AHA vs BHA vs PHA: which exfoliating acid is right for you?

AHAs, BHAs and PHAs are chemical exfoliants that loosen dead skin cells so they shed more evenly. AHAs such as glycolic and lactic acid suit dull, rough or dry skin. BHA (salicylic acid) dissolves in oil, so it reaches into pores and suits oily or blackhead-prone skin. PHAs work more gently and are a common pick for sensitive skin.

What is a chemical exfoliant?

Your skin sheds dead cells all the time, but the process can slow down or become uneven. The result can be a rough texture, a dull look or clogged pores. Exfoliating speeds up that shedding.

There are two ways to do it. Mechanical exfoliation uses scrubs, brushes or pads. Chemical exfoliation uses acids that dissolve the bonds holding dead cells together. Chemical exfoliants spread evenly and don't rely on rubbing, which is why many people with dry, sensitive or acne-prone skin find a mild acid easier to tolerate than a gritty scrub.

AHA vs BHA vs PHA: the comparison

AHAs: glycolic acid and lactic acid

Glycolic acid is the smallest AHA molecule, so it penetrates quickly and works fast. That makes it effective for rough texture and dullness, and also the AHA most likely to sting. Lactic acid is a slightly larger molecule and is often described as gentler. It also acts as a humectant, so it suits dry skin that needs smoothing. Mandelic acid is larger still and is often chosen by people who find glycolic acid too strong.

In the US, an expert safety panel concluded that glycolic and lactic acid are safe in consumer products at 10% or less, with a pH of 3.5 or higher, as long as the product protects against sun sensitivity or tells you to use sun protection. Stronger peels belong in a dermatologist's office.

BHA: salicylic acid for oily and acne-prone skin

Salicylic acid is fat-soluble, so it can mix with the oil in your pores and loosen the plug of sebum and dead cells that forms blackheads. It also has an anti-inflammatory effect, which is why it shows up in so many acne cleansers and leave-on treatments. Read more in our guide to salicylic acid for skin.

Ingredient scanners, Green Point included, flag salicylic acid because it's a regulated ingredient with concentration limits (EU rules cap how much a product can contain and restrict its use in products for young children). That flag is about using it safely, not about whether it works.

PHA for sensitive skin

Polyhydroxy acids such as gluconolactone and the related lactobionic acid are bigger molecules with more water-attracting groups. Research reviews describe effects similar to AHAs but with less irritation, plus moisturizing and antioxidant properties.

That makes PHAs a reasonable first acid if your skin is dry or easily irritated, or if glycolic acid has stung in the past. The trade-off is that results are usually slower and subtler. If you have rosacea or eczema, ask a dermatologist before using any acid.

How often should you exfoliate?

There's no single right number. The stronger the product, the less often you need it. A sensible way to start:

1. Pick one acid that fits your main concern and skin type.
2. Patch test it on a small area (for example, under the jaw) for a few days.
3. Use it once or twice a week at night, after cleansing and before moisturizer.
4. If your skin stays calm after two or three weeks, try every other night.
5. Daily use is only for gentle, low-strength products that your skin clearly tolerates.

Don't stack several exfoliants on the same night (an AHA toner, a BHA serum and a scrub). Combination products formulated with both AHA and BHA are fine if your skin tolerates them. Be careful with retinoids and benzoyl peroxide too: the American Academy of Dermatology notes that exfoliating while using them can worsen dryness or even trigger breakouts. Our guide to skincare ingredients you shouldn't mix explains how to alternate them.

Do exfoliating acids make you sensitive to the sun?

Yes, at least AHAs do. In a study cited by the FDA, four weeks of using an AHA cream increased skin's sensitivity to UV damage, and the effect faded about a week after stopping. The FDA recommends that AHA products carry a "Sunburn Alert" telling you to use sunscreen, cover up and limit sun exposure while using the product and for a week afterwards. It advises similar precautions for BHA products.

In practice: use acids at night, apply a broad-spectrum sunscreen every morning and reapply when you're outdoors.

Signs of over-exfoliation

Using acids too often, too strong or in too many products can damage your skin barrier. Watch for:

• Redness, stinging or burning, even with gentle products
• Tight, flaky or unusually shiny skin
• New breakouts or small bumps
• Products that never bothered you suddenly sting

If this happens, stop all acids and retinoids, use a gentle cleanser and a plain moisturizer, and give your skin a few weeks. Our guide to repairing a damaged skin barrier walks through the steps. If you have a darker skin tone, go slowly: the AAD warns that aggressive exfoliation can leave dark spots.