With so many options, how do you choose which sunscreen is best for you?

但是有琳瑯滿目的選擇，你到底要怎麼選出最適合你的防曬乳呢？

To answer that question, we first have to understand how sunscreens work.

回答這個問題之前，我們要先來看看防曬乳是怎麼作用的。

Sunlight is composed of electromagnetic waves and is our primary source of ultraviolet radiation, which has a shorter wavelength than visible light and carries more energy.

太陽由不同的電磁波構成，也是我們接收紫外線的主要來源，紫外線和可見光比起來有較短的波長，而且帶有更大的能量。

UVA, UVB, and UVC are classified according to their wavelengths.

近紫外線、中紫外線還有遠紫外線因不同的波長而有所區別。

Short wavelength UVC never reaches the Earth's surface, but UVB and UVA do.

短波長的遠紫外線完全沒辦法碰觸地球表面，但是中紫外線和近紫外線可以。

Medium-wavelength UVB rays can enter the skin's superficial layers and long-length UVA rays can penetrate into the deeper layers.

中波長的中紫外線輻射可以進入表層肌膚，而波長較長的近紫外線則可以穿透至更深層的肌膚組織。

UVB in small amounts actually helps us make vitamin D, which enables our bodies to build and maintain strong bones.

少量的中紫外線其實可以幫我們製造維生素 D，是一種可以協助我們的身體製造和維持強壯骨骼的物質。

However, prolonged exposure to UVA and UVB can damage DNA, age your skin, and promote the development of potentially deadly skin cancer.

但是長時間暴露在近紫外線和中紫外線下有損我們的基因、讓肌膚老化，甚至還會提高致命的皮膚癌的發生機率。

Sunscreen protects your skin either physically by deflecting UV rays with an inorganic blocker like zinc oxide or titanium dioxide,

防曬乳可以用物理原則反射紫外線來保護身體肌膚，用的是一種無機的阻隔，像是鋅氧化物或是二氧化鈦，

or chemically, by using carbon-based compounds to absorb UV photons that are then harmlessly dissipated as heat.

或是藉由化學的方式，也就是透過碳的化合物來吸收紫外線光子讓紫外線光子變成無害的熱能。

So, what differentiates one sunscreen from another?

現在來談談，這些防曬乳到底有什麼差別呢？

When we choose a sunscreen, we can compare application method, the SPF, and the active ingredients.

當我們選擇防曬乳的時候我們可以比較它們的使用方法、防曬係數，還有其中包含的有效成分。

Sprays can be convenient to put on, especially when you're wet, but a recent study found that most people don't apply a thick enough layer to get full protection.

噴霧型的防曬乳很方便，尤其是當你全身溼答答的時候，但是近期一個研究顯示，人們並沒有噴上足夠的防曬乳，因此沒辦法完全獲得保護。

And the possible health risks of inhaling sunscreen compounds from a spray cloud might make you consider reaching for that bottle of lotion instead.

而吸入飄來的防曬乳噴霧很有可能危害健康，因此用罐裝的防曬乳液可能會比較好。

Opt for a sunscreen with an SPF of at least 15, although 30 is better.

挑選防曬係數至少有 15 的防曬乳，雖然防曬係數 30 的會更好。

SPF is a nonlinear scale of how much UVB radiation is needed to give protected skin a sunburn.

防曬係數劃定一個非線性的範圍，讓你知道需要多少的中紫外線才會讓抹上防曬乳的肌膚被曬傷。

SPF 15 does a pretty good job by blocking 93% of UVB rays.

防曬係數 15 的防曬乳可以阻隔 93% 的中紫外線。

You get a slight increase as SPF goes up, with SPF 30 blocking 97%, and 50 blocking 98%.

當防曬係數變高，阻隔力也就變高一點，像是防曬係數 30 就能阻隔 97% 的中紫外線， 防曬係數 50 的話就能阻隔 98%。

SPF is based on the quantity of solar exposure.

防曬係數的多寡是由太陽光的量來界定。

So how much time you have before you start to burn really depends on a long list of factors, including your genetics, and when, where, and how you spend your time in the sun.

所以你要多久才會曬傷其實取決於很多因素，包含你的基因、時間、地點，還有你用什麼方式待在陽光底下。

Even though US-marketed sunscreens have been deemed safe by the FDA, scientists are still researching the effects of many active ingredients on the human body.

儘管美國販售的防曬乳都經過美國食品藥物管理局認定為安全合格的，科學家仍然在持續研究其中許多成分對人體的影響。

So if you're worried about potential irritants, look for mineral-based formulas with zinc oxide or titanium dioxide.

如果你擔心潛藏的刺激物，那就找礦物質配方的防曬乳，像是內含鋅氧化物或是二氧化鈦的防曬乳。

Even though they may go on a bit thick at first, they're less irritating than carbon-based chemical sunscreens.

即使一開始抹起來可能會有點厚重，不太舒服，但它們和碳組成的防曬乳比起來比較不具刺激性。

These mineral-based sunscreens are preferential for the environment, too.

這些礦物質組成的防曬乳對環境也比較友善。

If you plan on catching rays while splashing in a river or the ocean, keep in mind that carbon-based chemical sunscreens can harm marine life.

如果你打算到小河流或是大海玩耍，要注意碳組成的防曬乳會破壞海洋生態。

Take coral reefs for example.

以珊瑚礁為例，

Although they cover less than 1% of the Earth's underwater surface, they're home to nearly 25% of all fish species,

雖然它們只覆蓋了小於地球 1% 的水下表面，但它們是地球上約 25% 魚類的家，

making them the most diverse and productive marine ecosystems.

這使得珊瑚礁擁有最豐富的生物多樣性，也是最具有生產力的海洋生態系統。

Research shows that carbon-based chemical sunscreen ingredients,

研究顯示，碳組成防曬乳的成分，

like oxybenzone, butylparaben, octinoxate, and 4MBC contribute to a stress condition called coral bleaching in corals, which are living creatures.

像是氧苯酮、對羥基苯甲酸丁酯、甲氧基肉桂酸辛酯還有甲基亞芐亞基樟腦會造成珊瑚白化 ，而珊瑚其實是生物的一種。

Exposure to these organic compounds results in the death of the coral's symbiotic algae.

暴露於這些有機化合物之中會讓珊瑚中共生的藻類死亡。

In addition to providing a reliable food source, these algae give coral their brilliant rainbow of colors.

除了提供穩定的食物來源，這些藻類也讓珊瑚擁有彩虹般的顏色，

Without them, corals turn a bleached white and are susceptible to disease and possibly death.

如果沒有它們，珊瑚就會白化可能會生病甚至死亡。

And once the coral dies, the entire reef ecosystem is not far behind.

一旦這些珊瑚死掉，整個珊瑚礁生態系統很快也會瓦解。

So you're now ready to make an informed choice when picking out your next sunscreen.

希望你現在已經準備好，在選擇你下一罐防曬乳的時候做出明智的抉擇。

SPF is clearly labeled on the front.

防曬係數通常都很清楚的標示在正面。

On the back, under "active ingredients," you can find whether zinc oxide, titanium dioxide, and those coral-harming components are present.

防曬乳背面的有效成分那欄可以看到是否包含鋅氧化物、二氧化鈦，或是那些危害珊瑚的成分。

Taking a bit more time to check can be well worth it for both you and the environment.

多花點時間選擇是值得的，不僅為了自己也為了我們身處的環境。