AHA and BHA are two types of chemical exfoliants used in skincare to remove dead skin cells, unclog pores, and improve skin texture. The key difference: AHAs are water-soluble and work on the skin’s surface, while BHAs are oil-soluble and can penetrate deeper into pores. Which one you should use depends on your skin type and what you’re trying to fix.
How AHAs Work
Alpha hydroxy acids are derived from sugary fruits, milk, and other natural sources. The most common AHAs in skincare products are glycolic acid (from sugar cane), lactic acid (from milk), and mandelic acid (from almonds). Because they’re water-soluble, they work on the outermost layer of skin, loosening the bonds between dead cells so they shed more easily. Fresh, more evenly pigmented skin cells replace them.
This surface-level action makes AHAs particularly effective for concerns that live on top of the skin: dullness, uneven tone, fine lines, sun spots, and rough texture. AHAs also stimulate collagen production over time, which is why they’re a go-to for anti-aging routines. Lactic acid can smooth and soften fine lines and wrinkles, though it won’t do much for deeper creases. It also has hydrating properties that make it a good fit for dry or dehydrated skin.
Not all AHAs penetrate equally. Glycolic acid has the smallest molecular size, so it absorbs quickly and delivers the most intense exfoliation. Mandelic acid has a much larger molecule, which means it penetrates more slowly and causes less irritation. That makes mandelic acid a better starting point for sensitive skin or darker skin tones, where aggressive exfoliation carries a higher risk of post-inflammatory hyperpigmentation.
How BHAs Work
Beta hydroxy acid in skincare almost always means one ingredient: salicylic acid. Its oil-soluble structure is what sets it apart. While AHAs sit on the surface, salicylic acid can cut through the oily sebum inside your pores, dissolving the mix of dead skin cells and oil that forms blackheads, whiteheads, and deeper clogged pores.
Salicylic acid also has mild anti-inflammatory properties, which makes it effective for treating not just clogged pores but also the red, swollen pimples (papules and pustules) that come with inflammatory acne. This combination of pore-clearing and calming effects is why BHA shows up in so many acne-focused products.
Which One Matches Your Skin Concern
For acne, both acids help, but BHA generally has the edge. In a split-face study comparing 30% glycolic acid on one side and 30% salicylic acid on the other, both improved acne significantly. However, salicylic acid produced more lasting results with fewer side effects. That said, mandelic acid (an AHA) performed equally well as salicylic acid peels for mild-to-moderate acne in another trial, so AHAs aren’t off the table if your breakouts are on the milder side.
For hyperpigmentation and melasma, AHAs are the stronger choice. Glycolic acid peels have been shown to reduce melasma severity scores, and lactic acid peels produced significant improvement with no recorded adverse effects in one study. For uneven skin tone in general, mandelic acid pulls double duty on both acne and pigmentation.
For fine lines and wrinkles, reach for AHAs. Glycolic acid, lactic acid, malic acid, and tartaric acid all target fine lines by boosting cell turnover and collagen synthesis at the skin’s surface.
For oily, acne-prone skin, BHA is the more logical starting point since it works inside the pore where excess oil accumulates. For dry or sensitive skin, lactic acid is often the best entry into chemical exfoliation because it exfoliates gently while also attracting moisture to the skin.
Concentration and pH Matter
These acids only work properly within a specific pH range. The Cosmetic Ingredient Review Panel recommends that AHA products stay below 10% concentration with a pH at or above 3.5 to balance effectiveness with safety. Products below that pH threshold, or at very high concentrations, increase the risk of irritation, redness, and damage to the skin barrier. Most over-the-counter products are formulated within these guidelines, but it’s worth checking if you’re buying from smaller or less regulated brands.
Salicylic acid in consumer products typically ranges from 0.5% to 2%. A lower percentage is enough for daily maintenance, while 2% targets active breakouts more aggressively.
Sun Sensitivity Is Real
AHAs make your skin measurably more vulnerable to UV damage. FDA-sponsored research found that after four weeks of AHA use, skin sensitivity to UV-induced reddening increased by 18%, and sensitivity to cellular damage from UV roughly doubled on average. The FDA recommends that AHA products carry a sunburn alert label and advises using sunscreen, wearing protective clothing, and limiting sun exposure while using these products and for a week after stopping them.
BHA does not carry the same degree of UV sensitivity risk, partly because salicylic acid’s anti-inflammatory action offers a small buffer. Still, daily sunscreen is a smart baseline whenever you’re using any exfoliating acid.
Using Them With Other Active Ingredients
One of the most common concerns is whether AHAs or BHAs can be layered with retinol or vitamin C. The short answer: yes. The worry that acidic exfoliants would deactivate retinol came from a misunderstanding of skin chemistry. AHAs and BHAs do not actually lower the skin’s pH enough to interfere with the enzyme that converts retinol into its active form. Retinol itself is dissolved in oil, making it effectively a waterless ingredient with no pH to disrupt.
Vitamin C and retinol also pair well together. Both function in acidic environments, and vitamin C helps protect retinol from breaking down as it absorbs into the skin, potentially boosting its anti-aging effects. The one real caution is layering multiple retinoid products at the same time, which can over-sensitize skin. But combining a retinol with an AHA or BHA exfoliant, or with vitamin C, is supported by research and widely practiced.
That said, if you’re new to chemical exfoliation, introducing one active at a time lets you gauge how your skin responds before stacking products.
Purging vs. a Bad Reaction
When you start using an AHA or BHA, you may notice new breakouts in the first couple of weeks. This is often “purging,” where increased cell turnover pushes trapped oil and dead skin cells to the surface faster than usual. Purging typically shows up in areas where you already tend to break out, and it resolves within a few weeks as your skin adjusts.
A genuine adverse reaction looks different. If you experience burning, persistent redness, intense itching, or breakouts in areas where you don’t normally get them, the product is likely irritating your skin rather than renewing it. Location, duration, and appearance are the three things to pay attention to. Purging is temporary and predictable. Irritation tends to spread, worsen, or linger well beyond that initial adjustment window.