張恒

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.

如果你曾經(jīng)看過爺爺奶奶年輕時的照片，想知道是什么讓他們的頭發(fā)顏色由深變淺，那就加入我們吧，我們一起找出頭發(fā)變白的原因。

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 “l(fā)ong” 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.

你頭上的每根頭發(fā)都由兩部分組成：根和干。根把頭發(fā)固定在頭皮上，這樣它們不會從你的頭上脫落。干是頭發(fā)的“長長的”有色部分，你可以看到它長在你的頭上。每根頭發(fā)都嵌套在一個毛囊中。毛囊含有一種特殊的色素細胞，產(chǎn)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.

除了頭發(fā)的顏色，黑色素也能決定你皮膚和眼睛的顏色。皮膚較黑的人比皮膚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.

如果你曾經(jīng)曬黑，那黑色素已對你起作用了。它的一項重要工作是幫助吸收來自太陽的有害紫外線輻射，保護你的細胞免受損害。雖然黑色素做得很好，但并非多多益善，這就是為什么當你出門的時候總是要涂防曬霜的原因。

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.

說到頭發(fā)，不管你是金發(fā)、黑發(fā)還是紅發(fā)（或者介于兩者之間），你都可以感謝黑色素。隨著年齡的增長，毛囊中的色素細胞開始死亡。就像曬黑幾天后開始變白一樣，當毛囊中的色素細胞減少時，頭發(fā)的顏色也開始褪色。

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，這種褪色會使頭發(fā)變成透明的顏色，在人眼中呈現(xiàn)出銀色、白色或灰色。雖然灰發(fā)在深色頭發(fā)的人之中最為明顯，但頭發(fā)較淺的人也同樣可能長白發(fā)。

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.

這對于一個年輕人意味著什么呢？沒有辦法確切知道你什么時候會發(fā)現(xiàn)第一根白發(fā)。我們所知道的是，白發(fā)受遺傳學的影響。這意味著你可以根據(jù)你父母和祖父母頭發(fā)剛開始變白時的年齡來粗略估計。

（英語原文選自：wonderopolis.org）