AHAs vs. BHA
There are two primary topical ways to exfoliate skin, either with alpha hydroxy acids (AHAs), or with beta hydroxy acid (BHA). There is only one BHA (beta hydroxy acid) and that is salicylic acid, but there are a variety of AHAs. The five major types of AHAs that show up in skin-care products are glycolic, lactic, malic, citric, and tartaric acids. Of these, the most commonly used and most effective AHAs are glycolic and lactic acids. Both of these have the ability to penetrate the skin, plus they have the most accumulated research on their functionality and benefit for skin.
What glycolic, lactic, and salicylic acids both do is "unglue" the outer layer of dead skin cells, allowing healthier cells to come to the surface. Removing this dead layer can improve skin texture and color, unclog pores, and allow moisturizers to be better absorbed by the skin. Both AHAs and BHA affect the top layers of skin, and they help to improve the appearance of sun-damaged, dry, and/or thickened skin. Sun damage in particular causes the top layer of skin to become thicker, creating a dull, rough appearance on the surface of skin (Sources: Archives of Dermatologic Research, June 1997, pages 404–409; Dermatologic Surgery, May 1998, pages 573–577). There is even research showing AHAs can generate collagen production (Sources: Experimental Dermatology, April 2003, (Supplemental), pages 57-63 and Dermatologic Surgery, May 2001, page 429.)
Because AHAs and BHA work through chemical processes, they can penetrate the skin and produce better results than cosmetic scrubs, which work only on the exposed surface of the skin. And, there is no risk that AHAs and BHA will cause you to lose too much skin. Technically, there is a drop-off rate, meaning the AHA and BHA will exfoliate just the dead or damaged surface skin and leave the healthy skin alone. This is the main reason why you will see a drop-off in performance when using an AHA or BHA product. The dramatic results in the beginning of usage (when the thickened, discolored layers of skin are being removed) seem much more impressive than the results from continued use. This is to be expected, and it is important to note that continued use of an AHA or BHA product is required in order to maintain skin’s smooth, even-toned, healthy appearance.
The primary difference between AHAs and BHA is that AHAs are water-soluble, while BHA is lipid-(oil) soluble. This unique property of BHA allows it to penetrate the oil in the pores and exfoliate accumulated skin cells inside the oil gland that can clog pores. BHA is best used where blackheads and blemishes are the issue, and AHAs are best for sun-damaged, thickened, dry skin where breakouts are not a problem (Source: Global Cosmetic Industry, November 2000, pages 56–57).
pH Sensitive AHA and BHA
AHAs work best at concentrations of 5% to 10% with a pH of 3 to 4, and their effectiveness diminishes as you go above a pH of 4.5. BHA works best at concentrations of between 1% and 2%, and at an optimal pH of 3, diminishing in effectiveness as you go past a pH of 4. Both AHAs and BHA lose their effectiveness as a product's pH goes up or the concentration of the ingredient goes down. (Source: Cosmetic Dermatology, October 2001, pages 15–18).
If the cosmetics industry isn't forthcoming about the necessary percentages and pH for a BHA or AHA product (and most companies aren't), how can you tell if it provides decent or effective exfoliation? Consumers can't, not unless they are shopping with pH measuring paper in hand, which is exactly how I rate exfoliants when I review products for my book, newsletter, or Beauty Bulletin. As a general rule, it is best if the AHA ingredient is either second or third on the ingredient list, making it likely that the product contains a 5% or higher concentration of AHAs. For salicylic acid, because only a 2% to 0.5% concentration is required, it is fine if this ingredient is located toward the middle or end of the ingredient list.
It is interesting to not that at any pH, AHAs provide the added benefit of helping to keep water in the skin at the same time that exfoliation is taking place. This is due to the way they affect skin cells adding increased protection. AHAs can also increase the production of ceramides in the skin, which help keep it moist and healthy (Source: Dry Skin and Moisturizers Chemistry and Function, edited by Marie Loden and Howard Maibach, 2000, page 237).
While BHA penetrates deeper into the pore than AHAs, it can be less irritating than AHAs. This is due to BHA's relation to aspirin. Aspirin (acetylsalicylic acid) has anti-inflammatory properties and BHA, salicylic acid, is derived from aspirin and on the skin, retains some of its same anti-inflammatory benefits.
AHA and BHA products can definitely smooth the skin, fade signs of sun damage, correct uneven skin tone, improve texture, unclog pores, and give the appearance of plumper, firmer skin (because more healthy skin cells are now on the surface). Unfortunately, they have no residual effect—when you stop using them the skin will go back to the condition it was in before you started.
The above should answer your question
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