Sunscreen comes in many forms, each with its own impacts on your body and the environment.
防晒霜有各种形式的，每一种对于你的身体和大环境都有着不同的影响。
With so many options, how do you choose which sunscreen is best for you?
有这么多的选择，你应该如何选择最适合你的防晒霜呢？
To answer that question, we first have to understand how sunscreens work.
为了解答这个问题，我们首先需要知道防晒霜是如何起到它的作用的。
Sunlight is composed of electromagnetic waves and is our primary source of ultraviolet radiation,
太阳光是由电磁波组成的，也是我们接收到的紫外线的主要来源，
which has a shorter wavelength than visible light and carries more energy.
它有相较可见光更短的波长，也带有更多的能量。
UVA, UVB, and UVC are classified according to their wavelengths.
a波紫外线，b波紫外线和c波紫外线是由它们的波长来区分的。
Short wavelength UVC never reaches the Earth's surface, but UVB and UVA do.
短波长的c波紫外线到不了地球表面，但是a波和b波紫外线可以到达。
Medium wavelength UVB rays can enter the skin's superficial layers and long length UVA rays can penetrate into the deeper layers.
中等波长的b波紫外线可以进到皮肤的表层，而长波长的a波紫外线则可进入皮肤的深层。
UVB in small amounts actually helps us make vitamin D, which enables our bodies to build and maintain strong bones.
少量的b波紫外线可以帮助人体产生维他命D，它能让我们的身体保持强壮的骨骼。
However, prolonged exposure to UVA and UVB can damage DNA, age your skin,
然而，过长的暴露在a波和b波紫外线下会损坏DNA，并使皮肤衰老，
and promote the development of potentially deadly skin cancer.
还会增加得致命性皮肤癌的可能。
Sunscreen protects your skin either physically by deflecting UV rays with an inorganic blocker like zinc oxide or titanium dioxide,
防晒霜保护皮肤一是以物理护肤的方式偏转紫外线，运用的是一种无机的阻隔物，例如二氧化锌或二氧化钛，
or chemically by using carbon-based compounds to absorb UV photons that are then harmlessly dissipated as heat.
二是通过化学方式用碳化物类吸收紫外线中的光子，使它们以热的形式消散。
So, what differentiates one sunscreen from another?
那么，是怎样区分一种防晒霜呢？
When we choose a sunscreen, we can compare application method, the SPF, and the active ingredients.
当我们选择防晒霜时，我们可以比较它们的使用方法，比较防晒系数和其他的成分。
Sprays can be convenient to put on, especially when you're wet,
防晒喷雾使用起来很方便，尤其在你的肌肤是湿润的时候，
but a recent study found that most people don't apply a thick enough layer to get full protection.
然而，最近的一个研究发现大多数人并不会喷上足够保护皮肤的量。
And the possible health risks of inhaling sunscreen compounds from a spray
而且，也存在不小心吸入防晒喷雾而产生的健康隐患
cloud might make you consider reaching for that bottle of lotion instead.
这也许使你想去换用那瓶防晒乳液。
Opt for a sunscreen with an SPF of at least 15, although 30 is better.
选择至少有SPF15的防晒产品，当然SPF30的会更好。
SPF is a nonlinear scale of how much UVB radiation is needed to give protected skin a sunburn.
SPF是一种非线性的比例，关于的造成晒伤的b波紫外线的量。
SPF 15 does a pretty good job by blocking 93% of UVB rays.
SPF15的防晒可以阻挡93%的b波紫外线。
You get a slight increase as SPF goes up, with SPF 30 blocking 97%, and 50 blocking 98%.
SPF值的升高，它的阻挡力也会稍许增加，SPF30可以阻挡97%，而SPF50则可以阻挡98%。

SPF is based on the quantity of solar exposure.
SPF值根据的是太阳曝光的量。
So how much time you have before you start to burn really depends on a long list of factors,
所以你是否容易被晒伤有很多的因素，
including your genetics, and when, where, and how you spend your time in the sun.
包括你的基因、什么时间、什么地点、如何晒太阳的。
Even though US marketed sunscreens have been deemed safe by the FDA,
虽然美国认证的防晒霜被美国食品药品监督管理局认可为安全的，
scientists are still researching the effects of many active ingredients on the human body.
科学家仍旧在继续研究许多成分对人体会产生的影响。
So if you're worried about potential irritants, look for mineral-based formulas with zinc oxide or titanium dioxide.
所以如果你担心潜在的刺激，可以选用矿物配方以及有二氧化锌或二氧化钛的防晒产品。
Even though they may go on a bit thick at first, they're less irritating than carbon-based chemical sunscreens.
它们一开始可能会有一些厚，但是它们相对于碳配方的防晒霜，对皮肤刺激较小。
These mineral-based sunscreens are preferential for the environment, too.
矿物配方的防晒产品对环境也有保护。
If you plan on catching rays while splashing in a river or the ocean,
如果你想要一边沐浴阳光，又一边想在河或者海洋中戏水，
keep in mind that carbon-based chemical sunscreens can harm marine life.
要记住，碳配方的化学防晒霜对海洋生物有害。
Take coral reefs, for example.
就拿珊瑚礁来举例。
Although they cover less than 1% of the Earth's underwater surface,
虽然它们只覆盖了百分之一的地球水下面积，
they're home to nearly 25% of all fish species, making them the most diverse and productive marine ecosystems.
但是它们是百分之二十五的鱼类的家园，使得它们成为了最多样的和生产力最强的海洋生态系统。
Research shows that carbon-based chemical sunscreen ingredients, like oxybenzone, butylparaben, octinoxate, and 4MBC
研究显示，碳配方的化学防晒中的一些物质，例如说氧苯酮，尼泊金丁酯，桂皮酸钠和4MBC，
contribute to a stress condition called coral bleaching in corals, which are living creatures.
会给海洋带来很不好的影响，比如说珊瑚漂白，而它们都是有生命的生物。
Exposure to these organic compounds results in the death of the coral's symbiotic algae.
当珊瑚暴露在这些有机化合物前，会导致珊瑚的共生藻的死亡。
In addition to providing a reliable food source, these algae give coral their brilliant rainbow of colors.
这些共生藻除了提供食物源，它们还带给珊瑚彩虹般绚烂的色彩。
Without them, corals turn a bleached white and are susceptible to disease and possibly death.
没有了它们，珊瑚就像被漂白过一样，而且很容易会有染病和死亡的可能性。
And once the coral dies, the entire reef ecosystem is not far behind.
一旦珊瑚死去，整个珊瑚礁生态系统都将会崩溃。
So you're now ready to make an informed choice when picking out your next sunscreen.
你现在一定可以在选防晒霜时做出有所依据的选择了。
SPF is clearly labeled on the front.
SPF值在防晒产品上都会清楚地标明。
On the back under 'active ingredients,' you can find whether zinc oxide, titanium dioxide,
在背后的“成分表”中，你可以发现防晒产品是否含有二氧化锌、二氧化钛
and those coral-harming components are present.
或是对珊瑚有害的成分。
Taking a bit more time to check can be well worth it for both you and the environment.
花稍微多一点的时间来做出一个对你、对环境都好的选择吧。