Chemical exfoliation has become a cornerstone of modern skincare, and for good reason. Unlike physical scrubs that rely on abrasive particles to buff away dead skin cells, chemical exfoliants use acids to dissolve the bonds holding dead cells to the surface. The result is smoother texture, brighter tone, fewer breakouts, and better absorption of every product you apply afterward.
But walk into any skincare aisle and you will find a confusing array of options: glycolic acid, salicylic acid, lactic acid, mandelic acid, and more. These all fall into two main categories, AHAs and BHAs, and understanding the difference between them is essential for choosing the right exfoliant for your skin type and concerns. This guide breaks down what each type does, who should use which, how to determine the right concentration and frequency, and whether you can safely combine them.
What Are AHAs?
Alpha hydroxy acids (AHAs) are water-soluble acids derived primarily from natural sources. They work on the skin's surface by dissolving the "glue" (desmosomes) that holds dead skin cells together, allowing them to shed more efficiently. This process reveals the fresher, more evenly pigmented skin cells beneath.
AHAs also stimulate cell turnover in the epidermis and, at higher concentrations, can boost collagen production in the dermis. This makes them effective for addressing surface-level concerns like dullness, uneven texture, fine lines, and hyperpigmentation.
Glycolic Acid
Glycolic acid is the most well-researched and widely used AHA. It has the smallest molecular size of all AHAs, which means it penetrates the skin most effectively. This makes it the most potent AHA, but also the most likely to cause irritation if used at high concentrations or on sensitive skin. Glycolic acid is excellent for addressing dullness, sun damage, fine lines, and uneven tone. Typical concentrations in daily-use products range from 5% to 10%, while professional peels may go up to 30% to 70%. For a detailed exploration of this ingredient, see our glycolic acid guide.
Lactic Acid
Lactic acid has a larger molecular size than glycolic acid, so it penetrates less deeply and causes less irritation. It also has humectant properties, meaning it attracts moisture to the skin while exfoliating. This makes it the best AHA for dry or sensitive skin types that want the benefits of chemical exfoliation without harsh effects. Common concentrations range from 5% to 12%. Lactic acid is derived from milk (though most cosmetic formulations use synthetic lactic acid) and has been used for skin beautification since ancient times.
Mandelic Acid
Mandelic acid has the largest molecular size of the commonly used AHAs. It penetrates the skin slowly and evenly, making it the gentlest AHA option. It is particularly well suited for darker skin tones because the slow, even penetration reduces the risk of post-inflammatory hyperpigmentation (PIH), a common concern with more aggressive exfoliants on melanin-rich skin. Mandelic acid also has mild antibacterial properties, which gives it a slight edge for acne-prone skin compared to other AHAs.
What Is BHA?
Beta hydroxy acid (BHA) refers, in the skincare world, almost exclusively to one ingredient: salicylic acid. While there are technically other BHAs, salicylic acid is the only one used meaningfully in skincare products.
The defining characteristic of BHA is that it is oil-soluble. Unlike water-soluble AHAs that work on the skin's surface, salicylic acid can dissolve in oil and penetrate into the pores themselves. Once inside, it breaks down the mixture of sebum, dead skin cells, and debris that causes clogged pores, blackheads, and acne.
Salicylic acid is also anti-inflammatory, which helps reduce the redness and swelling associated with active breakouts. This combination of pore-penetrating exfoliation and anti-inflammatory action makes BHA the gold standard for oily, acne-prone skin.
How BHA Differs From AHA
The fundamental difference comes down to solubility and, therefore, where each type works.
- AHAs are water-soluble. They work on the skin's surface, dissolving the bonds between dead cells. They are best for surface concerns: dullness, rough texture, fine lines, and uneven pigmentation.
- BHA is oil-soluble. It penetrates into the pore lining and works from the inside out. It is best for pore-related concerns: blackheads, whiteheads, acne, and enlarged pores.
Think of it this way: AHAs resurface the top of your skin. BHA cleans out the inside of your pores. They address different problems through different mechanisms, which is why some people benefit from using both (on alternate days, not simultaneously).
Which Skin Types Benefit From Each
Choosing between AHA and BHA comes down to your primary skin concerns and skin type. Here is a clear breakdown.
Choose AHA If You Have:
- Dry skin: AHAs (especially lactic acid) provide exfoliation with added hydration. They do not strip oil or dry out the skin.
- Sun-damaged skin: Glycolic acid is particularly effective at reducing the appearance of sun spots, uneven pigmentation, and rough texture caused by UV exposure.
- Fine lines and early aging: AHAs stimulate collagen production in the dermis, which can reduce the depth of fine lines over time.
- Dull, rough texture: By removing the buildup of dead cells on the surface, AHAs reveal brighter, smoother skin immediately.
- Hyperpigmentation: AHAs accelerate the turnover of pigmented cells, fading dark spots faster than they would on their own.
Choose BHA If You Have:
- Oily skin: Salicylic acid regulates oil within the pore and reduces the shine that comes from excess sebum.
- Acne-prone skin: BHA penetrates pores to clear existing clogs and prevent new ones from forming. It is effective against both blackheads and inflammatory acne.
- Enlarged pores: By clearing the debris that stretches pores open, BHA helps minimize their appearance over time.
- Combination skin: If your T-zone is oily and congested but your cheeks are normal or dry, BHA can be applied selectively to problem areas.
- Sensitive, acne-prone skin: At low concentrations (0.5% to 1%), salicylic acid is one of the gentlest effective acne treatments available, partly because of its anti-inflammatory properties.
Concentration Guide
Using the right concentration is critical for getting results without causing irritation. Higher concentrations are not always better, and starting too strong is a common mistake.
AHA Concentrations
- Glycolic acid 5% to 8%: Good starting range for beginners and normal skin. Provides noticeable exfoliation with minimal irritation risk.
- Glycolic acid 10% to 15%: Intermediate strength for experienced users. More effective for stubborn hyperpigmentation and deeper texture issues.
- Glycolic acid 20%+: Professional-grade. Use only as directed by a dermatologist or esthetician. Risk of chemical burns at high concentrations.
- Lactic acid 5% to 10%: Effective range for most users. Even the lower end provides meaningful exfoliation due to lactic acid's favorable penetration profile.
- Mandelic acid 5% to 10%: The gentlest option. Suitable for sensitive skin and darker skin tones. Can be used more frequently than glycolic acid.
BHA Concentrations
- Salicylic acid 0.5%: Gentle enough for daily use. Good for mild congestion and maintenance after clearing breakouts.
- Salicylic acid 1%: Moderate strength. Suitable for regular use (every other day) for persistent blackheads and mild acne.
- Salicylic acid 2%: The maximum concentration available in over-the-counter products in most countries. Effective for moderate acne and oily skin. Use 2 to 3 times per week to start, increasing as tolerated.
Frequency Recommendations
How often you exfoliate is just as important as which exfoliant you choose. Over-exfoliation leads to barrier damage, increased sensitivity, and paradoxically worse skin. Under-exfoliation means you are not getting the full benefit of the product.
For Beginners
Start with once per week regardless of whether you choose AHA or BHA. Use the product in the evening (AHAs increase sun sensitivity, and nighttime application gives your skin hours to recover before UV exposure). Apply to clean, dry skin, wait for absorption (1 to 2 minutes), then continue with your usual moisturizer.
For Established Users
Most skin types do well with chemical exfoliation 2 to 3 times per week. Some resilient skin types can tolerate daily use of lower-concentration products (like a 0.5% salicylic acid toner or a 5% lactic acid serum), but this should be built up gradually over several weeks. If you notice any increased sensitivity, redness, or flaking, reduce the frequency immediately.
Maximum Safe Frequency
For most people, the upper limit is every other day for low-concentration products and 3 times per week for higher-concentration products. Daily use of high-concentration AHAs (10%+) or BHA (2%) is almost never necessary and significantly increases the risk of barrier damage. The goal is consistent, moderate exfoliation, not aggressive surface removal.
Can You Use AHA and BHA Together?
This is one of the most frequently asked questions in skincare, and the answer is nuanced. You can use both AHA and BHA in your routine, but how you combine them matters.
Alternating Days (Recommended)
The safest approach is to use AHA on some nights and BHA on others. For example, use salicylic acid on Monday and Thursday, and glycolic acid on Tuesday and Friday. This gives your skin the benefits of both without the irritation risk of layering them together. Leave at least one or two nights per week with no exfoliant at all to allow the skin barrier to maintain itself.
Same Night, Different Steps
Some experienced users apply BHA first (to clean out pores), wait 15 to 20 minutes, then apply AHA (to exfoliate the surface). This can be effective but carries a higher risk of irritation. Only attempt this if your skin has established tolerance to each product individually, and limit this combination to once per week at most.
Combination Products
Some products contain both AHA and BHA in a single formula. These are typically formulated at lower concentrations of each to reduce irritation risk. They can be convenient, but you lose the ability to adjust the ratio based on your skin's needs on any given day.
PHA: The Gentler Alternative
Polyhydroxy acids (PHAs) represent a third category of chemical exfoliant that deserves mention. The most common PHAs are gluconolactone and lactobionic acid. They function similarly to AHAs but have significantly larger molecular sizes, which means they penetrate the skin much more slowly and cause far less irritation.
PHAs also have humectant and antioxidant properties, making them multifunctional ingredients that exfoliate, hydrate, and protect simultaneously. They do not increase sun sensitivity to the same degree as AHAs, though sunscreen is still essential.
PHAs are ideal for people who cannot tolerate AHAs or BHAs, including those with rosacea, eczema, or extremely sensitive skin. They are also a good option for beginners who want to start chemical exfoliation with the lowest possible risk. The trade-off is that PHAs are less potent than AHAs or BHAs, so results take longer to appear and are more subtle. For skin that is not sensitive, AHAs and BHAs will deliver faster, more dramatic results.
Common Mistakes to Avoid
Even with the right product, errors in usage can undermine your results or damage your skin.
- Skipping sunscreen: AHAs increase photosensitivity for up to 7 days after use. BHA does this to a lesser extent. Failing to wear SPF 30+ daily while using chemical exfoliants will accelerate sun damage, the exact problem you may be trying to fix.
- Combining with retinol on the same night: Both retinol and chemical exfoliants accelerate cell turnover. Using them together can overwhelm the skin, causing redness, peeling, and barrier damage. Alternate them on different nights.
- Starting with high concentrations: A 10% glycolic acid toner might seem harmless, but if your skin has never been exposed to AHAs before, it can cause a significant reaction. Always start with the lowest available concentration.
- Exfoliating broken or sunburned skin: Chemical exfoliants should never be applied to compromised skin. Wait until your skin has fully healed before resuming use.
- Ignoring the pH factor: Chemical exfoliants work within a specific pH range. AHAs are most effective at a pH between 3.0 and 4.0. Salicylic acid works best between 3.0 and 4.0 as well. Products formulated at a pH above this range are less effective. If a product does not list its pH, it may not be optimized for efficacy.
Building Your Exfoliation Routine
To pull everything together, here is a practical framework for incorporating chemical exfoliants into your routine based on your skin type.
Oily, acne-prone skin: Start with 2% salicylic acid, 2 times per week. After 4 weeks, increase to 3 times per week if tolerated. If you also want surface smoothing, add a mandelic acid product on alternate nights.
Dry, dull skin: Start with 5% lactic acid, once per week. After 4 weeks, increase to twice per week. If well tolerated, you can gradually move to a 10% glycolic acid product for more intensive resurfacing.
Combination skin: Use BHA on oily areas (T-zone) and AHA on drier areas (cheeks), or alternate between AHA nights and BHA nights throughout the week.
Sensitive skin: Start with a PHA product (gluconolactone) once per week. After 6 weeks, if your skin responds well, you can try a 5% lactic acid or 0.5% salicylic acid on an alternating schedule.
Mature skin: Glycolic acid (5% to 10%) is the strongest choice for stimulating collagen and addressing fine lines. Use 2 to 3 times per week. Pair with retinol on alternate nights for maximum anti-aging benefit, but monitor for irritation closely.
Frequently Asked Questions
Can chemical exfoliants cause purging, and how is purging different from a breakout?
Yes, chemical exfoliants can cause purging. Purging happens because AHAs and BHAs accelerate cell turnover, which pushes existing clogs and micro-comedones (tiny, invisible blockages) to the surface faster than they would emerge on their own. Purging typically appears as small whiteheads or pimples in areas where you normally break out. It should begin within the first 1 to 2 weeks of starting a new exfoliant and resolve within 4 to 6 weeks. A true breakout, by contrast, appears in areas where you do not normally get pimples, persists beyond 6 weeks, or involves deep, painful cysts. If you are breaking out in new areas or the breakouts are getting worse after 6 weeks, stop using the product. That is not purging. That is a negative reaction to the product.
Should I apply chemical exfoliants to wet or dry skin?
Apply chemical exfoliants to clean, dry skin. Water on the skin's surface dilutes the acid, reducing its effectiveness and altering its pH. More importantly, applying acids to damp skin can increase penetration in an uneven way, leading to irritation in some areas while other areas receive insufficient exfoliation. After cleansing, pat your skin dry and wait about 30 seconds to a minute before applying your AHA or BHA. This ensures even distribution and consistent results. After the exfoliant has absorbed (typically 1 to 2 minutes for leave-on products), you can then apply your hydrating products and moisturizer.
Do AHA and BHA products expire, and does this affect their effectiveness?
Yes, chemical exfoliant products do expire, and expired products can be both less effective and potentially more irritating. AHAs and BHAs are active compounds whose potency degrades over time, especially when exposed to air, light, and heat. Most products have a PAO (Period After Opening) symbol on the packaging, typically 6 to 12 months. After this period, the acid concentration may have dropped below the effective threshold, or the product's pH may have shifted. An expired exfoliant that has degraded unevenly can also cause patchy irritation. Store your exfoliant products in a cool, dark place (not in the shower or next to a window), keep lids tightly closed, and replace them within the recommended timeframe.
Is it safe to use chemical exfoliants during pregnancy?
Glycolic acid and lactic acid (AHAs) are generally considered safe during pregnancy at the concentrations found in over-the-counter skincare products. These water-soluble acids have minimal systemic absorption when applied topically. Salicylic acid (BHA) is more nuanced. Low-concentration topical salicylic acid (2% or less in a face product) is considered low risk by most dermatologists and the American College of Obstetricians and Gynecologists. However, high-concentration salicylic acid peels (20%+) and oral salicylates should be avoided. As a precaution, many pregnant individuals switch to lactic acid or PHA exfoliants to avoid the salicylic acid question entirely. Always consult your healthcare provider before making changes to your skincare routine during pregnancy, as individual circumstances vary.