Three acid families cover almost every exfoliating product on a Korean skincare shelf, and they are not interchangeable. AHAs are water-soluble and the class the sun-sensitivity rules are written about: FDA describes consumer AHA products as safe at 10% or less with a final pH of 3.5 or greater when formulated or labeled for daily sun protection. BHA means salicylic acid in practice, and it is lipid-soluble, which is why the peer-reviewed literature treats it as the option for comedonal acne. PHAs are the polyhydroxy family, and the one head-to-head trial that compared a gluconolactone PHA regimen with a glycolic acid AHA regimen over 12 weeks found comparable antiaging results with significantly less reported stinging and burning. Format then decides the exposure: regulators treat a rinse-off product and a leave-on product as different risk situations, and the product's own directions are the only place the label states which one you bought.
This guide is assembled from regulatory texts, medical-organization guidance and peer-reviewed literature, cited inline. It is not a product test report, it names no products, and it is not medical advice. Where a number could not be traced to a published authority, the number is not stated.
The three classes in one table
The chemistry, not the marketing, defines the classes. Kornhauser, Coelho and Hearing, writing in Clinical, Cosmetic and Investigational Dermatology in 2010, define alpha hydroxy acids as "carboxylic acids with one hydroxyl group attached to the α-position," polyhydroxy acids as "carboxylic acids with two or more hydroxyl groups attached to carbon atoms or an alicyclic chain," bionic acids as PHA structures with a sugar molecule attached, and beta hydroxy acids as "carboxylic acids having one hydroxyl group attached to the β-position."
| AHA | BHA | PHA | |
|---|---|---|---|
| Acids most often named | Glycolic acid, lactic acid | Salicylic acid | Gluconolactone. Lactobionic acid is classed by the 2010 review as a bionic acid, a sugar attached to a PHA structure |
| Structure and solubility | One hydroxyl group at the α-position; water-soluble | One hydroxyl group at the β-position by definition, though the classification is disputed; lipid-soluble | Two or more hydroxyl groups. No solubility or penetration figure was found for this guide |
| Where the literature focuses it | Photoaging and surface renewal | Comedonal acne and the follicle | Antiaging benefit with reduced sensory irritation |
| Sun-sensitivity rules written about it | Yes, explicitly | Not in the same form | None found |
| Concentration ceiling in published guidance | 10% or less for consumer use, per the CIR conclusion FDA describes | No cosmetic percentage ceiling found in FDA's guidance; drug and Korean use limits exist | None found |
One caveat belongs at the top rather than buried. The same 2010 review notes that salicylic acid, the ingredient the whole BHA category is built on, "does not function physiologically or therapeutically as a βHA." A 2015 review of salicylic acid as a peeling agent in the same journal records the disagreement directly: "Kligman described SA as a β-hydroxy acid, but Yu and Van Scott have classified it as a phenolic aromatic acid." "BHA" on a Korean product label is a shopping category, not a settled chemical one.
AHA: the class the sun rules were written for
FDA's position on AHAs in cosmetics reflects a Cosmetic Ingredient Review Expert Panel conclusion rather than a binding percentage in United States law, and the distinction matters when reading a label. The CIR safety assessment of alpha hydroxy acids concluded that glycolic and lactic acid are safe for consumer use when the AHA concentration is 10% or less and the final product pH is 3.5 or greater, and when the product is either formulated to protect the skin from increased sun sensitivity or its directions tell consumers to use daily sun protection. The same panel treated salon products differently: up to 30% at a final pH of 3.0 or greater, in products designed for brief discontinuous use followed by thorough rinsing, applied by trained professionals, with directions for daily sun protection.
Read those two conclusions together and the logic of the whole category appears. Higher concentration was accepted only in exchange for shorter contact, immediate rinsing, and a trained applicator. Concentration and contact time are traded against each other, which is precisely why format is not a cosmetic detail.
The sun-sensitivity evidence is specific rather than vague. Kornhauser and colleagues report that "10% glycolic acid, applied 6 days/week for 4 weeks, enhanced the sensitivity to UVB radiation, measured as increased sunburn cell formation, and also lowered the minimal erythemal dose," that "glycolic acid (10%) increased the SSR sensitivity of human skin, whereas SA (2%) did not," and that the enhanced photosensitivity "was reversed within a week of terminating the treatments." That last clause is the reason the labeling language says what it says. FDA's Guidance for Industry: Labeling for Cosmetics Containing Alpha Hydroxy Acids recommends this statement: "This product contains an alpha hydroxy acid (AHA) that may increase your skin's sensitivity to the sun and particularly the possibility of sunburn. Use a sunscreen, wear protective clothing, and limit sun exposure while using this product and for a week afterwards."
Note what that experiment was and was not. It was a fixed regimen run to measure a photobiological endpoint. It is not a recommended frequency, and no authority found for this guide converts it into one.
BHA: the oil-soluble one, and why its sun profile reads differently
Salicylic acid's behavior follows from where it goes. The 2015 review states that "SA is a lipid-soluble agent, in contrast with the α-hydroxy acids (such as glycolic acid), and is therefore miscible with epidermal lipids and sebaceous gland lipids in hair follicles," and that "being a lipophilic agent and having an ability to concentrate in the pilosebaceous apparatus, SA peels are a good therapeutic option for comedonal acne." A 2012 review in the Jundishapur Journal of Natural Pharmaceutical Products makes the same contrast, noting that BHAs "are lipid-soluble in contrast to water solubility of AHAs" and therefore penetrate through sebaceous follicles.
The mechanism has been revised too. The 2015 review argues salicylic acid "should now be regarded as a desmolytic agent rather than as a keratolytic agent," because it works by disrupting cellular junctions rather than breaking down keratin. Labels that describe it as keratolytic are using older language.
On sun sensitivity, FDA's beta hydroxy acids page records that salicylic acid is the BHA most commonly used in cosmetics and that the CIR panel concluded salicylic acid and related substances are safe as used when formulated to avoid irritation and to avoid increased sun sensitivity, or, where increased sun sensitivity would be expected, when directions for use include daily sun protection. FDA also notes one reviewed study in which salicylic acid had a photoprotective effect in mice. The 2019 CIR amended safety assessment restates the conclusion in terms of present practices of use and concentration, formulated to be non-irritating and non-sensitizing.
The practical asymmetry is worth stating plainly: FDA publishes a numeric consumer ceiling and pH floor for AHAs and does not publish an equivalent cosmetic percentage ceiling for BHAs. Numbers for salicylic acid come from elsewhere.
| Where a salicylic acid number comes from | The number |
|---|---|
| United States over-the-counter acne monograph, 21 CFR 333.310 | 0.5 to 2 percent |
| Korea, general cosmetics (as a preservative) | 0.5% |
| Korea, body-cleansing products | 2% |
| Korea, rinse-off products | 3% |
| Korea, products for children under 13 | Not permitted |
The Korean figures are as recorded in regulatory reporting on the MFDS salicylic acid labeling rules, which also notes that in a general cosmetic the ingredient may serve only as a preservative and cannot itself be promoted for exfoliation-type purposes. That single rule explains a great deal of vague Korean label copy: a product without an acne functional-cosmetic filing has to talk around the ingredient.
PHA: the mildness claim, and the one trial that tested it head to head
The PHA case rests on a direct comparison rather than on inference. Edison, Green, Wildnauer and Sigler, in Cutis in 2004, compared a gluconolactone-based PHA regimen against a glycolic acid AHA regimen over 12 weeks, measuring outcomes by silicone replicas, clinical grading and pinch recoil for skin resiliency. Both regimens produced "significant antiaging benefits to skin." The AHA regimen was marginally ahead on two measures at week 12, improving sallowness 17.1% versus 12.4% and resiliency 13.5% versus 10.2%. The tolerability result went the other way: AHA subjects reported significantly worse stinging and burning at weeks 6 and 12, and the authors concluded the study demonstrated "the enhanced mildness of PHAs and their equivalence in providing antiaging benefits compared with an AHA regimen."
That is a real, checkable finding, and it is narrower than how PHAs are usually marketed. It compared two specific regimens on specific endpoints. It does not establish that any product labeled PHA is gentler than any product labeled AHA, and it sets no PHA concentration or pH guidance, because none was found for this guide.
The usual mechanical explanation for PHA mildness, that the larger molecule penetrates more slowly, appears widely in brand and retailer material. No peer-reviewed source was located for this guide that states a molecular size or penetration rate for gluconolactone or lactobionic acid, so the explanation is left out here and only the measured tolerability outcome is reported.
Format changes the exposure more than the acid name does
No published authority found for this guide assigns concentrations to product formats. What the authorities do establish is that rinsing changes the situation, and they establish it three separate times.
- The CIR salon conclusion permits up to 30% at pH 3.0 or greater only in products "designed for brief discontinuous use followed by thorough rinsing," by trained professionals.
- Korea's cosmetic precaution rules attach the AHA sunscreen warning to AHA products but exclude rinse-off products and hair products from that particular line.
- Both Korea and the European Union set fragrance-allergen naming thresholds ten times higher for rinse-off products than for products that stay on the skin: above 0.01% versus above 0.001%, the European figures sitting in Article 19(1)(g) and Annex III of Regulation 1223/2009 as amended by Commission Regulation (EU) 2023/1545.
With that established, formats can be compared on the variables a shopper actually controls.
| Format | Contact time | Dose control | What to check on the label |
|---|---|---|---|
| Toner or liquid exfoliant | Set by the directions; usually left on | Poor. The amount decanted onto a cotton pad varies per use | Whether the directions say to rinse, and whether an AHA precaution or Sunburn Alert appears |
| Pre-soaked pad | Set by the directions; wipe formats are commonly leave-on | Better. One pad is one dose, though how much is transferred still varies | Directions, and whether the pad is described as a wipe-off step |
| Serum or ampoule | Leave-on until the next cleanse | Best. Dispensed in drops or pumps | Where the step sits in the routine, and any instruction about sunscreen |
| Wash-off mask | Bounded by the directions, then rinsed | Moderate. Amount applied varies, duration is stated | The stated duration and the rinse instruction, which are the two numbers on the package |
| Cleanser | Shortest. Applied and rinsed in the same step | Low, and it matters less because the product leaves | Whether the acid is doing anything at cleanser contact time at all |
The last row deserves emphasis. A cleanser is the format where a listed acid is least likely to be the point of the product, and, in Korea, a body-cleansing product is also the category where salicylic acid is permitted at up to 2% rather than 0.5%. The presence of an acid in a cleanser tells you very little on its own.
How to tell a leave-on treatment from a rinse-off one on the label
There is no universal on-pack symbol. There are five tells, in descending order of reliability.
- The directions. The instruction to rinse, or the absence of one, is the only place the label states the answer directly. Read it before the ingredient list.
- A stated duration. A number of minutes followed by a rinse instruction is a wash-off treatment by definition.
- The AHA sunscreen precaution on a Korean package. Korea's precaution rules exclude rinse-off products and hair products from the "use with a sunscreen" line for AHA products. A Korean package carrying that line for an AHA is being labeled as a product that stays on the skin.
- A "consult a specialist" line on a Korean AHA product. Korea's precaution rules apply to AHA products other than those containing 0.5% or less AHA, require a patch test instruction and the sunscreen instruction, and require a statement to consult a specialist because of the side-effect risk of high-concentration AHA only on products containing more than 10% AHA or with a pH below 3.5. The presence of that specific line is therefore informative: it indicates the product is above 10% AHA or below pH 3.5. The rules are set out in Annex 3 to the Enforcement Rule of the Cosmetics Act.
- A Sunburn Alert statement on a United States package. FDA's AHA labeling guidance recommends it, so its presence signals the manufacturer treated the product as an AHA product for labeling purposes.
Two cautions on tells 3, 4 and 5. Their absence proves less than their presence, because a package may be printed for a different market, and the AHA precautions do not apply to products at 0.5% AHA or below. And none of these tells reveals a concentration by itself, with the single exception of the ">10% or pH below 3.5" inference in tell 4.
What published guidance actually says about frequency
Less than most shoppers expect. The American Academy of Dermatology's guidance on exfoliating at home states that "how often you exfoliate depends on your skin type and exfoliation method," and adds the operative principle: "Generally, the more aggressive the exfoliation, the less often it needs to be done." That is guidance about a relationship, not a schedule.
No frequency number for at-home acid use was found in FDA's AHA or BHA material, in the CIR conclusions, or in the AAD guidance consulted for this article. Any product that prints a specific weekly frequency is stating a manufacturer's direction, which is worth following, and not a published clinical recommendation.
The AAD guidance is specific about several other points:
- Method should follow skin type. Dry, sensitive or acne-prone skin is directed toward "just a washcloth and a mild chemical exfoliator," while oily, thicker skin may tolerate stronger chemical treatments or mechanical exfoliation.
- Do not exfoliate compromised skin. The AAD says never to exfoliate if you have open cuts or wounds, or if your skin is sunburned.
- Watch the overlap with retinoids and benzoyl peroxide. Exfoliating while using prescription retinoid creams, retinol, or products containing benzoyl peroxide may worsen dry skin or cause breakouts.
- Pigmentation risk is a real constraint. The AAD advises avoiding strong chemical or mechanical exfoliation with a darker skin tone, or if dark spots appear after burns, insect bites or breakouts, because more aggressive exfoliation can lead to increased pigmentation.
The AAD's chemical peels guidance reinforces the same theme at professional strength: apply sunscreen every day once skin has healed, with total sun avoidance for three to six months after a deep peel, and it warns that people with skin of color can develop permanent pigment problems when treated by someone without the relevant expertise.
Matching the acid class to a stated problem
The mapping below only lists uses this guide could source. Where the literature is silent, the row is silent.
| Stated problem | Class the sourced literature points to | The source |
|---|---|---|
| Comedonal acne, congested follicles | BHA (salicylic acid) | Lipophilic, concentrates in the pilosebaceous apparatus, described as a good therapeutic option for comedonal acne (2015 review) |
| Signs of photoaging, sallowness, resiliency | AHA (glycolic acid) or PHA (gluconolactone) | Both regimens produced significant antiaging benefits over 12 weeks (Cutis, 2004) |
| Stinging or burning on an AHA regimen | PHA (gluconolactone) | AHA subjects reported significantly worse stinging and burning at weeks 6 and 12 in the same trial |
| Dry, sensitive or acne-prone skin choosing a method | A mild chemical exfoliator over mechanical scrubbing | AAD guidance on exfoliating at home |
| Oily, thicker skin | May tolerate stronger chemical treatments or mechanical exfoliation | AAD guidance on exfoliating at home |
| Any acid use with sun exposure | Daily sun protection alongside, and for a week after stopping an AHA | FDA AHA guidance; photosensitivity reversed within a week of stopping in the 2010 review |
For a diagnosed skin condition, or for a persistent reaction to a product, published guidance consistently points to a dermatologist rather than to a different product, and this guide does the same.
FAQ
Is BHA better than AHA for acne?
The peer-reviewed case for salicylic acid in comedonal acne rests on solubility: it is lipid-soluble, mixes with sebaceous gland lipids in hair follicles, and concentrates in the pilosebaceous apparatus, which a 2015 review describes as making salicylic acid peels a good therapeutic option for comedonal acne. That is a mechanism-and-indication argument about the ingredient, not a ranking of finished products, and acne that does not respond belongs with a dermatologist.
Do I need sunscreen with a PHA or a BHA, or only with an AHA?
The explicit sun-sensitivity rules are written about AHAs, and the 2010 review found that 10% glycolic acid raised UVB sensitivity while 2% salicylic acid did not. FDA also notes the CIR position that salicylic acid products should include daily sun-protection directions where increased sun sensitivity would be expected. Daily sun protection is standard guidance regardless, so the difference concerns which labels are required to say so, not whether sunscreen is worth using.
Can I tell an exfoliant's strength from its ingredient list?
No. Ingredient declaration rules in both the United States and Korea allow everything used at 1% or less to be listed in any order, and they require ingredient identity rather than function or concentration. The one exception on a Korean AHA package is the "consult a specialist" precaution, which is printed only on products above 10% AHA or below pH 3.5.
Are acid pads gentler than acid serums?
Nothing found for this guide supports a general answer. Format governs dose control and contact time, not acid strength, and both pads and serums are commonly leave-on. Compare the directions, the stated duration if any, and the precautions, rather than the format name.
How often should I use an exfoliating toner?
No published clinical frequency was found for at-home acid exfoliation. The AAD says frequency depends on skin type and method, and that the more aggressive the exfoliation, the less often it needs to be done. Follow the manufacturer's stated directions, and note the AAD's guidance not to exfoliate skin that is sunburned or has open cuts or wounds.