AHA, BHA, PHA: Choosing an Exfoliating Acid Without Wrecking Your Skin

Alpha hydroxy acids (AHAs) all work the same way: they loosen the bonds holding dead skin cells together so they shed more evenly. But the size of the molecule changes everything. Glycolic acid is the smallest AHA and the most-studied, so it gets deep and delivers the strongest results on fine lines, dullness and texture. If you want visible change fast, glycolic is your fastest route, though it also carries the highest irritation risk and the most pronounced sun sensitivity.

Lactic acid is larger, penetrates more slowly, and doubles as a humectant, meaning it draws water to the surface while it exfoliates. This is why lactic acid tends to feel less drying than glycolic at the same percentage, a real advantage for drier or reactive skin. It still raises sun sensitivity, but usually a notch gentler in the process.

Mandelic acid is the largest of the three and works the slowest. It's marketed as the AHA for reactive skin, acne-prone skin, and darker skin tones at risk of post-inflammatory hyperpigmentation after too-aggressive exfoliation. Its evidence base is younger than glycolic or lactic, but it's studied specifically for melasma-prone and blemish-prone skin, which is its actual strength. Mandelic acid is not a substitute for glycolic; it's a different tool for different needs.

Salicylic acid, or BHA, is oil-soluble where AHAs are water-soluble. That one difference changes everything. Because it dissolves in oil, it can work inside a clogged pore the way a water-soluble exfoliant cannot. This makes BHA the first choice for oily, combination and congestion-prone skin, and the reason it shows up so often in treatments for visibly clogged pores and surface bumps.

You don't need high percentages of salicylic acid to see results. Most leave-on formulas sit at 0.5 to 2%, and higher percentages mainly increase the chance of dryness without doing more for the appearance of your pores. BHA doesn't raise sun sensitivity the way AHAs do, which is a practical advantage. If your main concern is pores or blemishes rather than tone or fine lines, salicylic acid is usually the right acid to reach for.

Polyhydroxy acids (PHAs) are the new kids, and gluconolactone is the most common. It's a much larger molecule than any AHA, which means it mostly works at the very surface and is generally considered suitable for skin that reacts poorly to other acids. It's also a humectant and mild antioxidant on top of its exfoliating action, part of why it turns up in lines built for sensitive or rosacea-prone skin.

The trade-off is real: gluconolactone works more gently, which means it delivers subtler results than a comparable percentage of glycolic or lactic acid. It's not meant to replace stronger acids if your skin can tolerate them. It's an entry point into exfoliation for reactive skin, or a maintenance option when you need something truly gentle. It's also less likely to raise sun sensitivity than AHAs, though daily sunscreen is still good practice with any exfoliating routine.

Azelaic acid is a dicarboxylic acid found naturally in grains, and it doesn't fit neatly into any family. It's studied for the appearance of an even, clear-looking complexion, which means it works on redness, tone, and post-blemish marks at the same time. Cosmetic formulas sit at around 10% and are formulated differently than prescription azelaic acid, which is a higher-strength drug.

What makes azelaic acid remarkable is that it's well tolerated on reactive skin while still delivering results. It doesn't raise sun sensitivity like AHAs do, and it doesn't risk the irritation that comes from stacking acids on top of retinoids. This makes azelaic acid the acid for people who find glycolic too harsh but lactic acid too slow, or for anyone trying to use both exfoliants and retinoids without wrecking their skin barrier.

Retinoids and exfoliating acids both work by encouraging cell turnover. Use them together in the same layer and you're asking your skin to do twice as much work, which shows up as dryness, peeling, redness, and irritation. The published research on AHAs specifically flags layering them with retinoids in the same application as a risk factor for visible irritation.

The sensible approach is alternating: retinoid on some nights, acid on others, or using acid in the morning and retinoid at night. If you're going to layer them, space them out or use gentler versions of each. Azelaic acid is the one exception; it's studied as compatible with retinoids since it doesn't have the same irritation profile as stronger AHAs. But even then, introduce both slowly and monitor how your skin responds.

AHAs (all three of them: glycolic, lactic, mandelic) raise your skin's sun sensitivity. This is not a marketing scare tactic; it's a documented effect tied to how they work. When you use an AHA, daily broad-spectrum sunscreen becomes non-negotiable, not optional. Skipping it while using these actives defeats the entire point and leaves skin more exposed to UV damage.

BHA doesn't raise sun sensitivity notably, and gluconolactone is even gentler in that regard. But daily sunscreen is still good practice if you're exfoliating regularly, since any active that encourages cell turnover is asking your skin to adapt, and UV protection supports that process. Azelaic acid doesn't raise sun sensitivity, another reason it's useful for people juggling multiple actives.

Glycolic and salicylic acids are typically used once daily or a few times per week, depending on concentration and skin tolerance. Lactic acid at lower concentrations can be used nightly, but starting a few nights a week and building up is the sensible approach. Mandelic acid works similarly, though its gentler profile means drier skin might tolerate it better. Gluconolactone, because of its mildness, is often formulated for daily use, but introducing any new active gradually is still wise.

Azelaic acid, at 10% cosmetic strength, is generally well tolerated and designed for regular use, though patch testing first is still sensible for anything new. The principle across all of these is the same: introduce slowly, use consistently, and give your skin time to adapt before adding anything else.