Acids used in skincare work by breaking apart the protein bonds that hold dead skin cells together, forcing them to shed faster than they would on their own. This process, called keratolysis, reveals fresher skin underneath and can improve texture, tone, and clarity over time. But the effects depend entirely on the type of acid, its concentration, and how long it stays on your skin.
How Acids Break Down Skin Cells
Your outermost layer of skin is made up of dead cells stacked together like bricks, held in place by protein structures called desmosomes. These connections keep your skin intact and act as a barrier against the outside world. When you apply an acid to your skin, it disrupts those connections, loosening the “glue” between cells so they detach and fall away.
At lower concentrations (the kind found in daily serums and cleansers), this shedding is gradual and gentle. At higher concentrations used in professional chemical peels, acids go deeper, denaturing proteins in both the outer and inner layers of skin. That triggers a more dramatic turnover, peeling away damaged cells and stimulating new cell growth from below.
The pH of a product determines how actively the acid works. Most exfoliating acids need a pH around 3.5 to 4 to function well. When the pH climbs above 4.5, the exfoliating power of many common acids drops significantly, sometimes by more than half. This is why a 10% glycolic acid serum formulated at pH 5 might do almost nothing, while a 5% serum at pH 3.5 delivers noticeable results.
Alpha Hydroxy Acids: Surface-Level Exfoliation
Alpha hydroxy acids (AHAs) are water-soluble, meaning they work on the skin’s surface rather than inside pores. Glycolic acid is the most popular AHA because it has the smallest molecular weight of the group, which lets it penetrate skin more easily and work faster. Lactic acid and mandelic acid have larger molecules, so they penetrate more slowly and tend to cause less irritation.
AHAs are best suited for concerns like dullness, rough texture, fine lines, and uneven skin tone. By speeding up cell turnover on the surface, they gradually fade dark spots and smooth out minor imperfections. They also help skin retain moisture by exposing newer cells that hold water more effectively.
One important tradeoff: AHAs make your skin more sensitive to UV light. Research on glycolic acid found that this increased sun sensitivity lasts for the entire time you use the product and persists for about one week after you stop. Daily sunscreen is essential while using any AHA, and you should continue applying it for at least a week after your last use.
Beta Hydroxy Acids: Clearing Pores From the Inside
Salicylic acid is the most common beta hydroxy acid (BHA), and it works differently from AHAs in one critical way: it dissolves in oil. This means salicylic acid can cut through the sebum inside your pores and penetrate the oily lining of hair follicles, reaching buildup that water-soluble acids never touch.
Once inside the pore, salicylic acid loosens the dead cells and oil plugs that form blackheads and whiteheads. It also reduces sebum production in people with acne, which helps prevent new breakouts from forming. Over-the-counter acne products in the U.S. contain salicylic acid at concentrations between 0.5% and 2%, the range approved by the FDA for non-prescription use.
Because salicylic acid concentrates its activity inside pores rather than stripping the entire skin surface, it typically causes less widespread dryness and irritation than AHAs at similar strengths. This makes it a better starting point if your main concern is acne or congested skin rather than surface texture.
Polyhydroxy Acids: A Gentler Alternative
Polyhydroxy acids like gluconolactone and lactobionic acid have much larger molecules than AHAs or BHAs. They still exfoliate, but they work more slowly and sit closer to the surface. Gluconolactone has been shown to improve skin barrier function by reducing water loss through the skin, and lactobionic acid acts as a moisturizer that can improve dryness, redness, and itching.
These properties make PHAs a practical option for people with rosacea or sensitive, reactive skin who can’t tolerate glycolic or salicylic acid. In studies on mild rosacea-related redness, a serum containing 5% gluconolactone and 1% lactobionic acid improved skin hydration and barrier strength while keeping irritation minimal.
Acids That Target Dark Spots
Some acids don’t exfoliate at all. Instead, they work by interfering with melanin production. Your skin produces melanin through an enzyme that acts like a factory assembling pigment molecules. Brightening acids, including certain phenolic compounds and aromatic acids, block that enzyme by mimicking the raw materials it normally uses, essentially tricking it into binding with the wrong ingredient so it can’t produce as much pigment.
This is a slower process than exfoliation. While sloughing off pigmented surface cells can show visible results in days, reducing melanin production at the source takes consistent use over weeks or months. Many products combine both approaches: an exfoliating acid to remove existing pigmented cells and a brightening acid to slow down new pigment formation.
What Happens When You Overdo It
Your skin’s outermost barrier exists for a reason. It keeps moisture in and irritants out. Every time you apply an acid, you’re temporarily weakening that barrier by removing some of its structural cells. Used appropriately, your skin rebuilds between applications and comes back stronger. Used too aggressively, the barrier breaks down faster than it can repair itself.
The early signs of over-exfoliation are subtle: skin that feels tight or stings when you apply products that never bothered you before. As damage progresses, you may notice persistent redness, flaking, or a shiny, almost waxy texture. In more severe cases, misuse of high-concentration acids can cause chemical burns with blistering, swelling, cracked skin, and discoloration that may take weeks to heal.
The risk is highest when people layer multiple acid products, use professional-strength peels at home, or apply acids daily without building up tolerance first. Starting with a low concentration two or three times per week, then gradually increasing frequency, gives your barrier time to adapt.
How Skin Type Changes the Equation
Oily, acne-prone skin generally tolerates acids well because the extra sebum provides a natural buffer. Salicylic acid is usually the best fit here, since it targets the oil-filled environment where breakouts start. For oily skin with surface texture issues, alternating a BHA with an AHA on different days covers both concerns without overloading the skin.
Dry or sensitive skin benefits more from gentler options. Lactic acid exfoliates while also drawing moisture into the skin, and PHAs provide mild turnover without compromising the barrier. If your skin is reactive, prone to redness, or easily irritated, starting with a PHA serum is the lowest-risk entry point.
Darker skin tones require extra caution with stronger acids. Aggressive exfoliation can trigger post-inflammatory hyperpigmentation, where the skin responds to irritation by producing even more melanin in the affected area. Lower concentrations, shorter contact times, and consistent sunscreen use reduce this risk.