AHA vs BHA vs PHA: Which Acid Does What

AHAs are water soluble and act at the skin surface, BHA is oil soluble and mixes with follicular lipids, and PHAs are larger AHA relatives that sting less. Solubility decides where an acid can go. Molecular size and the pH of the formula shape how quickly it gets there. Almost every practical difference between these three follows from those two facts.

What is the difference between AHA, BHA, and PHA?

All three are hydroxy acids. The prefix describes chemistry, not strength.

An alpha hydroxy acid carries its hydroxyl group on the carbon next to the acid group. A beta hydroxy acid carries it one carbon further along. A polyhydroxy acid carries several hydroxyl groups on the same molecule.

The mechanism they share was described in 1984. Van Scott and Yu argued that thickened, rough skin usually reflects reduced shedding. The cause is stronger bonding between corneocytes. Alpha hydroxy acids weaken that intercellular bonding (PMID 6096420). In other words, these acids do not scrub cells off. They loosen the glue holding dead cells in place. If the outer layer itself is unfamiliar, start with what the stratum corneum actually is.

AHA vs BHA vs PHA at a glance

Class	Common examples	Solubility	Where it mainly acts	Typical best fit	Sting profile
AHA	Glycolic, lactic, mandelic	Water soluble	Surface of the stratum corneum	Rough texture, dullness, uneven tone	Highest, especially glycolic
BHA	Salicylic acid	Oil soluble	Surface plus sebum rich follicles	Congested pores, oily skin, comedones	Moderate, often less than glycolic
PHA	Gluconolactone, lactobionic acid	Water soluble, strongly humectant	Surface of the stratum corneum	Sensitive or easily reddened skin	Lowest of the three

Water soluble versus oil soluble: the difference that decides the most

An oil soluble acid can mix with oil. A water soluble one cannot.

That single property is why salicylic acid gets discussed alongside pores and nothing else does. A 2015 review of salicylic acid as a peeling agent calls it lipid soluble. Alpha hydroxy acids such as glycolic acid are not. That solubility, the review says, makes salicylic acid miscible with epidermal lipids and with sebaceous gland lipids in hair follicles (PMID 26347269). Glycolic acid sits in the water phase of a product and stays largely where the water goes.

The same review reframes how salicylic acid works. It is now described as desmolytic rather than keratolytic, because it disrupts cellular junctions instead of lysing keratin filaments. It also reports that salicylic acid reduces sebum secretion in people with acne (PMID 26347269).

This is why oil soluble chemistry dominates pore talk. It comes up in discussions of sebaceous filaments, which are normal sebum lined structures rather than clogs. It also comes up in arguments about what actually changes how pores look.

PHAs bring a third behavior. Their extra hydroxyl groups let them bind water. A 2019 review assessed lactobionic acid, a PHA, alongside lactic acid, an AHA. It concluded that both bind large amounts of water and act as potent antioxidants. It also credits both with helping maintain epidermal barrier integrity during use (PMID 30270529).

So humectancy is not the clean dividing line between the classes. Lactic acid is an AHA and it is strongly humectant. What separates PHAs from AHAs is size and how they are tolerated, not whether they hold water.

Molecular size and how fast an acid moves

Size matters because a smaller molecule diffuses faster through a dense, lipid rich layer. The ranking below follows from molecular weight. It is a chemistry expectation, not a measured clinical result.

Acid	Class	Molecular weight (g/mol)	Expected diffusion speed
Glycolic acid	AHA	76.05	Fastest
Lactic acid	AHA	90.08	Fast
Salicylic acid	BHA	138.12	Moderate, but oil seeking
Mandelic acid	AHA	152.15	Slower, aromatic ring
Gluconolactone	PHA	178.14	Slow
Lactobionic acid	PHA	358.30	Slowest

Lactobionic acid weighs 358 g/mol against 76 g/mol for glycolic acid, roughly four and a half times as much. Molecular size is the explanation usually offered for the tolerability gap between the classes. It is a hypothesis, not a tested finding.

Formulation matters too. One comparison of lactobionic acid against glycolic acid ran both in a gel and in an emulsion. Both acids performed better in the emulsion built on a new alkylpolyglucoside emulsifier. The authors emphasized the importance of vehicle on the effects of topical actives (PMID 20367666). That study used skin bioengineering measures in 77 healthy volunteers. The lactobionic samples produced neither skin irritation nor barrier impairment.

A 12 week study compared a gluconolactone based regimen against a glycolic acid based one. Both improved photoaging measures on clinical grading, silicone replicas, and pinch recoil. Only two antiaging endpoints differed significantly, and both favored the AHA at week 12 only. Sallowness improved 17.1 percent with the AHA versus 12.4 percent with the PHA. Pinch recoil improved 13.5 percent versus 10.2 percent. Stinging and burning were significantly worse in the AHA group at both week 6 and week 12 (PMID 15002657).

A companion review reported that PHAs provide effects similar to AHAs without the same sensory irritation. It credits them with added humectant and moisturizing properties. It also reports they were found compatible with clinically sensitive skin (PMID 15002656).

Which acid fits which concern?

AHA for surface texture and tone. Roughness you can feel with a fingertip, dullness, or flat looking tone all sit in the surface layer. An AHA targets that layer. That is the same territory covered in the guide to what causes uneven skin texture.

BHA for pores and sebum. Salicylic acid is the only widely used BHA in skincare. In the United States it is a recognized over the counter acne active at 0.5 to 2 percent under 21 CFR 333.310(d). Higher strengths exist as in office peels and are a different category of procedure. For background on pore biology, see the pore science page.

PHA for skin that reacts to everything else. Self reported sensitivity is common. A meta analysis pooled 26 studies across 18 countries and 51,783 people. It found that 71 percent reported sensitive skin to some degree. Forty percent described their skin as very or moderately sensitive (PMID 31869523). For a large share of people, the gentlest option is the only one that gets used consistently.

A caution on the evidence base. A 2020 Cochrane review covered 49 acne trials and 3,880 participants. It rated most hydroxy acid comparisons as low or very low quality. Salicylic acid versus tretinoin showed a risk ratio of 1.00 for treatment response. The 95 percent confidence interval ran from 0.92 to 1.09, in a single 46 person study (PMID 32356369). These acids are widely used. The head to head trial evidence behind them is thinner than the marketing suggests.

Why pH matters as much as the percentage on the label

A hydroxy acid only behaves like an acid in its free, unneutralized form. Raise the pH of a formula and more of the acid converts to its salt. The percentage on the front of the bottle does not tell you that ratio.

pH also drives the enzymes that do the shedding. When 50 percent glycolic acid at pH 0.9 was applied to human skin for 30 minutes, surface pH fell to about 2. It recovered to roughly 3 within a day. In vitro, desquamation regulating proteinase activity rose markedly around pH 3. Cathepsin D sits near its optimal pH there (PMID 15660917).

For reference, healthy skin is already acidic. A study of 330 people measured average forearm surface pH at 5.12. It fell to 4.93 after 24 hours without products. The authors estimated the natural value at about 4.7 (PMID 18489300). A leave on exfoliant sits below that. Skin then has to climb back.

Concentration is not a simple dial either. A 2018 review of alpha hydroxy acids concluded that whether an AHA helps or harms depends on its concentration. It also leaves the photodamage question unsettled (PMID 29642579). Read pH and percentage together. Neither number is meaningful alone.

Glycolic acid and sun sensitivity

This part is measured, not theoretical, and it separates the classes.

One randomized double blind study treated the backs of 29 Caucasian subjects. Sites got 10 percent glycolic acid at pH 3.5 or placebo. Application was once daily, six days a week, for four weeks. Sunburn cells and minimal erythema dose were measured in 16 of them. Cyclobutyl pyrimidine dimers were measured in the other 13. The glycolic sites showed more sunburn cells and a lowered minimal erythema dose. The DNA dimers were elevated but not to a statistically significant level. The sensitivity was gone one week after treatment stopped (PMID 12713551).

A later study used a within subject design. Each of 14 people had four test sites on the mid back. Three sites were treated for 3.5 weeks with 10 percent glycolic acid, 2 percent salicylic acid, or vehicle. The fourth site was left untreated. Glycolic acid increased erythema, DNA damage, and sunburn cell formation after solar simulated radiation. Salicylic acid produced no significant change in any of those markers (PMID 19411163).

Two takeaways. Daily broad spectrum sunscreen belongs with any AHA routine. And the effect appears reversible within about a week of stopping.

Frequency versus strength

Western exfoliation defaulted for years to high strength and low frequency. One strong weekly peel pad, then nothing.

Korean routines popularized the opposite pattern. Low concentrations go on in thin, watery layers, applied more often. Acids show up in toner format rather than as a weekly event. The 7 skin method is the clearest expression of that layering habit. It is usually done with hydrating toners rather than exfoliating ones.

There is observational support for routine consistency mattering, though not for acids specifically. A cross sectional study of 361 Korean and Chinese women measured biophysical skin parameters. The Korean cohort had higher hydration, lower transepidermal water loss, and less severe wrinkling. The study also adjusted for age and region. Daily sunscreen, daily base makeup, and moisturizer use were linked to better hydration, water loss, and elasticity readings (PMID 33911566). Those were the habits measured. No acid or toner habit was assessed. Read it as support for consistency, not as proof about frequent low strength exfoliation.

What over-exfoliation looks like

The most common mistake is not choosing the wrong acid. It is using several at once, or one too often. Then the resulting damage gets read as a sign that something is working.

Normal adjustment	Warning sign
Mild tingling for a minute after application	Stinging from products that never used to sting, including plain moisturizer
Slight dryness in week one that settles	Tightness that returns within an hour of moisturizing
Fine, even flaking that stops	Flaking that concentrates around the nose and mouth and keeps returning
Skin looks brighter under normal light	Skin looks tight and shiny in a waxy way rather than dewy
Transient pinkness after use	Redness that is still there the next morning
No change in sensitivity to weather	New reactivity to wind, heat, or sweat

If several of the right hand column apply, the fix is subtraction. Stop every acid, retinoid, scrub, and cleansing brush. Run a plain routine of gentle cleanser, a bland moisturizer, and sunscreen for two to four weeks. Reintroduce one acid at a lower frequency than before. Wait until skin has stopped reacting first. The full recovery sequence is covered in the guide to repairing a compromised skin barrier.

Two limits worth stating plainly. Persistent redness, burning, or a rash is not something to manage with product swaps. And if you have a diagnosed condition such as rosacea or eczema, talk to a board certified dermatologist first. A general guide is not the right input for that decision.

How to build a routine around one acid

Pick one. Running an AHA and a BHA on the same night is the fastest route to the table above. It also makes it impossible to tell which one did anything.

Structured schedules exist for this reason. Skin cycling spaces an exfoliant and a retinoid across a four night rotation. Recovery nights sit between them. That caps weekly exposure without any tracking.

A reasonable starting cadence is once or twice a week for an AHA or BHA. A PHA can go on more often if it is well tolerated. Patch test on the jawline for several days first. Give any acid eight to twelve weeks before judging it, since surface turnover and pigment changes are slow.

FAQ

Can you use AHA and BHA together?

You can, but there is little reason to when starting out. Combining them raises irritation risk without a clear payoff, and it removes your ability to tell which acid helped. If you want both, alternate nights rather than layering. Skin that already stings from a single acid will not tolerate two.

Is PHA actually strong enough to do anything?

Yes. In the one direct comparison, a gluconolactone regimen matched a glycolic acid regimen on most photoaging measures over 12 weeks. It differed significantly on only two antiaging endpoints, and caused significantly less stinging and burning (PMID 15002657). Tolerability was the clearer difference between the two regimens.

What pH should a chemical exfoliant be?

Most leave on exfoliants on the market are formulated in the low 3s to low 4s. That is a market observation rather than a rule. Raise the pH and more of the acid exists as a salt, so less is available in free form. Natural skin surface pH is estimated at about 4.7 (PMID 18489300). Any exfoliant is a deliberate, temporary dip below that.

Do you need sunscreen with a BHA or PHA too?

Wear sunscreen daily regardless. That said, the evidence differs by class. Ten percent glycolic acid increased erythema, DNA damage, and sunburn cells in treated human skin. Two percent salicylic acid produced no significant change in the same study (PMID 19411163). Glycolic acid users have the strongest reason to be strict about it.

How long does it take for a chemical exfoliant to work?

Texture usually shifts first, over roughly two to four weeks. Tone and pigment take longer, often eight to twelve weeks or more. They depend on turnover cycles rather than surface shedding. Judging an acid after a week is the most common reason people abandon one that was working.

Which acid should a beginner start with?

Start with the mildest option that addresses your actual complaint. Rough texture points to a low strength AHA such as lactic or mandelic. Congested pores point to salicylic acid, recognized as an over the counter acne active at 0.5 to 2 percent. Reactive or easily reddened skin points to a PHA. Never start with three.