AHA, BHA and PHA: which acid does what

The acid aisle is where beginners get hurt, usually not because they picked the wrong acid but because they used the right one four times a week. The three families do different jobs, and Korea adds one regulatory wrinkle worth knowing.

The Korean salicylic acid rules

Korea regulates salicylic acid by use rather than treating it as one ingredient with one limit. As a preservative it is capped at 0.5 percent. Rinse-off products have their own allowance, and hair products another. There is also a restriction on products for children under three, with rinse-off products such as shampoo excepted.

For a shopper the useful consequence is that salicylic acid on a Korean ingredient list is not automatically an exfoliating dose. At preservative levels it is doing preservation. The claim on the front of the box, and the position on the list, are what separate the two cases.

Frequency beats strength

The most common acid mistake is treating a low concentration as permission for high frequency. A gentle acid used five nights a week does more damage than a stronger one used once, because the barrier does not get a chance to recover in between.

Set a weekly budget instead. One or two acid nights to start, held for a few weeks before changing anything. If you want to run both an AHA and a BHA, they share that budget rather than each getting their own.

Signs you have gone too far

Skin that feels tight and looks shiny at the same time, stinging from products that never stung before, flaking alongside redness. That combination is the barrier asking you to stop, and the answer is to stop all acids and retinoids for a couple of weeks rather than to find a gentler acid.

Recovery is boring and effective: gentle cleanser, a calming and barrier-focused moisturizer, sunscreen, nothing else. Korean calming formulas are well suited to this stretch, which is much of why that category exists.

What not to stack

Using an exfoliating acid and a retinoid in the same routine can raise cumulative irritation for some users. Separating them across the week is a conservative option, not a universal incompatibility rule.

Sunscreen is not optional during an acid routine. Korean AHA products above certain levels carry a sun warning on the packaging by rule, which is a regulator telling you the same thing this paragraph is.

Choosing your first acid

If you have never used one, start with a PHA or a low-strength lactic acid rather than glycolic. The larger molecules are associated with less sting, and the point of a first acid is to learn how your skin responds rather than to get a result in week one.

If your concern is congestion and oiliness rather than texture, BHA is the more logical starting point because oil solubility is the property that distinguishes it. Korean blemish toners commonly pair it with calming ingredients, which makes them a gentler introduction than a bare acid solution.

Either way, buy a small size. An acid you use twice a week lasts a long time, and finding out in month two that it does not suit you is cheaper with a small bottle.

For this update: I removed the single-variable “frequency decides strength” rule and rewrote the comparison around acid identity, concentration, pH, vehicle and use pattern.

Frequently asked