AHA versus BHA versus PHA: which acid your skin actually needs and why

The shared mechanism and the critical differences between the three acid families

AHA (alpha-hydroxy acid), BHA (beta-hydroxy acid) and PHA (polyhydroxy acid) all work by accelerating the natural desquamation process — the shedding of the outermost layer of dead skin cells that normally occurs every twenty-eight days but slows with age, sun damage and certain skin conditions. By dissolving the protein bonds (primarily desmosomes) between corneocytes at the surface, exfoliating acids produce faster cell turnover, clearer surface texture and improved product absorption. The differences are molecular: AHA is water-soluble (glycolic acid, lactic acid, mandelic acid) and works primarily at the skin surface; BHA is lipophilic (salicylic acid) and penetrates the oil inside follicles; PHA (gluconolactone, lactobionic acid) has a larger molecular size than AHA, penetrating more slowly and more gently. These structural differences determine which skin type and concern each acid serves most effectively.

AHA: for surface texture, tone and anti-aging

Alpha-hydroxy acids are the most versatile exfoliating category for addressing surface concerns. Glycolic acid, the smallest AHA molecule, penetrates deepest and produces the fastest results in surface texture and fine line reduction — it is also the most likely to cause irritation in sensitive or dehydrated skin. Lactic acid, larger than glycolic and also a humectant at the concentrations used in skincare, provides comparable exfoliation with less irritation potential and adds hydration as a secondary benefit. Mandelic acid, the largest common AHA, is the gentlest option and is specifically recommended for darker skin tones because its slower action reduces the post-inflammatory hyperpigmentation risk that faster-acting AHAs can trigger when used on melanin-rich skin. AHA is the right acid family for skin with uneven surface texture, dullness, fine lines from sun damage, or post-breakout marks on a stable barrier.

BHA: for congested pores and acne-prone skin

Salicylic acid's lipophilicity is the property that makes it irreplaceable for follicular congestion. Unlike AHAs that work at the skin surface, salicylic acid can pass through the oil-filled follicle wall and dissolve the mixture of sebum and dead cells that forms blackheads and the microcomedones that precede acne. At concentrations between 0.5% and 2%, salicylic acid also has anti-inflammatory properties that reduce the redness and swelling of existing inflammatory acne lesions while clearing the congestion that would produce new ones. BHA is the right choice for skin with visible blackheads, consistent breakouts, enlarged pores from congestion, or oily skin that does not respond well to surface-only exfoliation. It is not the best choice for very dry or sensitive skin, where the lipid-dissolving action of salicylic acid can disrupt an already depleted barrier.

PHA: for sensitive skin and barrier-conscious routines

PHAs were developed specifically for skin that cannot tolerate the penetration speed of AHA or the lipid-dissolving activity of BHA. The larger molecular size of gluconolactone and lactobionic acid means they work at the outermost layers of the stratum corneum rather than penetrating to the depth that AHAs reach — producing gentler exfoliation with a lower irritation ceiling. PHAs also have humectant properties and have been documented to have antioxidant activity in skin tissue, making them one of the few exfoliating actives that simultaneously improve surface cell turnover and provide protective benefits. For skin with eczema, rosacea or persistent reactive sensitivity, PHA is often the only acid tolerated consistently. Including PHA alongside AHA and BHA at lower concentrations in a combined formula covers the exfoliation need across different skin depths while PHA moderates the irritation potential of the more aggressive acids.

When a PDRN ampule fits into an acid routine

The logical partner for an exfoliating acid routine is a PDRN or collagen-stimulating ampoule applied on the evenings between acid sessions. Exfoliating acids accelerate cell turnover but do not directly stimulate the underlying collagen network — and the temporary barrier disruption that accompanies any acid application creates a window during which PDRN's adenosine A2A receptor activity and peptide signalling work most efficiently, because the partially cleared surface layer allows better penetration. On exfoliation nights, apply acid first, wait 15 minutes for pH normalisation, then apply PDRN ampoule as the treatment step. On off-exfoliation nights, apply PDRN as the treatment in a standard routine without acid. This alternating protocol delivers both surface cell turnover acceleration and dermal collagen rebuilding without the compounding irritation risk of applying acid and PDRN simultaneously at high frequency.