Mascara 睫毛膏

This is the makeup¹ item you should be most vigilant² about: As the brush is taken out, applied³, and put back in the tube, it brings with it any bacteria that has collected along the way, explains cosmetic⁴ 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最好。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 bugs⁵ 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 irritation⁶ 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 blemishes⁷, 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。