AHA vs BHA vs PHA — Telling Them Apart by Ingredient Name and Label Cautions
In Korea, “use a sunscreen with it” is the line an MFDS notice requires on leave-on products with more than 0.5% AHA. This guide sorts glycolic and lactic acid (AHA), salicylic acid (BHA) and PHA by ingredient name and back-label lines, then covers the inner-arm patch test, night and day order, and the week after you stop.
Turn two toners over and set them side by side, and sometimes only one of them has the line “This may increase the skin’s sensitivity to sunlight, so use a sunscreen with it” on it. Under a notice from Korea’s Ministry of Food and Drug Safety (MFDS), a Korean labeling regulation, AHA is the one ingredient this wording is set for.
A leave-on product with more than 0.5% AHA carries this line together with “Test on a small area first to check for skin problems.” Even when both fronts say “exfoliating,” one line on the back tells the two bottles apart.
This article sorts AHA, BHA and PHA by their ingredient-list names and the caution lines on the back, and covers what to do from the first day you apply one to the week after you stop.
For the difference between pads you wipe with and pads you leave on the skin, see Korean toner pad types; for managing blackheads on your nose without squeezing, see Blackheads Without Squeezing; and for where retinol goes in your night routine, see the order for applying retinol.
Charne
AHA · BHA · PHA Peeling Pads
35 pads
Pads labeled with AHA, BHA and PHA together. Read the ingredient list and cautions, and test a small area before first use.
View on NaverHow do you sort the three by ingredient name?
Glycolic and lactic acid are AHAs; salicylic acid is a BHA. PHA gets its own short look.
AHA — the acids from fruit and milk
AHAs are grouped as glycolic acid from sugar cane, lactic acid from milk and pickled vegetables, citric acid from citrus fruit, malic acid from apples and tartaric acid from grapes (DermNet, a New Zealand skin information site written by dermatologists). Glycolic acid and lactic acid are the AHAs most commonly used in cosmetics (the U.S. Food and Drug Administration, FDA).
AHAs loosen the glue-like substances that hold surface skin cells to each other, so old dead skin comes away gently (DermNet). The FDA also notes that AHA products are marketed to smooth fine lines and surface wrinkles.
AHA
BHABHA, in cosmetics, essentially salicylic acid
The BHA in cosmetics is essentially salicylic acid, though chemically it isn’t a true BHA (FDA). Still, the industry and consumers call it BHA, so on the back you only need to look for one name: “salicylic acid.”
Salicylic acid softens keratin, a protein that makes up the skin’s structure (DermNet). In acne, it slows the shedding of cells inside the pores to prevent clogging and helps break down blackheads and whiteheads (DermNet).
PHA, the one a 2010 review paper covered
PHA is the group that a 2010 review of hydroxy acids in a dermatology journal (Kornhauser et al.) summarized as giving effects similar to AHAs with fewer irritation reactions. Unlike the two groups above, Korea’s notice sets no caution wording under the PHA name, so the back-label lines won’t sort it out. PHA gets a short section of its own below.
PHA| Group | Name to look for in the ingredients | What the sources say it does | Korean notice wording |
|---|---|---|---|
| AHA | Glycolic acid · lactic acid (citric · malic · tartaric acid) | Loosens the substances holding old surface cells together so dead skin comes away (DermNet) | Above 0.5%: test on a small area · leave-on: sunscreen (except rinse-off and hair products) / above 10% or pH below 3.5: consult a specialist |
| BHA | Salicylic acid | Softens keratin and slows cell shedding inside pores to prevent clogging (DermNet) | Not for children aged 3 or younger (except products rinsed off right away, such as shampoo) |
| PHA | “PHA” in the product description | Summarized as having effects similar to AHA, but with fewer irritation reactions (2010 review) | No set wording, so follow the product’s directions |
Here are all three side by side in one table, and the column on what each one does names its source.
Which bottles carry “use a sunscreen with it”?
AHA above 0.5%, in a leave-on product. When both conditions apply, that sunscreen line goes on the back.
0.5%
Products with more AHA than this carry caution lines
18%
Rise in sensitivity to UV reddening after 4 weeks of AHA use (a study the FDA describes)
1week
How long after stopping the FDA’s recommended wording still says to protect from the sun
Sources · MFDS Notice No. 2026-56 [Annex 1] (Korean regulation) · U.S. FDA: Alpha Hydroxy Acids
Two lines in Korea’s notice, and the exceptions
[Annex 1] of the notice requires two lines on products containing AHA: “This may increase the skin’s sensitivity to sunlight, so use a sunscreen with it,” and “Test on a small area first to check for skin problems.” Products with 0.5% AHA or less fall outside this item.
The sunscreen line has one more exception, since rinse-off products and hair products don’t carry it. So even among AHA products, a cleanser has no sunscreen line, while a leave-on toner or serum may.
The U.S. FDA’s recommended “Sunburn Alert” wording
The FDA has issued wording it recommends for AHA cosmetics. It says the AHA in the product may increase your skin’s sensitivity to the sun, particularly the possibility of sunburn, so use a sunscreen, wear protective clothing, and limit sun exposure while using it and for a week afterward. Because it’s a U.S. recommendation, not every product has it printed on the label.
In a study the FDA describes, people who applied AHA for four weeks showed an 18% rise in sensitivity to UV reddening, and measurements taken a week after they stopped showed no difference.
The FDA’s 2005 guidance for industry also cites research showing that glycolic acid could change the skin’s UV response at a concentration as low as 4% and after as little as four days of use.
What if you see “consult a specialist”?
AHA above 10% or pH below 3.5. This line marks a high concentration.
When the third line applies
The AHA item in [Annex 1] of the notice has a third line: “Contains a high concentration of AHA that may cause side effects, so consult a specialist or other professional.” It appears only on products with more than 10 percent AHA or a pH below 3.5.
These numbers aren’t a line that stops sales; they’re the threshold for adding one more line. If you see it on the back, read the product as having more AHA or a lower pH than one carrying only the first two lines.
With a bottle carrying this line, I suggest asking a dermatologist before you start using it.
In the U.S., 10% and pH 3.5 come from an industry review
The FDA page also mentions 10% or less and a pH of 3.5 or greater. But these numbers are presented not as an FDA-set limit, but as a conclusion of the CIR (Cosmetic Ingredient Review) panel, the U.S. cosmetics industry’s self-regulatory review body. The same conclusion calls for either building sun protection into the product or having the package directions tell consumers to use daily sun protection.
Low concentrations of 4–10% are common in over-the-counter creams and lotions, while high concentrations above 20% are used for chemical peels (the review paper mentioned above).
The glycolic acid peels DermNet describes are also in-clinic procedures at 20–70%, a different matter from products you apply at home.
Which lines should you look for on a salicylic acid bottle?
If it’s a leave-on product, “Do not use on children aged 3 or under.” That 0.5% figure isn’t an exfoliation limit.
The children line, and rinse-off products
[Annex 1] of the notice requires “Do not use on children aged 3 or under” on products containing salicylic acid and its salts. Products you rinse off right after use, such as shampoo, are exempt. If you have young children at home, keep leave-on BHA toners and serums out of their reach.
Salicylic acid can irritate or cause a burning feeling even on healthy skin, and mild stinging can occur when it’s applied to broken skin or at higher concentrations (DermNet). DermNet also says to keep it away from the eyes and mucous membranes.
The 0.5% in the notices is a number for other purposes
That 0.5% salicylic acid figure actually appears in two Korean notices. One is the preservative table in the “Regulation on Safety Standards for Cosmetics,” where it serves as the limit for use as a preservative. The other is the acne-prone skin table in the “Regulation on Review of Functional Cosmetics” (functional cosmetics are Korea’s regulated category for products with specific claims), where it sets the ingredient level for rinse-off cleansing products that don’t need to submit review data.
Neither number is a limit for leave-on exfoliating products. So rather than reading it as “salicylic acid is allowed only up to 0.5%,” first check whether your product is rinse-off or leave-on. For blackhead care as a whole, see Blackheads Without Squeezing.
Rinse-off and leave-on products carry different lines on the back.
What are the two things a review paper summarized about PHA?
Reported as less irritating
The 2010 review summarizes PHA as giving effects similar to AHA, but with fewer irritation reactions. The same paper describes a study where sunburn cells increased significantly at sites treated with glycolic acid, but not at sites treated with gluconolactone.
Korea’s notice [Annex 1] has no separate PHA item. So with a PHA product, follow that product’s directions and cautions on the back as written, and if it’s new to you, test it on your inner arm first, as in the section below. “Less irritating” isn’t a reason to skip the test.
How do you test on your inner arm for the first 7–10 days?
Twice a day, 7–10 days, on a coin-sized spot where it won’t rub or wash off. This is how the American Academy of Dermatology (AAD) says to test a new skin care product.
Patch testWhere to test and how much to apply
Apply a new product to an area the size of a U.S. quarter twice a day for 7 to 10 days (AAD). Pick a spot where the product won’t get rubbed or washed off, like the underside of your forearm or the inside of your elbow, and use the same amount and thickness you normally would (AAD).
Leave it on as long as you usually would; with rinse-off products like cleansers, leave it on for 5 minutes or for the time given in the directions (AAD).
This is also how you can follow the “test on a small area” line on the back of AHA products. BHA and PHA products might not carry that line, but if a product is new to you, start the same way.
If you see a reaction
If your skin reacts during the test, gently wash the product off as soon as possible and don’t use it again (AAD). Keep the test spot on one arm and look at it each time you apply, morning and night.
medicube
Zero Pore Pad 2.0
70 pads
Read the acid names in the ingredient list rather than relying on the product name. Check for overlap with any exfoliant or retinol you already use.
View on NaverWhen do you apply it, and what about sunscreen by day?
Leave-on AHA at night, sunscreen the next morning. Timing is the key with this ingredient.
Where the case for nighttime use comes from
DermNet starts from the premise that AHA products differ in concentration, type of acid, and base formula, then explains that leave-on products are best applied at night, or twice daily if needed. In the end, your product’s directions set the frequency, so follow the number written in the directions on the back first.
AHA products can be alternated with other anti-aging products such as retinoid creams and vitamin C (DermNet). Since that means alternating, one way is not to layer the two on the same night but to use them on different days.
Sun protection the next morning
Even when you apply AHA at night, the notice wording is “use a sunscreen with it,” and the FDA’s recommended wording also says to apply sunscreen while using the product. So while you’re using AHA, finish the next morning’s routine with sunscreen, and on days you’ll be outside for a long time, cut down on sun exposure with covering clothes and a hat.
Retinol follows the same timing rule. Retinoids can make skin more sensitive to the sun, so apply them only at night and always protect your skin from the sun during the day (AAD).
To start retinol, see the retinol beginner’s guide.
What if it overlaps with other acids or retinol?
Avoid masks with active ingredients you already use. That’s what the AAD says in its guidance on masks.
If you already use an exfoliating product
The AAD says that when choosing a facial mask, you should avoid masks containing active ingredients you already use in other skin care products, especially if those products contain retinol or acids.
The AAD also quotes a dermatologist saying that adding a mask with similar ingredients when an exfoliating product is already in your routine can cause dryness and irritation.
That advice is about masks, but the same standard can apply to toner pads and serums. On days you use exfoliating wipe pads, for example, skip adding an AHA serum on top. For how to choose pads, see Korean toner pad types.
Sensitive skin, and prescribed salicylic acid
The AAD’s mask guidance also says that if your skin is sensitive, avoid masks with strong exfoliating ingredients, such as AHA or BHA at some concentrations. It also says to wash a mask off right away if it itches, burns or stings while it’s on.
A salicylic acid preparation prescribed by a dermatologist is a separate case. DermNet says not to use other topical medicines such as retinoids, or cosmetics that dry and peel the skin, on areas treated with salicylic acid unless your doctor tells you to.
This is guidance for a medicine, so if you have a prescription, I suggest telling your doctor about the cosmetics you currently use.
Why keep up sun protection for a week after you stop?
The FDA’s suggested wording covers “while using this product and for a week afterwards.” Putting the cap back on doesn’t switch it off right away.
The week after stopping
In the study the FDA describes, the rise in UV sensitivity from AHA no longer showed a difference in measurements taken a week after stopping. The recommended wording, too, calls for sun protection “for a week afterwards.”
I suggest noting the day you finish an AHA bottle or decide to take a break, and for the following week keeping up morning sunscreen and a hat as before.
The FDA notes that adverse reaction reports it received on AHA products from 1992 to February 2004 included 45 cases of burning and 35 of dermatitis or rash, among others.
The more serious reactions appear to occur most often with the products that exfoliate the most, the FDA explains.
When to wash it off and not use it again
Whether you’re testing a product or already using it, if burning or a rash appears, gently wash off what you applied. The AAD says not to use a product again if it caused a reaction during testing, and if your skin got worse after a mask, to find the cause with a dermatologist.
For white flakes on your face, there’s a separate guide: Flaky skin on your face, what to check before you rub.
What are the common mix-ups about acids?
Here are four things you often hear, set beside what the sources actually say.
| Easy to assume | What the sources say | What to check on the back |
|---|---|---|
| Korea allows AHA only up to 10% | The notice adds “consult a specialist” when AHA is above 10% or the pH is below 3.5 | The third line in the cautions |
| Salicylic acid can only be used up to 0.5% | 0.5% is the preservative limit and the reference level for rinse-off acne cleansing functional products exempt from review data | Whether it’s rinse-off or leave-on |
| You only need sunscreen on the days you apply it | The FDA’s recommended wording covers the period of use and the week after | The day you start a break from AHA |
| Retinol and acids can’t be used in the same week | DermNet says AHA can be alternated with retinoid creams, and the AAD says to avoid masks with active ingredients you already use | Whether acids and retinol overlap across two products on one night |
Dr.G
Green Mild Up Sun+
50 ml × 2 · SPF50+ PA++++
A sunscreen worth considering for your morning routine while using a leave-on AHA. Keep protecting your skin from the sun for a full week after you stop using the AHA.
View on NaverWhat do you check when you line the three bottles up again?
Back to the two bottles from the start. For the one with “use a sunscreen with it,” look for an AHA name in the ingredient list, because the notice puts this line on leave-on products with more than 0.5% AHA. If “consult a specialist” is also there below it, that signals a product that, under the notice, has more than 10% AHA or a pH below 3.5.
On a bottle showing salicylic acid, check for the age-3-and-under line, and with a bottle labeled PHA, follow that product’s directions as written.
If a bottle is new to you, test any of the three on your inner arm for 7–10 days first. Apply leave-on AHA at night, and wear sunscreen every morning while you use it and for a week after you stop. While keeping to this order, have only one exfoliating product out at a time.
What do people often ask about acid exfoliants?
By the names in the ingredient list. If you see a name that the U.S. Food and Drug Administration (FDA) and DermNet, a New Zealand skin information site, list as an AHA, such as glycolic acid or lactic acid, it’s an AHA; if you see salicylic acid, it’s what’s commonly called BHA. With Korean products, check the cautions too. A leave-on product with more than 0.5% AHA carries “use a sunscreen with it.”
Korea’s MFDS notice puts this line on products with more than 10% AHA or a pH below 3.5. It isn’t a standard that stops sales; it marks a high concentration. With a product carrying this line, asking a dermatologist before you first use it is the order the line itself sets out.
The “Sunburn Alert” wording the U.S. FDA recommends says to use sunscreen, cover up with clothing and limit sun exposure while using the product and for a week afterward. Korea’s notice also requires a line telling you to use sunscreen on products with more than 0.5% AHA (except rinse-off and hair products).
A 2010 review of hydroxy acids summarized PHA as giving effects similar to AHA with fewer irritation reactions. Korea’s notice has no separate PHA item, so follow the product’s directions and cautions, and if it’s new to you, test it on your inner arm for 7–10 days.
DermNet, a New Zealand skin information site, explains that you can alternate AHA products with retinoid creams. And the American Academy of Dermatology (AAD) notes that when you pick a mask, if you’re already using products with retinol or acids, you shouldn’t double up on those same active ingredients. One way is to avoid layering the two on the same night and use them on different days instead.
Which three lines does Sein read first on the back?
Sort acid exfoliants into AHA, BHA and PHA by the acid name in the ingredient list, and decide what to do from the sunscreen, consult-a-specialist and age-3-and-under lines in the cautions. Give a new product 7–10 days on your inner arm as a test, apply leave-on AHA at night, and protect your skin from the sun the next day and for a week after you stop.
Read next
- Korean Toner Pads: Exfoliating, Calming and Hydrating, How to Tell Them Apart
- Blackheads Without Squeezing
- White Flakes on Your Face? Check This Before You Exfoliate
- Vitamin C Serum for Beginners: Order, Frequency and Color Changes
- All Glowmeo skincare guides
