张恒

If youve ever seen a picture of a grandparent when they are young and wondered what made their hair go from dark to light， join us as we find out why hair goes gray.

如果你曾经看过爷爷奶奶年轻时的照片，想知道是什么让他们的头发颜色由深变浅，那就加入我们吧，我们一起找出头发变白的原因。

Each hair on your head has two parts： a root1 and a shaft2. The root keeps the hairs on your head so they will stay on your scalp3. The shaft is the “long” colored part of hair you see growing on your head. Each root of hair is in a follicle4. Follicles have special kinds of pigment cells5 that make melanin6.

你头上的每根头发都由两部分组成：根和干。根把头发固定在头皮上，这样它们不会从你的头上脱落。干是头发的“长长的”有色部分，你可以看到它长在你的头上。每根头发都嵌套在一个毛囊中。毛囊含有一种特殊的色素细胞，产生黑色素。

Whats more， melanin is also responsible7 for the color of your skin and eyes. People who have darker skin have more melanin than those with fair skin.

除了头发的颜色，黑色素也能决定你皮肤和眼睛的颜色。皮肤较黑的人比皮肤白皙的人黑色素含量高。

If you have ever had a suntan8， youve already experienced melanin in action. One of its important jobs is to help take in ultraviolet radiation9 from the sun and protect10 your cells. Though melanin does a great job， it isnt perfect， which is why it is important to always wear sunscreen11 when you go outdoors.

如果你曾经晒黑，那黑色素已对你起作用了。它的一项重要工作是帮助吸收来自太阳的有害紫外线辐射，保护你的细胞免受损害。虽然黑色素做得很好，但并非多多益善，这就是为什么当你出门的时候总是要涂防晒霜的原因。

When it comes to hair， whether you have blonde， dark， or red hair （or somewhere in between）， you can thank melanin. As a person grows up， the pigment cells in the hair follicles begin to die. Much like a suntan begins to fade12 after a few days， when there are fewer pigment cells in the hair follicle， hair color begins to fade， too.

说到头发，不管你是金发、黑发还是红发（或者介于两者之间），你都可以感谢黑色素。随着年龄的增长，毛囊中的色素细胞开始死亡。就像晒黑几天后开始变白一样，当毛囊中的色素细胞减少时，头发的颜色也开始褪色。

In many older people， this fading gives the hair a transparent13 color. It shows silver， white， or gray to the human eye. Gray hairs are most noticeable14 in those with dark hair， but people with lighter hair are likely to go gray， too.

在許多老年人中，这种褪色会使头发变成透明的颜色，在人眼中呈现出银色、白色或灰色。虽然灰发在深色头发的人之中最为明显，但头发较浅的人也同样可能长白发。

What does this mean for young people？ There is no way to know when you will find your first gray hair. What we do know is that going gray is influenced15 by genetics16. You may find it at the same age as your parents and grandparents did.

这对于一个年轻人意味着什么呢？没有办法确切知道你什么时候会发现第一根白发。我们所知道的是，白发受遗传学的影响。这意味着你可以根据你父母和祖父母头发刚开始变白时的年龄来粗略估计。

（英语原文选自：wonderopolis.org）