AHA, BHA and PHA: which acid suits whom?

Chemical exfoliating acids smooth the skin by gently dissolving dead surface cells rather than scrubbing them away. On the label you will see three main families: AHA, BHA and PHA. All three are acids, but they behave differently, and the right choice depends on your skin type. This guide separates them in plain language.

First, what does exfoliation actually do?

The outermost layer of skin renews itself constantly. Cells produced underneath travel upwards, flatten and form a lifeless layer at the very top. In healthy skin that journey takes roughly four weeks; as we age, as sun damage accumulates and in certain skin conditions, the cycle slows down. When it slows, dead cells pile up on the surface, reflect light unevenly, leave the skin looking dull and rough, and make any moisturiser you apply harder to absorb. Chemical acids loosen the glue-like bonds holding those cells together so they shed more easily. That is the core difference from mechanical scrubs: acids work without pressure, without the risk of scratching, and more evenly across the face.

AHA: for the surface (glycolic, lactic acid)

AHAs (alpha hydroxy acids) are water-soluble and work on the SURFACE of the skin. The best known are glycolic acid (small molecule, strong) and lactic acid (larger molecule, gentler). This is the family that first comes to mind for dull, dry, rough skin and uneven tone. Glycolic acid is as irritating as it is effective; lactic acid is a safer first step for dry skin that is new to acids.

The members of the AHA family are not interchangeable

AHA is spoken about on the shelf as if it were one ingredient, but the differences between its members are real. Glycolic acid has the smallest molecule of the family; it moves quickly, shows results early and, in return, carries the highest chance of irritation. Lactic acid has a bulkier molecule, works more slowly and brings a moisture-attracting quality with it, which is why dry skin tends to prefer it. Mandelic acid is the bulkiest member of all, penetrating gradually, and it is considered the more cautious option on deeper skin tones. Citric, malic and tartaric acids, meanwhile, appear in most products not to exfoliate but to adjust the formula's acidity, so treating a product as an exfoliant just because you recognise those names will mislead you.

BHA: for the pore (salicylic acid)

In practice the BHA family has a single member, salicylic acid. Here is the difference: it is oil-soluble, so it can pass through the sebum on the skin and get INSIDE THE PORE. That makes it the classic choice for blackheads, congestion and oily, breakout-prone skin. It also has a soothing side, so inflamed spots often tolerate it better than an AHA. The common strength is 1-2%.

Two practical details about salicylic acid

The first is contact time. A cleanser carrying two per cent salicylic acid is rinsed off within thirty seconds and leaves very little work behind; a toner or serum at the same strength sits on the face for hours. The same percentage means a completely different level of power depending on the product type, which is why reading the number on the box alone falls short. The second is tolerance. Many people find salicylic acid easier to live with than AHAs on inflamed spots, because the molecule itself has a calming character. That said, if you know you are sensitive to aspirin or similar salicylate derivatives, it is sensible to speak to your pharmacist before adding these products to your routine.

PHA: for sensitive skin (gluconolactone, lactobionic acid)

PHAs (poly hydroxy acids) are the gentlest members of this group. Their molecules are large, they act slowly and superficially, and the risk of stinging and redness is noticeably lower. They also attract moisture. For sensitive, dry or barrier-damaged skin that cannot tolerate an AHA, the sensible option is a PHA. Results arrive more slowly, so patience is required.

The three families side by side

Which one for whom? A short summary

• Dull, rough, dry skin and uneven tone: AHA (lactic acid to begin with).
• Oily skin, blackheads, congested pores and a tendency to break out: BHA (salicylic acid).
• Sensitive skin, skin that reddens easily, or anyone new to acids: PHA.
• Combination skin: use them on separate areas, or pick a single low-strength product; layering two on the same night is not required.

How do they appear on the label?

On the ingredients list these acids are written with their English INCI names: glycolic acid, lactic acid and mandelic acid for the AHA family; salicylic acid for BHA; gluconolactone and lactobionic acid for PHA. One tip: an ingredient near the top of the list signals a higher concentration, while one near the end usually means it is there for pH adjustment or as a low-dose supporting act. So not every product that says 'with glycolic acid' delivers a genuine exfoliating effect; you need to read the position on the list together with any strength claim.

What the percentage does not tell you: pH and free acid

The number on the packaging is not the only thing that decides how hard an acid actually works. The pH of the formula matters just as much, because an acid is only present in its free, active form in a sufficiently acidic environment. As the formula moves towards neutral, the same percentage behaves far more mildly. This is why five per cent glycolic acid from two different brands can feel like two different strengths on the skin, and why user reviews contradict each other. Most products do not print their pH; the ones that do tend to be the brands that value transparency. If you want to refresh the basics of these abbreviations, have a look at our what AHA and BHA are page.

Where it belongs in the routine, and how often

An acid goes on after cleansing and before moisturiser, applied to dry skin. Starting with a single night a week and watching the response for a fortnight causes far fewer problems than jumping straight to every other day. Tightness, redness and a stinging sensation on contact with water are signs you are using too much; in that case, a few days off with nothing but moisturiser is the most practical fix. Make it a habit not to pile strong actives into the same evening: an acid one night, a retinoid another. The idea that niacinamide and acids cannot be used together is a widespread rumour; in modern formulas the two frequently appear in the same product.

When do results show up?

Managing expectations is half the job here. A smoother surface is usually noticeable within the first few applications, because that is purely about the top layer. Even tone, the look of marks and the appearance of pores, on the other hand, require several turns of the renewal cycle, which means weeks rather than days. Trying a product for ten days and shelving it as useless is the single most common judgement error. Equally, if two months of consistent use produce no change at all, the problem may lie with the formula rather than with you: the concentration may be low, or the acid may be sitting at the very end of the list.

How to start, and the sun warning

The golden rule is low strength, infrequent use. Begin with 1-2 nights a week and increase slowly according to how your skin responds; do not crowd it into the same night as other strong actives such as retinol. Sensible starting ranges are 5-8% for AHA, 1-2% for BHA and 3-10% for PHA. The most critical point is sunlight: AHAs in particular make skin more sensitive to the sun, and that sensitivity can last for days after use. While you are using an acid, sunscreen every morning is essential; otherwise the very product you bought to even out your tone can increase the risk of marks. If you want to see which acid sits where in a given product, you can scan its label.

Widely believed, but wrong

• Tingling is not proof that something is working. A mild, brief sensation is considered normal, but persistent stinging is a warning that the barrier is under strain.
• Acids do not thin the skin. The action takes place in the lifeless cell layer; the living layers underneath are not the target. The worry that regular use will wear your skin away is therefore misplaced.
• Coming from fruit does not make an acid safer. Lactic acid made in a laboratory and lactic acid formed in milk are the same molecule, and skin cannot tell them apart.
• A higher percentage does not mean a better result. Irritated skin cannot benefit from any active as intended, and it takes weeks to settle again.
• There is no rule that oily skin can handle an acid daily. Producing plenty of oil is not armour against irritation.

Mistakes people fall into

• Applying an acid and scrubbing with a grainy product on the same night; when the two methods stack up, the surface is pushed too far.
• Applying a new acid to the whole face on the very first try. Testing a small patch along the jawline for a few days is a far more cautious start.
• Not noticing that two different products contain the same acid. If salicylic acid is in both your cleanser and your toner, you have doubled the dose without realising it.
• Carrying on while the skin is already red, in the belief that it will get used to it.
• Applying an acid in the morning and then skipping sun protection during the day.
• Swapping products every week and giving none of them enough time; you then have no way of telling which product did what.

Thirty seconds at the shelf: what to check

• Look at where the acid sits on the ingredients list rather than at the name on the front.
• Is there a strength claim on the box, and if so, does it say which acid it refers to?
• Is the product rinsed off or left on? The same percentage means two entirely different things in those two cases.
• If the box contains more than one acid, factor in the combined load; three low percentages still add up.
• Does the packaging carry any warning about sun sensitivity? A serious formula does not shy away from saying so.

Who does this choice matter most for?

An exfoliating acid does not have to be in everyone's routine. If your skin is smooth and untroubled, you can build a perfectly good regime without this step at all; cleansing, moisturising and sun protection are the backbone of the whole thing anyway. For some groups, however, picking the right family makes a much bigger difference:

• Sensitive skin that reddens easily: starting with a PHA and using it sparingly makes sense; we have gathered the details in our sensitive skin guide.
• Deeper skin tones: the tendency to leave dark marks after irritation is higher, so moving slowly directly affects the outcome.
• Those pregnant or breastfeeding: decisions about actives, salicylic acid included, should be made together with a doctor.
• Anyone on prescription acne treatment: those medicines already speed up renewal, so adding an acid on top can amplify irritation.
• People who work outdoors all day: adding an acid before the sun protection habit is established can produce the opposite of the desired result.

What do the same acids do on a food label?

These names are not confined to the cosmetics aisle. Lactic acid is a natural product of yogurt, pickles and sourdough bread, and it appears on food labels as acidity regulator E270. Citric acid shows up as E330 in almost every fruit-flavoured product, and malic acid as E296 in sour sweets. In other words, the molecule dissolving lifeless cells on your face can be the same one keeping a jar fresh; what decides the job is not the name but the environment and the amount. We explain the logic of these codes on our what an E-number is page.

In short

If the three families had to be separated in one sentence: AHA refreshes the surface, BHA gets inside the pore, and PHA does the same job by the slowest and gentlest route. The right choice is determined by how your skin responds rather than by what the product costs, and in this field using less often beats using more. Everything here is general information; if you are dealing with persistent breakouts, rosacea or a diagnosed skin condition, the best course is to decide together with your doctor or pharmacist.