This is the makeup item you should be most vigilant  about: As the brush is taken out, applied, and put back in the tube, it brings with it any bacteria that has collected along the way, explains cosmetic chemist Jim Hammer. Lingering bacteria can cause redness and itchiness, or even conjunctivitis (a.k.a. pinkeye) and sties. Switch mascara every two to three months to be safe. "It will also have a distinct gasoline-like smell once it's gone bad," says makeup artist Pati Dubroff. If you develop any negative symptoms , immediately throw out any of your eye makeup that may have been contaminated. If your eyes are still irritated a day or two later, see an eye doctor.

tips：刷子一进一出，就会带进很多细菌。每两三个月更换一支睫毛膏。当睫毛膏闻起来有汽油味的时候就说明坏了。

 

Foundation 粉底

With the exception of powder formulas, all foundations are water-based, which means bacteria love them. Unopened, foundation can last for a couple of years, but once the seal is broken, Hammer says, it's best to replace it after 6 to 12 months. Keep foundations out of moist environments (such as your bathroom) and away from heat, as high temperatures encourage the growth of the bugs and can speed up the spoiling—true for beauty products in general. An off smell or a change in color tells you the formula is finished—applying it could cause irritation or breakouts.

tips：粉底都含水，所以易滋生细菌。没开封的粉底可保质几年时间，一旦拆封，6到12个月就该换了。变味和变色是粉底变质的征兆。

 

Concealer 遮瑕膏

Concealers are meant to cover blemishes, not cause them—which is what can happen if you use a concealer that's too old. "You'll know it's gone bad because the color will start to shift," says cosmetic  chemist Ni'Kita Wilson. Concealers in powder and stick form can last for up to two years, while liquids should be tossed after one.

tips：粉状或棒状的遮瑕可维持2年左右，液态遮瑕1年后就该更换。

 

Face Powder 散粉

Powders are the Energizer Bunny of makeup. They keep going...and going—at least, for up to two years. Still, as Wilson points out, "some powders contain small amounts of water from botanical extracts, so there's the risk of it growing bacteria if you leave it lying around your bathroom." Look on the ingredients panel  for long Latin names—these are often botanicals, and their common names (such as oat extract, chamomile, aloe, bamboo, and green-tea extracts) are typically listed in parentheses.

tips：散粉可维持2年，但一些含植物精华的散粉不宜放在洗手间之类地方。

 

Eye Shadow 眼影

Eye-enhancing shadows? Yes, please. Eye-irritating ones? No, thanks. To keep your eyes from getting red or itchy, replace powder eye shadows after about three months, according to Hammer. Even though they are similar in formulation to other powders, because they're constantly in contact with a mucous membrane, there's a higher risk of transferring bacteria to the product and then back to the eyes. Pay extra attention to cream-based shadows, which tend to grow bacteria more quickly than powders. If you use your fingers to apply, wash your hands before doing so, and be aware of any color switches or off-putting smells.

tips：粉状眼影3个月就该换了，膏状眼影更容易滋生细菌

 

Eyeliner 眼线

Liquid or pencil, these eye definers should be replaced about every three months. Like eye shadows, they are applied near a sensitive area and can pick up bacteria and bring it back to the eye very easily "without any visible signs of contamination," Hammer warns. If you've used a dingy liner, you could experience redness, itchiness, or, in extreme cases, conjunctivitis. Pencils will last a bit longer than liquid if you're steadfast about regular sharpening, but you'll know they're dead once a white film starts to develop on the tip that can't be sharpened off, says Dubroff.

tips：眼线液，眼线笔和眼影一样，每个月就该更换一次。相比之下，眼线笔比眼线液寿命长一些。

 

Lipstick & Lip Gloss 唇膏唇彩

The wrong lipstick color may spoil your mood—but the risk of the formula spoiling (or growing bacteria) is low since they don't contain any water, says Hammer. But since they're repeatedly exposed to the mouth area, he suggests replacing lip products after a year, or if you've recently been sick. Lipsticks and lip glosses contain oily ingredients that start to smell like stale cooking oil over time, according to Dubroff, who also says to look out for changes in texture. "You'll know a lipstick has gone bad when it gets hard and you can't spread the pigment on your lips; lip gloss will feel streaky and globby if you try to apply it."

tips：唇部用品1年更换一次。当唇油散发出不新鲜的烹饪油味道就说明已变质。

 

Nail Polish 指甲油

Nail polish may just be the Twinkie of the beauty world: It won't go bad from bacteria. Eventually though, lacquer will dry out and become thick and clumpy, with the pigments settling along the bottom of the bottle. Shake it, and if the formula remains separated, it's done-zo.

tips：指甲油不会滋生细菌，但是时间一长会干。如果摇晃之后，沉淀和油状还是分离态，就变质了。