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Sunscreen comes in many forms, each with its own impacts on your body and the environment.
防曬霜有很多種，每種對你的身體和環境影響各有不同。
With so many options, how do you choose which sunscreen is best for you?
有了這么多的選擇，你如何選擇最適合你的防曬霜？
To answer that question, we first have to understand how sunscreens work.
要回答這個問題，我們首先要了解防曬霜是怎么起作用的。
Sunlight is composed of electromagnetic waves and is our primary source of ultraviolet radiation,
陽光由電磁波構成的，是我們紫外線輻射的主源，
which has a shorter wavelength than visible light and carries more energy.
其具有比可見光更短的波長并攜帶著更多的能量。
UVA, UVB, and UVC are classified according to their wavelengths.
UVA，UVB和UVC是根據其波長來分類的。
Short wavelength UVC never reaches the Earth's surface, but UVB and UVA do.
短波長的UVC永遠不會到達地球表面，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.
中等波長的UVB射線能進入皮膚的淺層，長波長的UVA射線可以滲透到更深的皮膚層。
UVB in small amounts actually helps us make vitamin D, which enables our bodies to build and maintain strong bones.
少量UVB實際上可以幫助我們制造維生素D，使我們構建身體和保持強健的骨骼。
However, prolonged exposure to UVA and UVB can damage DNA, age your skin, and promote the development of potentially deadly skin cancer.
但是，長期暴露于UVA和UVB之中會破壞DNA，皮膚老化，激發潛在致命皮膚癌的發展。
Sunscreen protects your skin either physically by deflecting UV rays with an inorganic blocker like zinc oxide or titanium dioxide,
防曬在物理上通過像氧化鋅或二氧化鈦的無機阻斷劑來偏轉紫外線從而保護肌膚，
or chemically by using carbon-based compounds to absorb UV photons that are then harmlessly dissipated as heat.
或在化學上通過使用碳基化合物來吸收紫外線光子，隨后無害的作為熱量耗散。
So, what differentiates one sunscreen from another?
那么，從什么來區分防曬呢？
