuned for Part IV, which will include product recommendations for all four different HAs mentioned in Part II. Speaking of which, LHA and Gluconolactone being relatively new, don’t have much research pertaining to interactions with UVR. All the more reason for them to be further studied! Don’t forget to enter my brush giveaway, which closes in less than a week (http://thetriplehelixliaison.wordpress.com/2012/05/07/win-an-assortment-of-brushes-v-0-02/)!


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  • @Nadia

    Ooh nice, good catch! I went in and fixed the typo.

    What I mean to say (and what the article now states) is that generally, the higher the percentage, the stronger the product. However, that’s not always the case. For example, a product with a pH of say 4 and 10% glycolic acid, would be less potent than a product with a pH of 2.5 and 8% glycolic acid. In this scenario, the 8% WOULD be stronger than the 10%, not the other way around.

    Furthermore, as I noted in the article, the vehicle is important too. Lotions and creams tend to be less effective than liquids and gels. So in the same scenario, if the 8% product was a liquid and the 10% one was a cream, the difference in potency would be even more noticeable!

    I hope that all makes sense now. Once again, thanks!

  • Hey John!

    I love your articles – sound evidence and great writing! Just a quick question; was this a typo: “…say that an 8% HA cream is more potent than a 10% HA”?

    Just trying to compare glycolic acid percentages within product lines and I was under the impression that the higher the percentage (ie 10% vs 8%) the more potent the product effects however I may be missing something here.


  • @Alyzze

    Um, I’m not sure how I can simplify the explanation about pH and pkA. It’s all right up there in the post. I mean, I’d talk about titration curves, but I think that would make it even more confusing for you.

    When the pH is less than the pKa, it means that there is more free acid available, which will allow for additional exfoliation. Also, the pKa is not a measurement of any given product; rather it’s an absolute number that describes a particular acid like glycolic acid. And you can find the pKa of any acid on compound websites, or even Wikipedia.

    Basically, the lower the pH, the most exfoliation you’ll get, assuming that everything else is the same, including the acid concentration. But remember, more exfoliation means more irritation.

    I hope that makes sense.

    Also, remember that medical grade peels are different than professional in-office peels. Anything can be medical grade, but in-office peels are the super strong ones that have to be done by a physician or qualified individual so that you don’t literally burn your face off.

  • Alyzze


    Question- In the section discussing HA’s and PH vs pKa.. can you elaborate on the results the formulations you provided would yield? Ie. Is a Ph <pKa is that better for exfoliation vs hydration ? and how would one find out the pKa of a HA product?

    As a consumer what does this mean to me when i'm shopping for HA products or even considering Medical Grade Chemical peels, as i know they are not all made the same ( ie. Bioderma vs Skinceuticals – the latter being far stronger and more effective even in the same concentration)


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