Your skin prefers things a little acidic.

The pH of the outer layer of the skin, or the stratum corneum, hovers slightly acidic at four to six; derms call this the acid mantle. "Acidic conditions maintain skin-barrier function and the body's ability to ward off infection," says Joshua Zeichner, the director of cosmetic and clinical research at Mount Sinai Hospital in New York City. Two major factors alter your skin's acidity. For starters, true soaps are naturally alkaline. (One study found patients who used alkaline soap broke out more than those who used an acidic one.) Look for soaps that balance alkaline detergents, such as sodium laurel sulfate or sodium lauroyl isethionate, with acids like stearic or citric acid (we like the good old Dove White Beauty Bar). Lots of cleansers, like this one from La Roche-Posay, also now list their pH, adds Zeichner.

Aging also compromises the body's ability to maintain acidity, which is why skin often gets dry or itchy over the years. In that case, acidic moisturizers—such as those with alpha hydroxy acids, like lactic acid—can lower pH levels and rev up production of the skin's natural moisturizers. We like Philosophy Hope in a Jar with lactic acid and Neutrogena Healthy Skin Face Lotion with alpha hydroxy acids.

Hair pH is slightly less important.

Hair is also slightly acidic, says Paradi Mirmirani, a dermatologist in California who specializes in hair. "Most shampoos contain detergents, which are basic," she says. But chemists have long known that "to keep the hair from feeling dry or dull, they've had to balance the pH with acidic ingredients." This goes for pretty much all formulas, but if you want to make sure your shampoo makes the cut, look for sulfate-free shampoos or cleansing conditioners. We like Purely Perfect Cleansing Creme.

And nail pH doesn't seem to matter at all.

Surprise, surprise: Nails, too, have a slightly acidic pH. "We don't generally get concerned with pH when we consider nail health," says Dana Stern, a New York City dermatologist who specializes in the topic. (In fact, she couldn't think of a single nail condition that had anything to do with pH at all.) Why, then, all the pH-balanced basecoats? "Nail dehydrators degrease the surface of the nail to allow for better adhesion," she explains. These products—acetone included—just so happen to be in the pH range of five to seven, so they're marketed as pH balanced. "In reality, the pH has nothing to do with it," she says. "They're just good at drying the surface of the nail."