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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, and promote the development of potentially deadly skin cancer.
但是，长期暴露于UVA和UVB之中会破坏DNA，皮肤老化，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？
