AHA, BHA, PHA: the acid family tree

Quick answer

AHAs (glycolic, lactic, mandelic) are water-soluble and work on the surface — tone, texture, pigmentation. BHAs, which in practice means salicylic acid, are oil-soluble and get into the pore, which is why they’re the acne family. PHAs (gluconolactone, lactobionic) are larger molecules that work only on the surface but barely irritate anyone, which makes them the sensitive-skin and post-procedure family. One family at a time. Two to three nights a week. Sunscreen the next morning, every time, no exceptions.

The AHAs

Glycolic acid is the smallest molecule in the family, from sugar cane. The size is the whole story — it penetrates the deepest and works the fastest, and it also irritates the most. Five to ten percent in serums; thirty to seventy percent in professional peels (don’t do those at home). Strong evidence for fading pigmentation, smoothing texture, and softening fine lines over weeks. Best fit: normal-to-oily skin from the twenties on, no reactive history.

Lactic acid comes from milk fermentation. The molecule is larger than glycolic, the action is gentler, and it has a useful side property — it’s a humectant. So it pulls moisture into the skin while it’s exfoliating, which is why dry skin tolerates it better than glycolic. Five to ten percent. Good for dull skin and mild texture concerns without the irritation glycolic can bring.

Mandelic acid comes from bitter almonds. Largest of the common AHA molecules, slowest penetration, gentlest action. Two specific use cases where it shines: darker skin tones, because the slower action means less risk of post-inflammatory pigmentation; and acne-prone skin, because it has a mild antibacterial effect that the other AHAs don’t.

Citric, tartaric, and malic acids show up in formulations but rarely as the main act. Worth knowing they exist; not worth optimizing around.

The BHA

In practice, BHA means salicylic acid. It’s oil-soluble, which is the whole point — it can travel down into the sebaceous gland and the pore in a way AHAs can’t. Half a percent to two percent in OTC products. Strong evidence for blackheads, comedonal acne, and oily congestion. It’s also anti-inflammatory, which helps with active acne lesions. One footnote: it’s aspirin-derived. If you have a salicylate allergy, skip the whole family.

Betaine salicylate is a gentler cousin that shows up in some K-beauty products. Similar benefits, lower irritation. Worth a mention if salicylic acid has historically been too much for your skin.

The PHAs

Gluconolactone is the most common PHA. The molecule is much larger than the AHAs, which means it doesn’t penetrate deeply — it works only on the very surface. The trade-off is that almost nobody finds it irritating, it doesn’t measurably increase sun sensitivity, and it’s compatible with skin that flares on everything else. Slower visible results, but consistent.

Lactobionic acid is similar — gentle, surface-acting, with bonus antioxidant and humectant properties.

PHAs have become the default for sensitive skin, rosacea-prone skin, and the period after professional treatments (Botox, peels, microneedling) when the skin doesn’t want to be challenged.

Picking the right one

Match the acid to the actual goal.

Dull or pigmented skin with normal tolerance: glycolic 7 to 10 percent, twice a week.

Dry or sensitive skin with surface texture concerns: lactic 5 to 10 percent, twice a week.

Acne, oily skin, blackheads: salicylic 1 to 2 percent, three times a week.

Darker skin tone, hyperpigmentation prone: mandelic 5 to 10 percent, twice a week.

Reactive, rosacea-prone, very sensitive: PHAs (gluconolactone), up to daily if tolerated.

The mistake here is trying to use multiple acids to “cover all bases.” Two acids stacked in the same routine rarely outperform one acid used consistently. They do reliably raise irritation risk.

How to introduce one without trashing your skin

Once a week for the first two weeks, on a clean face at night. Wait twenty minutes after application, then a gentle moisturizer. If your skin handles it — no redness, no tightness, no stinging — twice a week from week three. Three times a week by week five if you actually need it. Most people settle at two to three nights and stop there. Daily acid is rarely necessary outside specific clinical reasons.

What not to put in the same slot

Retinoids — alternate nights. High-concentration vitamin C — vitamin C in the morning, acids at night. Benzoyl peroxide — alternate nights. Other acids — don’t stack AHA and BHA in one routine.

Sunscreen the next morning. Acids increase sun sensitivity for the next twenty-four to forty-eight hours. Unprotected exposure during that window cancels the brightening benefit you came for.

Mistakes I see often

Going daily too fast. Most acids aren’t designed for daily use. Two to three times a week is the sustainable cadence for almost every skin type.

Mixing an acid serum with an acid toner and an acid exfoliating pad. That’s not a routine, that’s a barrier breakdown in progress.

Expecting acne to clear in two weeks of salicylic. It takes six to eight weeks to show clear improvement on stubborn acne. Quit before that and you’ll never know what worked.

Skipping moisturizer after acids. The skin loses water more easily for hours after exfoliation. Always seal with moisturizer.

Using acids while recovering from a chemical peel or microneedling. Pause for at least a week after any professional treatment.

FAQ

Can I use an AHA and a BHA in the same routine? Generally not. Use one family at a time on alternate nights. Some products do combine them at low concentrations (The Ordinary’s AHA 30% + BHA 2% peel is the classic) but those are explicitly weekly, not daily.

Are acid pads as effective as serums? Pads are convenient but the dose isn’t precise. Serums give you control over coverage and concentration, which matters more than the time you save with pads.

Do acids thin the skin? No. This is one of the most durable myths in skincare. Acids speed up the shedding of dead surface cells. They don’t thin the living layers. People who use them long-term actually have measurably stronger barriers because of the regenerative response.

Can I exfoliate while using retinol? Yes, on alternating nights. Never the same night, for most people.

Sources

Kornhauser A et al. Applications of hydroxy acids. Clinical, Cosmetic and Investigational Dermatology, 2010. Tang SC, Yang JH. Dual effects of alpha-hydroxy acids on the skin. Molecules, 2018.

Keep reading

• Glycolic vs lactic acid: which AHA belongs in your routine
• BHA explained
• Body acne: chest, back, and what's actually on your butt