AHAs and BHAs: chemical exfoliants explained

“Chemical exfoliant” sounds alarming until you realize it just means an acid that loosens dead skin cells instead of a scrub that rubs them off. The two families you’ll see on labels are AHAs and BHAs, and they don’t do the same job. Here’s how each one works, who it suits, why sunscreen becomes non-negotiable, and what Health Canada allows in a bottle you can buy off the shelf.

What exfoliation actually is

Your outermost skin layer is made of flattened dead cells stuck together and constantly shed from the top as new ones arrive from below. When that shedding slows or becomes uneven, skin looks dull and rough, pores clog, and moisturizer sits on top of a layer of old cells.

Exfoliants speed up the shedding. Physical ones (scrubs, brushes) do it by friction. Chemical ones do it by loosening the “glue” between the dead cells so they let go on their own. Done gently, that’s all a chemical exfoliant is. Done too hard or too often, it’s an irritant.

AHAs: glycolic and lactic acid

AHA stands for alpha hydroxy acid. The two you’ll meet most are glycolic acid and lactic acid. Both dissolve in water, which means they work mostly at the surface, on the dead outer layer. They don’t get into oily pores.

What they do:

• Loosen dead surface cells so they shed, which is why skin feels smoother within a couple of weeks. The US FDA describes AHAs plainly as ingredients that cause exfoliation by shedding surface skin. The precise biology is still being worked out.
• AHAs have a second job: they’re humectants, ingredients that attract and hold water in the outer skin. DermNet lists them alongside glycerin and urea in that category. That’s part of why lactic acid is so common in body lotions for rough, dry skin.

AHAs are the go-to for rough texture, dullness, mild sun damage and uneven tone. They suit normal, dry and sun-damaged skin better than oily, breakout-prone skin.

BHA: salicylic acid

BHA stands for beta hydroxy acid, and in skin care it means one ingredient: salicylic acid. Its defining feature is that it dissolves in oil rather than water. A 2015 review describes it as “miscible with epidermal lipids and sebaceous gland lipids”, which is a technical way of saying it can follow the oil down into your pores.

Inside a pore, it does two things:

• Loosens the connections between dead cells lining the follicle, so plugs break up. The review is careful to call this “desmolytic” (it disrupts the junctions between cells) rather than dissolving keratin itself.
• Slows the shedding of cells inside the follicle that would otherwise pile up into new plugs, as DermNet describes.

That combination makes salicylic acid the exfoliant for blackheads, whiteheads, oily skin and mild acne. The 2015 review notes it also helps with the dark marks that acne leaves behind.

The trade-off: it can be drying. In the 2015 review, dryness was the most frequently noted side effect of salicylic acid peels, along with temporary redness and flaking that settled within a week or so.

Irritation and sun sensitivity

Both acids can sting, redden, dry out and peel skin, especially at higher strengths, on damaged skin, or when combined with other actives.

AHAs have an extra issue. Studies sponsored by the US FDA found that after four weeks of applying an AHA cream, volunteers’ skin was 18 percent more sensitive to UV-induced redness, and UV-induced cell damage roughly doubled. The good news is that the effect was reversible: one week after stopping, there was no significant difference in sensitivity. Between 1992 and 2004, the FDA also logged 114 adverse reports for AHA products, including burning, dermatitis, swelling, blisters and chemical burns.

That’s why AHA products in Canada must carry cautions, and why the practical advice is simple: if you use an AHA, wear sunscreen every day, and keep using it for at least a week after you stop.

A few more cautions:

• Salicylic acid absorbs through skin. Over large areas, at high strength or on broken skin, that can add up. DermNet advises it isn’t suitable for children under two and that pregnant or breastfeeding people should talk to their doctor first.
• Don’t combine acids with retinoids, benzoyl peroxide, alcohol-based toners or scrubs on the same night.
• Don’t use either on eczema flares, sunburn or broken skin.

Canada’s rules on strength

Health Canada regulates hydroxy acids through two lists, and the numbers are worth knowing.

The Cosmetic Ingredient Hotlist, which sets what cosmetics may contain, allows:

• AHAs in consumer products from 3 to 18 percent, at a pH of 3.5 or higher. Products between 18 and 30 percent, or with a pH between 3.0 and 3.5, are for professional use.
• Required cautions on AHA products: use only as directed, avoid contact with eyes, stop if irritation persists, and cover treated areas with sunscreen before sun exposure.
• Salicylic acid in cosmetics at a maximum of 2 percent.

The Prescription Drug List takes over at the top end: AHAs above 30 percent or below pH 3.0, and salicylic acid above 20 percent or below pH 3.0, are prescription drugs (with exceptions for wart, corn and callus products).

So a “10 percent glycolic toner” is a cosmetic; a 30 percent peel belongs in a clinic. And a 2 percent salicylic acid product is as strong as a cosmetic gets, which is more than enough for daily use.

How often, and how to start

• Pick one acid to begin with. AHA for texture and dryness, BHA for pores and breakouts.
• Start with two or three nights a week. A brief tingle is normal; burning is not.
• Apply to clean, dry skin, wait a minute, then moisturize. Using a moisturizer afterward reduces dryness.
• Sunscreen every morning, especially with AHAs, in a Canadian winter as much as in summer.
• If irritation lasts more than a few days after stopping, or acne is getting worse rather than better after two months, see your doctor or pharmacist. Stronger options exist by prescription.

You can browse the panel’s reviewed exfoliators or filter for products suited to acne-prone skin.

The short version: AHAs work on the surface and suit rough, dry or sun-damaged skin; salicylic acid works in the pore and suits oily, congested skin. Both are effective at modest strengths, both need respect, and AHAs need sunscreen. Start slow, use one at a time, and let the Health Canada limits reassure you that the products on the shelf are meant for home use.