Sunscreen comes in many forms, each with its own impacts on your body and the environment.
防晒霜有很多种，每种对你的身体和环境影响各有不同。
With so many options, how do you choose which sunscreen is best for you?
有了这么多的选择，你如何选择最适合你的防晒霜？
To answer that question, we first have to understand how sunscreens work.
要回答这个问题，我们首先要了解防晒霜是怎么起作用的。
Sunlight is composed of electromagnetic waves and is our primary source of ultraviolet radiation,
阳光由电磁波构成的，是我们紫外线辐射的主源，
which has a shorter wavelength than visible light and carries more energy.
其具有比可见光更短的波长并携带着更多的能量。
UVA, UVB, and UVC are classified according to their wavelengths.
UVA，UVB和UVC是根据其波长来分类的。
Short wavelength UVC never reaches the Earth's surface, but UVB and UVA do.
短波长的UVC永远不会到达地球表面，但UVB和UVA可以。
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UVB射线能进入皮肤的浅层，长波长的UVA射线可以渗透到更深的皮肤层。
UVB in small amounts actually helps us make vitamin D, which enables our bodies to build and maintain strong bones.
少量UVB实际上可以帮助我们制造维生素D，使我们构建身体和保持强健的骨骼。
However, prolonged exposure to UVA and UVB can damage DNA, age your skin,
但是，长期暴露于UVA和UVB之中会破坏DNA，皮肤老化，
and promote the development of potentially deadly skin cancer.
激发潜在致命皮肤癌的发展。
Sunscreen protects your skin either physically by deflecting UV rays with an inorganic blocker like zinc oxide or titanium dioxide,
防晒在物理上通过像氧化锌或二氧化钛的无机阻断剂来偏转紫外线从而保护肌肤，
or chemically by using carbon-based compounds to absorb UV photons that are then harmlessly dissipated as heat.
或在化学上通过使用碳基化合物来吸收紫外线光子，随后无害的作为热量耗散。
So, what differentiates one sunscreen from another?
那么，从什么来区分防晒呢？
When we choose a sunscreen, we can compare application method, the SPF, and the active ingredients.
当我们选择防晒品时可以比较一下使用方法，SPF值，和活性成分。
Sprays can be convenient to put on, especially when you're wet,
喷雾使用起来比较方便，尤其是当你身上湿了时候，
but a recent study found that most people don't apply a thick enough layer to get full protection.
但最近的一项研究发现，大多数人没有喷到足够厚以得到充分的保护。
And the possible health risks of inhaling sunscreen compounds from a spray cloud
而且吸入防晒化合物可能带来的健康风险
might make you consider reaching for that bottle of lotion instead.
也许会让你考虑用一瓶霜来代替。
Opt for a sunscreen with an SPF of at least 15, although 30 is better.
选择具有至少15的SPF防晒剂，虽然30更好。

SPF is a nonlinear scale of how much UVB radiation is needed to give protected skin a sunburn.
SPF是指需要多少非线性标度给皮肤保护以防被UVB照射晒伤。
SPF 15 does a pretty good job by blocking 93% of UVB rays.
SPF15通过阻断93％的UVB射线起到了很好的作用。
You get a slight increase as SPF goes up, with SPF 30 blocking 97%, and 50 blocking 98%.
随着SPF上升略有提高，SPF30可以阻挡97％，50可以阻断98％。
SPF is based on the quantity of solar exposure.
SPF是基于太阳能曝光量来定的。
So how much time you have before you start to burn really depends on a long list of factors, including your genetics,
所以，你在晒伤之前有多少时间，实际上取决于一长串的因素，包括你的遗传，
and when, where, and how you spend your time in the sun.
何时，何地以及你在阳光下干什么。
Even though US marketed sunscreens have been deemed safe by the FDA,
尽管美国市场上销售的防晒霜已被FDA认定为安全，
scientists are still researching the effects of many active ingredients on the human body.
科学家们仍在研究多种活性成分对人体的影响。
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，那就找找含有氧化锌或二氧化钛的矿物质公式。
Even though they may go on a bit thick at first, they're less irritating than carbon-based chemical sunscreens.
尽管开始的时候可能会厚一点，但是他们比碳基化学防晒剂的刺激性较小。
These mineral-based sunscreens are preferential for the environment, too.
这些矿基物防晒霜对环境也更好。
If you plan on catching rays while splashing in a river or the ocean,
如果你打算在河流或海洋里玩耍时防晒，
keep in mind that carbon-based chemical sunscreens can harm marine life.
请记住，碳基的化学防晒剂会伤害海洋生物。
Take coral reefs, for example. Although they cover less than 1% of the Earth's underwater surface,
以珊瑚礁为例。尽管它们覆盖了小于1％的地球水下地表，
they're home to nearly 25% of all fish species, making them the most diverse and productive marine ecosystems.
但却是近25％的鱼类物种的家园，构成了最多样化和富有成效的海洋生态系统。
Research shows that carbon-based chemical sunscreen ingredients, like oxybenzone, butylparaben,
研究表明碳基的化学防晒成分，像氧苯酮，对羟基苯甲酸丁酯，
octinoxate, and 4MBC contribute to a stress condition called coral bleaching in corals, which are living creatures.
甲氧基肉桂酸辛酯和4MBC，会导致所谓的珊瑚白化困境，而珊瑚都是生命体。
Exposure to these organic compounds results in the death of the coral's symbiotic algae.
暴露于这些有机化合物会导致珊瑚共生藻类的死亡。
In addition to providing a reliable food source, these algae give coral their brilliant rainbow of colors.
除了提供可靠的食物来源，这些藻类给予了珊瑚彩虹般的缤纷色彩。
Without them, corals turn a bleached white and are susceptible to disease and possibly death.
如果没有他们，珊瑚会漂白，容易害病甚至死亡。
And once the coral dies, the entire reef ecosystem is not far behind.
而一旦珊瑚死亡，整个珊瑚礁生态系统也就不远了。
So you're now ready to make an informed choice when picking out your next sunscreen.
所以，你在挑选下一个防晒品时请做出明智的选择。
SPF is clearly labeled on the front.
SPF值被清楚地标示在前面。
On the back under "active ingredients," you can find whether zinc oxide, titanium dioxide,
背面标注着“活性成分”，你可以找到是否含有氧化锌，
and those coral-harming components are present.
二氧化钛和那些珊瑚贻害成分。
Taking a bit more time to check can be well worth it for both you and the environment.
多花一点的时间来检查对您和环境来说意义非同一般。