Why lips go dry？
为什么嘴唇会干呢？
Let’s start by reviewing the ways in which the surface of our lips is different from the surface of the skin on the rest of our bodies. Basically, our lips are made of the same mucusmembrane that coats the inside of our mouths. This means two things.
首先，我们需要知道嘴唇的表皮有别于身体其他部位的。基本而言，嘴唇有相同的粘液膜组成，保护我们的嘴巴。这就说明了两件事首先。
First, our lips don’t have the same protective outer layer as the skin elsewhere on our bodies. This layer is part of the barrier to evaporation, and its absence means our lip skin retains less fluid. Second, our lips don’t have the same glands and pigments as ordinary skin. For example, lips don’t have oil glands, which also help keep moisture from evaporating, and they contain less melanin, the pigment that helps protect our skin from sunburn.
第一，我们身体其他部位的肌肤有一层“保护层”，这是嘴唇所没有的。“保护层”能有效保湿，嘴唇因为没有而不易保湿。第二，嘴唇不像其他肌肤有腺体和色素。比如，嘴唇不能分泌油脂，不易保湿。此外，嘴唇缺少黑色素等色素保护皮肤免受太阳灼伤。

What this means is that our lips have little to no protection from the environment, and their main source of moisture is the saliva that comes from our mouths. What’s more, unlike much of the rest of our body, our lips are almost always on display, and rarely covered by clothing. As a result, the moisture on our lips is continually evaporating. And that’s before we take into account exposure to sun, wind, cold, and air-conditioning. Plus, if you’re sick or have allergies, and your nose is congested, your lips end up drying more quickly because you tend to breathe through your mouth.
这就说明嘴唇易受环境影响，而嘴唇水分大多来源于嘴里的唾液。此外，嘴唇有别于身体其他部位的皮肤（有衣蔽体），嘴唇经常暴露在外面。这样一来，嘴唇的水分一直在蒸发，这还不算阳光、大风、天冷、空调的“摧残”。此外，如果你生病了或者过敏而鼻塞，不得不用嘴巴呼吸，嘴唇会干得更快！