A chemical peel is a solution applied to the skin that contains one or more acids (or sometimes enzymes) designed to dissolve the bonds holding dead skin cells together, causing controlled shedding and renewal. The exact ingredients depend on the peel’s depth: superficial peels use milder acids at lower concentrations, medium peels use stronger formulations, and deep peels contain potent compounds that penetrate well below the surface. Beyond the active acid itself, most peel solutions also include solvents and carriers that affect how evenly and deeply the acid works.
Alpha Hydroxy Acids (AHAs)
AHAs are the most common ingredients in superficial chemical peels. They’re water-soluble acids derived from natural sources like sugarcane, milk, and almonds. Their job is to break down the “glue” between dead skin cells on the surface, encouraging them to shed more quickly. The three AHAs you’ll encounter most often are glycolic acid, lactic acid, and mandelic acid, and each behaves a little differently on skin.
Glycolic acid is the smallest and simplest AHA, which means it penetrates skin more easily than the others. Professional peels typically use concentrations between 20% and 50%. It’s considered a first-line choice for treating dark spots, melasma, fine lines, and general sun damage. Because of its small molecular size, it works quickly but can also cause more stinging and irritation than larger AHAs.
Lactic acid is structurally almost identical to glycolic acid but slightly larger. Peels use it at 10% to 30% concentrations. It performs comparably to glycolic acid for sun damage, uneven pigmentation, and fine lines, but tends to be gentler. It also has mild moisturizing properties, making it a common pick for people with drier or more sensitive skin.
Mandelic acid is the largest of the three. It has a benzene ring in its structure that makes it bulkier, so it absorbs into skin more slowly and uniformly. Professional peels use it at around 40%. It’s effective for redness, uneven skin tone, and oily skin because it can reduce the amount of oil your skin produces.
Salicylic Acid (BHA)
Salicylic acid is the main beta hydroxy acid used in chemical peels, and it works differently from AHAs in a key way: it’s oil-soluble. That means it can cut through the oily buildup inside pores, making it especially effective for acne-prone skin. Professional peels use it at 20% to 30% concentrations.
For years, salicylic acid was described as a keratolytic, meaning it dissolved the protein (keratin) in dead skin. That understanding has been updated. It actually works as a desmolytic agent: rather than dissolving skin cells themselves, it disrupts the junctions between cells, loosening them so they detach cleanly. The distinction matters because it explains why salicylic acid peels can exfoliate effectively without as much raw irritation as some AHA peels.
Trichloroacetic Acid (TCA)
TCA is the workhorse ingredient in medium-depth peels. At concentrations of 30% to 35%, it penetrates past the outermost skin layer and into the upper portion of the dermis, the thicker layer beneath. This deeper reach makes TCA peels effective for moderate sun damage, deeper wrinkles, and precancerous skin changes that superficial peels can’t address.
TCA is sometimes combined with other agents to improve results. One well-known approach pairs it with Jessner’s solution, a mixture of salicylic acid, lactic acid, and resorcinol in an ethanol base. The Jessner’s solution is applied first to break down the skin’s surface barrier, allowing the TCA to penetrate more evenly and predictably. This combination lets practitioners use a lower TCA concentration while still achieving a reliable medium-depth peel.
Phenol and the Baker-Gordon Formula
Deep peels reach further into the skin than any other type, and the classic deep peel formula is the Baker-Gordon solution. It contains five ingredients: 88% phenol, croton oil, hexachlorophene (an antiseptic soap), liquid soap, and distilled water. Phenol causes a controlled chemical burn that destroys damaged skin down through the deeper dermis, triggering significant collagen remodeling as the skin heals.
Croton oil is a critical component. It acts as a penetration enhancer, driving the phenol deeper and more uniformly into the skin. Even small changes in the amount of croton oil significantly alter how deep the peel goes. The soap ingredients help emulsify the solution so it applies evenly. Deep phenol peels are reserved for severe sun damage, deep wrinkles, and precancerous lesions. They require sedation or anesthesia and carry risks that lighter peels don’t, including potential effects on heart rhythm from phenol absorption, which is why they’re only performed under medical supervision.
Solvents and Carriers
The active acid is only part of what’s in the bottle. The solvent it’s dissolved in changes how the peel behaves on your skin. Ethanol is a traditional carrier for salicylic acid peels, but it evaporates unevenly and can leave concentrated patches of acid on the skin. Polyethylene glycol (PEG) is an alternative carrier that distributes salicylic acid more uniformly while also moisturizing the skin. In studies, 30% salicylic acid in a PEG base produced significant improvement in acne lesions without the redness, bleeding, or post-inflammatory darkening that ethanol-based versions sometimes cause.
Other formulations use propylene glycol or water-based gels as carriers. These vehicles also function as penetration enhancers, increasing how easily the active acid passes through the skin’s outer barrier. When comparing two peels with the same active ingredient at the same concentration, differences in the carrier can meaningfully change the results and side effects.
Enzyme Peels
Not all chemical peels use acids. Enzyme peels rely on protein-dissolving enzymes, most commonly bromelain (from pineapple) and papain (from papaya). These enzymes break down the proteins in dead skin cells directly, loosening them from the surface. The exfoliation they produce is gentler than acid-based peels because the enzymes work on a narrower range of targets. They don’t disrupt the bonds between living cells the way acids can. This makes enzyme peels a common choice for sensitive skin that reacts poorly to acids.
Enzyme peels are strictly superficial. They won’t address deeper concerns like moderate wrinkles, significant sun damage, or scarring. But for mild dullness and texture issues, they offer a lower-risk entry point.
Why pH and Concentration Both Matter
Two peel solutions can list the same acid at the same percentage and still differ in strength because of pH. Every acid has a pKa value, which is the pH at which half of the acid molecules are in their active, free form. When a peel’s pH sits below the acid’s pKa, more of the acid is in its active state and the peel is stronger. When the pH is higher, less acid is active and the peel is milder.
This is why professional peels aren’t just “stronger versions” of the acids in your at-home products. A dermatologist can manipulate both the concentration and the pH of the solution to control exactly how deeply it penetrates. At-home peels typically have higher pH values that keep the acid less active, even when the percentage on the label looks impressive. It’s the combination of concentration, pH, and the carrier solvent that determines what a peel actually does to your skin.