Most of us think of the Sun as our friend.
多数人认为太阳使我们的朋友。
It helps plants grow, keeps us warm, and who doesn't love to lie on the beach on a sunny day?
它帮助植物生长，使我们暖和，谁不喜欢在阳光普照的日子里躺在沙滩上晒太阳呢？
But for all of it's good qualities, the Sun can also be harmful in large amounts.
不过除了它的好处外，太阳在很多方面也会造成伤害。
That's why we invented sunscreen.
这就是发明防晒霜的原因。
The purpose of sunscreen is to shield the body from the Sun's ultraviolet rays,
防晒霜的功用是保护皮肤，隔离太阳的紫外线，
which have several harmful effects, including sunburn, aging, and skin cancer promotion.
紫外线有不少害处，其中包括晒伤、皮肤老化，也可能造成皮肤癌。
These rays are separated by their different wave lengths, into types such as UVA and UVB,
这些辐射光因为有不同波长，而分散成UVA（长波紫外线）与UVB（中波紫外线），
which exert a variety of effects in the skin due to the absorption patterns of chromophores,
这对皮肤产生了各种影响，这是由于色基发色团的吸收方式中，
the parts of the molecules responsible for their color.
部分分子对颜色产生反应。
The primary two chromophores are hemoglobin, found in our red blood cells, and melanin, which gives our skin its pigment.
两种主要的色基发色团是血红蛋白--存在于红血球中，以及黑色素，它使我们的皮肤产生色素。
We know that UVB rays cause the skin to burn.
我们知道UVB会导致皮肤晒伤。
The role of UVA rays is less well understood and appears to have an effect on our tanning response, carcinogenesis, and aging.
而对UVA射线的作用了解还不够全面，它会引发照射反应，癌变和老化。
So, how does the sunscreen protect us from these rays?
那么防晒霜如何保护我们免于辐射光的危害呢？
There are two basic types of sunscreen, physical and chemical blockers.
常见的防晒霜有两类，物理性防护和化学性防护。
Physical blockers, like zinc oxide or titanium dioxide, reflect the Sun's rays by acting as a physical barrier.
物理性防护，像是氧化锌或是二氧化钛，借由物理性阻隔物反射了太阳的辐射光。
If you've seen lifeguards with noses covered in white, then you know what this looks like.
如果你有看过救生员的鼻子上涂了一层白白的东西，你就会知道物理性防晒的样子。
The same ingredients are primary components of diaper creams, where the goal is also to create a physical barrier.
相同的配方被用作制作婴儿用的痱子粉，目的也在于产生物理屏障。
Historically, they haven't always been easy to apply and were conspicuously visible on the skin,
一直以来，他们不是那么容易涂抹，而且在皮肤上也很显眼，
but new formulations have made this less of an issue.
不过新的配方减少了这些不便。
Chemical blockers, on the other hand, absorb the Sun's rays.
另一方面，化学性防护，吸收了太阳的辐射光。
They deteriorate more quickly than physical sunscreens because their ability to absorb the Sun diminishes.
它会比物理性防晒霜更快地退化掉，因为化学防晒有能力吸收太阳光的微小分子。
Generally, these are more transparent when rubbed on the skin,
一般说来，当涂抹于皮肤之上时，这种防晒霜更加透明，
but some people develop allergric reactions to some of the chemicals.
不过有些人会对某些化学品产生过敏反应。
Regardless of the type of sunscreen, all are subjected to testing to determine their sunburn protection factor, or SPF.
无论何种防晒霜，都需要经过测试以衡量其防辐射系数，或简称SPF。
This is essentially a measure of the protection that the sunscreen will provide from UVB rays before one begins to burn.
这实际上是防护效果的单位，用来衡量当一个人开始受到照射时，防晒霜能提供的对于UVB射线的保护。
But even if you don't burn, you still need to use sunscreen
但即使你并未感到皮肤烧烫，你仍须使用防晒霜，
because unless you live in a cave, you're not immune to the effects of the Sun.
因为除非你住在洞穴里，不然你对太阳所产生的影响没有免疫力。
It is true that darker skinned people and those who tan easily have more built-in protection from sunburns,
事实上，深色皮肤的人和那些容易晒黑的人不容易被晒伤，
but they are still vulnerable to the effects of UVA.
不过他们仍然无法幸免于UVA带来的伤害。

Children under the age of six months, on the other hand,
另一方面，六个月以下的孩童，
should have almost no sun exposure as their protective mechanisms are not fully functioning,
不适合直接暴晒到太阳之下，因为他们的保护机能还没有完全发育，
and their skin is more likely to absorb any sunscreen that is applied.
另外他们的皮肤也更易于吸收任何涂抹的防晒霜。
Wearing sunscreen helps protect against the development of all three types of skin cancer:
涂抹防晒霜有助于避免这三类皮肤癌：
basal cell carcinoma, squamous cell carcinoma, and melanoma.
基底细胞癌，鳞状细胞癌以及黑色素瘤。
On a daily basis, the DNA in your cells is developing mutations and errors
日常情况下，你细胞中的DNA产生的突变和错误，
that are generally handled by machinery within your cells,
通常都会在细胞中自动处理，
but ultraviolet rays from the Sun lead to mutations that the cell may not be able to overcome,
不过阳光中紫外线引发的大量变异使细胞的自我修复无法承受，
leading to uncontrolled growth and eventual skin cancer.
导致产生无法控制的成长，最终导致皮肤癌。
The scariest thing about this is that usually you can't even see it happening until its too late.
最恐怖的事情是通常你根本观察不到任何事情，直到为时已晚。
But if these concrete risks to your health are not enough to convince you to use sunscreen,
如果这些危害健康的事实还是无法说服你使用防晒霜，
there are aesthetic reasons as well.
还有一个审美上的原因。
Along with cigarette smoking, sun damage is the leading cause of premature aging.
除了吸烟以外，阳光伤害是提早老化的主因。
Photoaging from chronic sun exposure leads to a loss of elasticity in the skin, in other words, making it look saggy.
长期暴晒在阳光下会导致皮肤失去弹性，或者说，使皮肤看上去下垂。
Take a look at this truck driver who's left side was chronically exposed to the sun and notice the difference.
看看这位卡车司机，他的左侧长期在阳光下曝晒，你可以看出两边的差异。
This is an important point.
这是一个很重要的要点。
Car windows block UVB, the burn rays, but not UVA, the aging rays.
车窗可以隔绝UVB烧伤射线，不过挡不住UVA老化射线。
It is recommended to use sunscreen daily,
建议防晒霜每天都要使用，
but you should pay special attention before prolonged sun exposure or when at the beach or among snow
不过你也要特别注意在长时间暴露在阳光下时，或在沙滩上，或在雪地中，
since the reflectivity of water and ice amplifies the Sun's rays.
因为水和冰也会反射太阳辐射光。
For these cases, apply about an ounce fifteen to thirty minutes before you go out and once again soon after you get outside.
在这些情况下，于出门前十五到三十分钟涂抹大约一盎司防晒霜，并在出门后再涂抹一次。
After that, you should reapply it every two to three hours, especially after swimming or sweating.
然后，每隔两到三小时再涂抹一次，特别是在游泳和出汗之后。
Otherwise you should wear protective clothing with ultraviolet protection factor, or UPF.
否则，你也应该穿上有防晒系数UPF的防晒外套。
Stay in shaded areas, such as under trees or an umbrella, and avoid the sun at the peak hours of 10 a.m. to 4 p.m.
待在有遮蔽物的地方，像是大树或雨伞底下，避免日照强烈的时段，如上午10点到下午4点。
And what's the best kind of sunscreen?
那么最好的防晒霜是什么呢？
Everyone will have their preference, but look for the following things: broad spectra, SPF of at least 30, and water-resistant.
每个人都有自己的偏好，不过请考虑以下几点：广谱，防晒系数SPF至少30，并且防水。
A light moisturizer with SPF 30 should be good for daily use.
防晒系数SPF30的轻量润肤霜就很适合日常使用。
Take note if you decide to use a spray.
如果你决定使用防晒喷雾，请记得。
They take several coats to effectively cover your skin,
需要不少涂层才能有效保护皮肤，
like painting a wall with a spray can versus a paint brush.
就像用喷漆的方式彩绘墙壁与用油漆刷彩绘墙壁的区别。
So, enjoy the sun, but enjoy it with sunscreen.
所以，尽情享受阳光吧，但也请在使用防晒霜的情况下享受。