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The Skin-Brightening, KP-Calming Acid
Comes from: milk
Strength: 2/5
Why: It's hard to miss lactic acid when it's in your skin-care products. (The faint sour-milk smell? Unavoidable.) But the dairy-derived acid has its perks. It's similar to glycolic acid but is made up of larger molecules that don't penetrate the skin as deeply, making it better tolerated by irritation-prone skin.
If you have keratosis pilaris (aka chicken skin), you've probably seen it in KP treatments — it breaks down the keratin that causes those annoying bumps to form without over-drying skin.
Pregnant women are told to avoid using products with salicylic acid, so if you're preggers and blemish-prone, lactic acid products can help by gently exfoliating the dead skin cells that contribute to pimple formation.
Buy: Paula's Choice Clinical KP Treatment Cloths, $32; Belli Skin Care Anti-Blemish Facial Wash, $22
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The Rosacea-Relieving, Hyperpigmentation Busting Acid
Found in: grains like wheat and barley
Strength: 2/5 or 3/5, depending on concentration
Why: Soothing, antibacterial azeleic acid is the perfect match for rosacea sufferers. Azeleic acid is ideal for calming the inflammation that triggers redness and pimple-like bumps, two of rosacea's biggest bummers.
You might also find azeleic acid in products for treating dark spots, which is caused and exacerbated by inflammation. "[It] has lightening properties and anti-inflammatory properties," Tanzi points out, "so [it's] good for people with discoloration or melasma," she says.
Buy: Murad Redness Therapy Recovery Treatment Gel, $54; Philosophy the Microdelivery Triple-Acid Brightening Peel, $70
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The "Holy-S&%$-That's-Strong" Anti-Aging Acid
Comes from: a lab (it's synthetic)
Strength: 5/5
Why: If you want the strongest chemical peel available, you're probably looking at trichloroacetic acid, a powerful ingredient more commonly known as TCA. Tanzi says TCA is best for patients with extensive scars or damage because of its ability to "retexturize" pores and reduce the look of sun damage.
"The concentrations [of in-office treatments] range depending on the depth of the peel," says Tanzi. A dermatologist may use as little as 15 percent TCA for a light peel or as much as 50 percent for a deep peel that requires several weeks of post-treatment downtime.
TCA is somewhat rare in store-bought treatments because of its inherent strength. "[You'll] see no more than 3 percent in over-the-counter products," says Tanzi.
Buy: Peter Thomas Roth 40% Triple Acid Peel, $88
Alpha hydroxy acids and beta hydroxy acids are the workhorses of your skin care routine. They exfoliate dull skin and fade imperfections, minus scratchy microbeads. Pretty much everyone can benefit from having acid in her routines -- but you shouldn't pick just any ol' AHA cleanser. If you wanna get nerdy about it, the various alpha hydroxy acids and beta hydroxy acids (like salicylic acid, glycolic acid, lactic acid and malic acid) work differently on a molecular level, and choosing which one to use can make or break your skin. We asked Dr. Elizabeth Tanzi, M.D., a Washington, D.C.-based dermatologist, to decode some of the most common AHAs and BHAs and tell which one you should be using.
