Sunscreen or sunblock, SPF 15, SPF 60, UVA/UVB—sunscreen is covered with strange letters and numbers. Do you know how sunscreen really works?

First, let's define sunscreen. Sunscreen is different from sunblock: unlike sunblock, sunscreen allows some light to reach the skin. The ingredients in sunscreen filter some light, unlike those in sunblock, which block all of it. Some sunscreen ingredients absorb light and turn it into heat. Others bounce it off your skin. Different ingredients act on different kinds of light rays.

Let’s move on to SPF. SPF stands for “sun protection factor” and tells you how much protection a sunscreen provides against UVB rays. The higher the SPF number, the more protection you get. Experts generally recommend SPF 15 or higher.

Most sunscreens don't protect against UVA rays. This is controversial: UVA rays don't cause sunburns, but they are linked to skin cancer. Many people feel safer blocking these rays, too, so they use broad-spectrum sunblock. On the other hand, sunscreen blocks UVB rays; however, we get vitamin D from UVB rays. Blocking too many of them can be unhealthy. Choose your sun protection, and then use it correctly. Experts recommend putting sunscreen on 15-30 minutes before you go out into the sun. Some recommend reapplying sunscreen every hour or so after that. Others say you should reapply 30 minutes after going out into the sun, but then leave it alone unless you sweat or swim. An average adult in a swimsuit should use about an ounce of sunscreen for his or her whole body. About one-quarter to one-third of a teaspoon is enough for the face.

Using sunscreen correctly can be the difference between healthy or unhealthy skin. Think about what you want, and get the right sun protection for you.

用防曬乳或防曬霜？SPF 15（防曬係數15）、SPF 60、UVA（長波紫外線）／UVB（中波紫外線）。防曬乳的包裝上滿是奇怪的字母及數字。你知道防曬乳的作用是什麼嗎？

首先，我們來定義防曬乳是什麼。防曬乳和防曬霜不一樣：防曬乳不像防曬霜，它能讓皮膚接觸部分光線。防曬乳中的成分會過濾部分光線，而防曬霜的成分會將光線完全阻隔。防曬乳的部分成分會吸收光線，將其轉成熱能，有些成分則會使光線從皮膚上反彈回去。不同的成分，會對不同的光線起作用。

我們再來談談SPF。SPF 代表「防曬係數」，它告訴你防曬乳對抗「中波紫外線」所提供的保護程度。防曬係數愈高，你得到的保護就愈多。專家通常建議用SPF 15以上的防曬乳。

多數的防曬乳不抗長波紫外線。這點頗具爭議：長波紫外線不會造成曬傷，但它跟皮膚癌有關。許多人也認為擋掉這些光線比較安全，因此他們用全效型防曬霜。另一方面，防曬乳能擋住中波紫外線。然而，人照射中波紫外線能得到維他命D，擋掉太多中波紫外線可能對你有害。

選擇你要的防曬產品，並正確地使用它。專家建議在曬太陽前的15 到30 分鐘塗上防曬乳。一些專家建議塗上後約每小時補擦一次。有人則說，你在陽光下應每30 分鐘就要補擦一次，但之後除非流汗或游泳，否則這樣就夠了。一般成人穿著泳裝應該要用一盎司的防曬乳塗抹全身。臉部約用三分之一到四分之一茶匙就夠了。

使用防曬乳的正確與否，可能影響皮膚健康或不健康。想想看你要什麼，然後為自己選擇正確的防曬產品。