By Jessica B. Matlin

Safeguard your looks with our what-and-when-to-toss guide

"Unopened, well-formulated cosmetics can remain stable for a couple of years at room temperature," says Ni'Kita Wilson, a cosmetic chemist at Cosmetech Laboratories in Fairfield, NJ. "But the clock starts once you bring a product home and open it. When air hits the formula, certain ingredients start to oxidize and degrade." What's more, every time you touch your makeup or skin-care lotions and potions, you transfer germs to them - and, subsequently, to your face. Heat and humidity, which promote the growth of mold and yeast, are factors, too. That's one reason the bathroom, though convenient, isn't the ideal spot to store cosmetics. (High levels of airborne bacteria can contaminate beauty products, too.) A better place: a cool, dry linen closet.

Beyond the obvious signs - dried mascara or separated foundation - it can be tough to tell when something's past its prime. (U.S. labeling regulations don't require an expiration date on most cosmetics.) So read on for easy, expert beauty-protecting tips on when to throw away what.

Face Makeup

Toss-it time: Six months for liquids; two years for powders
Insider info: You increase the odds of bacterial growth - and, hence, of breakouts or irritation - when you repeatedly dip your brushes and fingers into liquid foundation. Also, as it ages, foundation can go on unevenly, creating a streaky, inconsistent finish. "Oils rise to the top, and the consistency thickens," explains New York City makeup artist Mathew Nigara. Powders present less of a problem because bacteria can't grow where there's no water. However, over time, powders with botanical ingredients like aloe or jojoba can become harder to blend and are more likely to crumble, as their trace amounts of water evaporate.

Mascara

Toss-it time: Three months
Insider info: "A mascara tube is a dark, wet environment - the perfect breeding ground for bacteria," says New York City optometrist Andrea Thau, O.D. "Preservatives in a mascara only work for so long." Dr. Thau knows from firsthand experience: She once developed a sty from a makeup artist's mascara wand. Plus, three-month-old mascara is a nonperformer. "It's chalky and powdery, and any lengthening or thickening fibers often separate from the fluid, so the mascara stops going on in a smooth, even coat," says makeup artist Cristina Bartolucci. To avoid hastening the demise of your mascara, never pump the wand - that pushes air into the tube, causing it to dry out faster. Instead, slowly draw out and twist the brush to scrape the tube's interior and pick up product.