Chemical exfoliation is one of the most effective things you can add to a skincare routine, and two categories of acids do most of the heavy lifting: AHAs (alpha hydroxy acids) and BHAs (beta hydroxy acids). If you’ve ever stared at a product label wondering whether to reach for glycolic acid or salicylic acid, you’re not alone. The difference between these two types of exfoliants is fundamental, and picking the wrong one for your skin type won’t deliver the results you’re looking for.
This guide covers exactly how AHAs and BHAs work, which concerns each one targets best, how to use them safely, and when using both makes more sense than choosing one.
What Is Chemical Exfoliation and Why Does It Matter?
Your skin naturally sheds dead cells every day, a process called desquamation. As you age, this process slows considerably. The result is a gradual build-up of dead skin cells on the surface, leading to dullness, uneven texture, clogged pores, and a more pronounced appearance of fine lines.
Physical exfoliants (scrubs, brushes) remove dead skin manually, but they’re inconsistent and can cause micro-tears in the skin, particularly in sensitive or active-breakout skin. Chemical exfoliants work differently. They dissolve the bonds holding dead skin cells together, allowing them to shed more efficiently and evenly without abrasion.
AHAs and BHAs are the two main categories of chemical exfoliants. Both work by breaking down these inter-cellular bonds, but where they work, how deeply they penetrate, and which concerns they target differ in important ways.
For effective action, both AHAs and BHAs need to be formulated at a pH between 3 and 4. Below this range they become too aggressive; above it, they lose efficacy. This is why formulation quality matters more than concentration alone.
What Are AHAs (Alpha Hydroxy Acids)?
AHAs are water-soluble acids derived primarily from natural sources like sugarcane, milk, fruit, and almonds. Because they are water-soluble, they work on the skin’s surface, loosening and lifting the bonds between dead skin cells in the outermost layer (the stratum corneum) to reveal fresher, brighter skin underneath.
AHAs are the go-to exfoliant for concerns rooted at the skin’s surface: dullness, uneven tone, rough texture, fine lines, and hyperpigmentation.
The Main Types of AHAs
Not all AHAs are the same. Molecular size determines how deeply and quickly each one penetrates, which directly affects both its potency and its irritation potential.
Glycolic Acid Derived from sugarcane, glycolic acid has the smallest molecular size of all AHAs. Small molecules penetrate more readily and quickly, which makes glycolic acid the most potent and fastest-acting AHA available. It is highly effective at resurfacing the skin, stimulating collagen production, reducing fine lines, and fading hyperpigmentation. The trade-off is that its quick penetration makes it more likely to cause irritation, particularly in sensitive or reactive skin. Ideal for normal, oily, or thicker skin types with experience in chemical exfoliation.
Lactic Acid Derived from milk (or synthetically produced), lactic acid has a slightly larger molecule than glycolic acid, meaning it penetrates more slowly and stays closer to the skin’s surface. This makes it a gentler exfoliant while still being effective at smoothing texture and improving tone. Crucially, lactic acid is the only AHA with documented humectant properties, meaning it draws moisture into the skin as it exfoliates. This makes it particularly valuable for dry or dehydrated skin, and it’s a strong option for anyone new to AHA exfoliants.
Mandelic Acid Derived from bitter almonds, mandelic acid has the largest molecular structure of the common AHAs. It penetrates the most slowly, providing a gradual and gentle exfoliation that is significantly less irritating than glycolic or lactic acid. Mandelic acid also has antibacterial and anti-inflammatory properties and can help regulate sebum production, making it uniquely suited to sensitive, acne-prone, or darker skin tones where other AHAs risk triggering post-inflammatory hyperpigmentation. If you’ve had bad experiences with stronger acids, mandelic acid is often the right place to restart.
Other AHAs Citric acid, tartaric acid, and malic acid appear in formulations as secondary AHAs, usually at lower concentrations to support a product’s primary exfoliant rather than doing the main work themselves.
What Are BHAs (Beta Hydroxy Acids)?
BHAs are oil-soluble acids, and this single chemical property is what separates them fundamentally from AHAs. Because they dissolve in oil rather than water, BHAs can penetrate through the sebum (natural oil) that lines the inside of pores and reach the pore lining itself. This is something AHAs simply cannot do.
The primary BHA in skincare is salicylic acid, originally derived from willow bark and now typically synthesised. It is available in over-the-counter products at concentrations between 0.5% and 2%, with higher concentrations reserved for professional treatments.
What Salicylic Acid Does Inside the Pore
Inside the pore, salicylic acid dissolves the keratin plugs and sebum deposits that lead to blackheads, whiteheads, and acne congestion. It exfoliates the pore lining, which helps prevent the build-up that causes breakouts in the first place. Beyond this, salicylic acid has two additional properties that make it particularly valuable for acne-prone skin:
- Anti-inflammatory: Salicylic acid reduces inflammation at the site of breakouts, helping to calm active blemishes rather than just preventing future ones
- Antibacterial: It creates an environment inside the pore that is hostile to acne-causing bacteria (Cutibacterium acnes), adding another mechanism of action against breakouts
This combination of pore-clearing, anti-inflammatory, and antibacterial activity is why salicylic acid has remained the most clinically validated over-the-counter acne ingredient for decades.
AHAs vs BHAs: The Core Differences
| AHAs | BHAs | |
|---|---|---|
| Solubility | Water-soluble | Oil-soluble |
| Where they work | Skin surface (stratum corneum) | Surface + deep inside pores |
| Primary targets | Dullness, texture, fine lines, hyperpigmentation, dry/dehydrated skin | Oily skin, blackheads, whiteheads, acne, enlarged pores |
| Best for | Dry, dull, sun-damaged, or ageing skin | Oily, acne-prone, congested, or combination skin |
| Irritation potential | Moderate (depends on type and concentration) | Generally lower than AHAs |
| Sun sensitivity | Yes, increases photosensitivity | Yes, increases photosensitivity |
| pH range needed | 3 to 4 | 3 to 4 |
| Most common examples | Glycolic acid, lactic acid, mandelic acid | Salicylic acid |
Which Exfoliant Is Right for Your Skin?
Dry or Dehydrated Skin
AHAs are the stronger choice, specifically lactic acid. Its hydrating properties alongside its exfoliating action make it the most appropriate AHA for skin that lacks moisture. Lactic acid smooths the surface and draws moisture in simultaneously. Avoid high concentrations of glycolic acid to start: dry skin is more prone to irritation and barrier disruption with stronger acids.
Oily or Combination Skin
BHA (salicylic acid) is the clear first choice. It penetrates into pores to clear sebum build-up, which is the root cause of the congestion, enlarged pores, and breakouts that oily skin tends to develop. If texture and tone are also concerns, incorporating a gentle AHA (lactic acid) on alternate days can address the surface while salicylic acid handles what’s deeper.
Acne-Prone Skin
Salicylic acid is the primary recommendation. Its combination of pore-clearing, anti-inflammatory, and antibacterial activity addresses acne through multiple mechanisms. For post-breakout marks and hyperpigmentation, layering in a gentle AHA like mandelic acid (which also has anti-acne properties) can help fade the discolouration that lingers after blemishes heal.
Sensitive or Reactive Skin
Mandelic acid is the most accessible AHA: its large molecule size, slow penetration, and anti-inflammatory properties make it the gentlest effective exfoliant available. For BHA, salicylic acid at 0.5% to 1% is generally well-tolerated even by sensitive skin. Start with once or twice a week and build up slowly. Avoid glycolic acid until your skin has built up tolerance.
Dull or Uneven Skin Tone
AHAs are best suited here. Glycolic or lactic acid accelerate cell turnover, lifting surface pigmentation and revealing brighter skin underneath. Both also directly inhibit melanin production to some degree. For darker skin tones where the risk of post-inflammatory hyperpigmentation is higher, mandelic acid is often the safer choice for tone-evening work.
Skin with Fine Lines and Early Signs of Ageing
AHAs, particularly glycolic acid, have the strongest evidence for anti-ageing exfoliation. Glycolic acid stimulates collagen production, improves skin elasticity, and directly resurfaces the skin to reduce fine lines over consistent use. Salicylic acid does not have the same anti-ageing profile, though smoother, less congested pores do contribute to a more youthful appearance.
Normal or Combination Skin
Either works, and many people with balanced skin alternate between both. A BHA 2 to 3 times per week to keep pores clear, and an AHA 2 to 3 times per week for surface texture and tone, is a sustainable approach once the skin has adjusted to chemical exfoliation.
AHA vs BHA Strength and Concentration Guide
Concentration matters, but it doesn’t tell the whole story. The type of AHA, the product’s pH, and the form of the product (rinse-off vs leave-on) all affect how strong the exfoliation actually is.
AHA concentrations:
- 5% or below: Very gentle; suitable for beginners, sensitive skin, and daily use in some formulas
- 5% to 10%: Moderate; effective for most skin types in leave-on toners and serums
- 10% to 15%: Strong; best for experienced users with resilient skin; often in targeted treatment formulas
- Above 20%: Professional-grade peels; not for at-home daily use
BHA concentrations:
- 0.5% to 1%: Gentle; suitable for daily use and sensitive skin
- 2%: The standard for most effective over-the-counter salicylic acid products
- Above 2%: Typically reserved for professional treatments and prescription formulations
Leave-on products (toners, serums, pads that aren’t rinsed off) are generally more effective than rinse-off products like cleansers, because the acid has more contact time with the skin to work at the required pH.
Can You Use AHAs and BHAs Together?
Yes, and for many people with multiple concerns, using both makes sense. They work on different skin structures through different mechanisms, so they’re complementary rather than redundant.
The key is not to overdo it. Over-exfoliation is one of the most common skincare mistakes, and the signs of a damaged moisture barrier (persistent redness, tightness, stinging, increased sensitivity, or paradoxically, more breakouts) are easy to miss until the damage is already done.
Safe approaches to using both:
Alternating days: Use AHA on some evenings and BHA on others. This is the most widely recommended approach for beginners using both.
Morning and evening split: Apply one in the morning and the other at night, though both require sunscreen regardless.
Zone application: Apply BHA to oilier areas (T-zone, nose, chin) and AHA to the rest of the face where pore-congestion is less of an issue.
Combined formula: Several well-formulated products blend low concentrations of both in a single product, which delivers a balanced, less aggressive exfoliation than two separate full-strength products applied simultaneously.
If you’re using retinol or tretinoin, be particularly cautious. Retinoids already increase cell turnover significantly. Adding both an AHA and a BHA on top creates a high risk of barrier damage. If you want to combine actives, choose one acid, not both, and alternate it with retinol on different evenings.
How to Introduce Chemical Exfoliants Into Your Routine
Whether you’re starting with AHA, BHA, or both, the same principle applies: start low and slow, and let your skin set the pace.
Step-by-step introduction:
- Begin with once or twice a week, at a lower concentration, in the evening
- Apply after cleansing and before serums and moisturiser
- Follow with a hydrating moisturiser to support the skin barrier
- Use broad spectrum SPF every morning without exception: both AHAs and BHAs increase photosensitivity
- After two to four weeks with no irritation, increase frequency or concentration as needed
- If you experience persistent redness, stinging, or tightness, reduce frequency and support the barrier with a simple, fragrance-free moisturiser
Do not exfoliate compromised skin. If your barrier is already damaged, broken, or sunburned, introduce acids only after the skin has recovered.
Ingredients to Avoid Pairing with AHAs and BHAs
Some combinations increase the risk of irritation significantly:
Retinol or tretinoin on the same evening: Both increase cell turnover. Using them on the same night regularly is likely to cause barrier disruption. Alternate evenings instead.
Benzoyl peroxide at the same time as BHA: Both are strong acne-targeted actives. Using them together can cause excessive dryness and irritation; separate AM and PM application is safer.
High-concentration vitamin C (ascorbic acid) layered with acids: All operate at a low pH. Layering them can cause irritation without additional benefit. Use vitamin C in the morning and acids in the evening.
Multiple exfoliants at once: Using a physical scrub and a chemical exfoliant on the same day, or two different acid products together, is almost always too much, particularly while you’re still establishing tolerance.
What to Expect: Results and Timeline
Chemical exfoliants are not overnight fixes, but they do produce visible results faster than many other actives.
Short-term (one to two weeks): Improved surface smoothness, more even skin tone, a brighter overall appearance.
Medium-term (four to eight weeks): Noticeable reduction in pore visibility (particularly with BHA), fading of surface pigmentation, improved skin texture.
Long-term (three to six months of consistent use): Meaningful improvement in fine lines and skin firmness (particularly with AHA), significant reduction in recurring breakouts and congestion (with BHA), and a more consistently even and radiant complexion.
Consistency matters more than frequency. Using your exfoliant three times a week without fail will deliver better results than daily use that you abandon after two weeks due to irritation.
Common Questions
Is BHA or AHA better for blackheads? BHA (salicylic acid) is clearly better for blackheads. It’s the only exfoliant that penetrates into the pore to dissolve the sebum and keratin deposits that form them. AHAs work on the surface and cannot reach inside the pore.
Can I use AHA and BHA every day? Most people don’t need to, and daily use of both increases the risk of over-exfoliation. Once daily of a single well-formulated exfoliant is sufficient for most skin types. Three to four times a week is often more than adequate.
Which is better for hyperpigmentation? AHAs, particularly glycolic and mandelic acid, are more targeted for hyperpigmentation. They inhibit melanin production and increase cell turnover to fade existing pigmentation. For darker skin tones, mandelic acid is often recommended over glycolic acid to reduce the risk of triggering more pigmentation through irritation.
Do AHAs and BHAs increase sun sensitivity? Yes. Both categories of chemical exfoliants increase photosensitivity by removing the top layer of skin. Daily SPF is non-negotiable when using either. Skipping sunscreen while exfoliating actively works against you, accelerating pigmentation and UV damage rather than reducing it.
Can people with rosacea use chemical exfoliants? With caution. Salicylic acid at low concentrations (0.5% to 1%) is often tolerated and can help manage the congestion component of rosacea. High concentrations of AHAs can trigger rosacea flares. Patch testing and a dermatologist consultation are strongly recommended before introducing any chemical exfoliant to rosacea-prone skin.
The Bottom Line
AHAs and BHAs are both highly effective exfoliants that work at different depths and address different concerns:
Choose AHAs if your primary concerns are surface-level: dullness, uneven texture, fine lines, and hyperpigmentation. Lactic acid is the most forgiving starting point; glycolic acid delivers faster, more dramatic results for those whose skin can handle it; mandelic acid is the gentlest and best suited to sensitive or darker skin tones.
Choose BHAs if your primary concerns are pore-related: blackheads, whiteheads, oily skin, acne, and congestion. Salicylic acid is the BHA, and its ability to work inside the pore is unique among common exfoliants.
Use both if you have multiple concerns and your skin can tolerate it, alternating rather than stacking, and always with a daily SPF and attentive barrier support.
The exfoliant that stays in your routine and doesn’t cause irritation will always outperform the stronger one that your skin can’t handle. Start gentle, build slowly, and your skin will show you what it needs.
