It makes sense in theory: If you’re super pale and you burn easily, you should use the highest SPF you can find when you go out in the sun, right? Not necessarily. The reality of what various SPFs offer in the way of protection — and which ones you should be using — is a bit more complicated than that.

　　理论上是这样的：如果你皮肤很白，但又容易晒伤，出门时就应该涂抹高防晒系数(SPF)的防晒霜，对吗?其实不然，不同SPF值的防晒霜都有什么作用，你应该选择哪种防晒霜，这些问题比你想得要复杂。

　　Sunscreens can offer protection from two types of ultraviolet (UV) rays: UVB, which according to the Skin Cancer Foundation, are the chief cause of sunburn and play a key role in the development of skin cancer, and UVA, which penetrate the skin more deeply and are largely responsible for skin aging and wrinkling, as well as skin cancer risk.The SPF measurement on sunscreens refers specifically to the UVB protection level — and the number may not mean what you think it does. SPF 15 doesn’t protect you from 15% of UVB rays, just as SPF 100 doesn’t protect you from 100% of the rays. In fact, as you get into the higher numbered SPF sunscreens (think: 50 and above), the difference in UVB protection is actually pretty minimal. “The relationship is not a linear thing,” dermatologist Dr. Mona Gohara, MD tells Teen Vogue. “For example, [SPF] 15 isn’t blocking out half of 30.” Rather, as Dr. Gohara explains, SPF 15 blocks about 93% of UVB rays, SPF 30 blocks about 95%, SPF 50 blocks about 97%, and SPF 100 blocks about 99%.

　　防晒霜可以隔离两种紫外线(UV)：第一种是紫外线B光谱(UVB)。皮肤癌基金会的研究表明，这种紫外线是导致晒伤的主要原因，并且是皮肤癌的主要诱因。另一种是长波紫外线(UVA)，这种紫外线会穿透皮肤，导致皮肤老化并出现皱纹，当然也有诱发皮肤癌的风险。防晒指数SPF的测量主要是依据该防晒霜隔离UVB的能力，SPF数值的意义可能和你想的不同。SPF数值为15并不意味着这个该防晒霜可以隔绝15%的UVB，正如SPF为100的防晒霜并不是能够完全隔绝UVB一样。事实上，SPF值比较高(大于等于50)的防晒霜，它们隔绝UVB的能力是大同小异的。马里兰州的皮肤病博士Mona Gohara告诉《Teen Vogue》时尚杂志说：“二者并不呈现线性关系。比如，SPF值为15的防晒霜并不是隔绝15%的紫外线，反之，它可以隔绝93%的UVB。SPF值为30的防晒霜可以隔绝95%的UVB，SPF值为50的可以隔绝97%的UVB，而SPF值为100的可以隔绝99%的UVB。”

　　That said, though the UVA protection isn’t technically represented by the SPF number on a sunscreen bottle. Steven Q. Wang, MD, a dermatologist and spokesperson for the Skin Cancer Foundation, tells Teen Vogue that the difference in UVA protection between a broad-spectrum (which represents both UVB and UVA protection) SPF 50 and an SPF 100 sunscreen, for example, may be significant enough to make the increase worthwhile. “If you’re looking at SPF 50 versus 100, the...incremental benefit [of the higher one] is small, but the UVA protection may be much higher,” he says.

　　也就是说，SPF系数并不能代表隔绝UVA的能力。马里兰州皮肤专家及皮肤癌基金会的发言人Steven Q. Wang向《Teen Vogue》杂志透漏，在广谱(代表UVB和UVA的隔绝能力)中，SPF50和SPF100的UVA隔绝能力是差别巨大的。也就是说，如果你在比较这两种防晒霜，SPF值高的可能在UVB防护上没有什么大差别，但是UVA的隔绝能力会高很多。

　　So, theoretically, though the increase in UVB protection with a super-high SPF sunscreen may be negligible, it may help, and certainly can’t hurt, to use it — that is, as long as you do so correctly. “The only downside [of using a higher SPF] is if you’re lulling yourself into a sense of false security,” Dr. Gohara says.

　　因此，理论上说，防晒霜的SPF数值增高并不会在其UVB的隔绝能力上增加多少，但是使用一个SPF值高的防晒霜还是有帮助的。只要学会正确使用，就不会有害处。Gohara 指出：“SPF值高的防晒霜唯一的缺点就是会给你营造一种虚假的安全感。”

　　Indeed, as Dr. Wang also pointed out, when people opt for increased SPFs, they may actually feel so protected by the product that they ultimately neglect proper sun protection. “When people use really high SPF products, they tend not to reapply, they tend to stay out in the sun much longer, and they don’t use other protective modalities such as clothing and hats,” he says. Regardless of any increase in protection that a high SPF may or may not offer, one thing is certain: It doesn’t give your body a free pass from sunburns, aging, or cancers, and it doesn’t protect you from those things for any greater length of time than the lower SPF options. Both Dr. Wang and Dr. Gohara emphasized that, no matter what the SPF, it’s crucial to reapply your sunscreen every two hours.

　　事实上，Wang博士也指出，当人们选择高SPF值的防晒霜时，他们就觉得很有安全感，从而忽略了必要的防晒措施。“当使用高SPF防晒产品时，人们一般就不会补擦，而且经常在太阳底下待很长时间，也不采取其他的防晒措施，诸如穿防晒服，戴防晒帽等。”不论高SPF的防晒霜是否在隔绝紫外线能力上有所加强，有一件事是肯定的，即它们无法完全避免晒伤，皮肤老化或者皮肤癌。而且它们也不会比其他低SPF值得防晒霜防晒效果更持久。Wang博士和Gohara博士都强调，无论防晒霜的SPF值是多少，一定要每隔两小时补擦。

　　Which brings us to what is potentially the most important factor of the SPF conversation: Human error. True, an SPF 30 sunscreen technically blocks about 95% of UVB rays, but that’s only when it’s used correctly — and it’s usually not. “Most people don’t use enough sunscreen, so the real SPF protection they’re getting is about one third of the labeled SPF,” Dr. Wang says. “So, if you have a product that says [SPF] 60, you’re really getting an SPF of maybe 20 or 30.” Though there’s plenty of available information on how to properly apply sunscreen — including instructions on the bottles themselves — most of us simply don’t lather on as much product as is used in testing and labeling SPFs. On top of that, we miss spots, we forget to reapply, we go in the water too soon, and we don’t compensate for these errors with supplemental protection.

　　这就引申出使用防晒霜一个最关键的方面：人为失误。的确，SPF值为30的防晒霜可以隔绝93%的紫外线，但前提是人们的使用方法必须正确。可是通常人不会正确使用防晒霜。Wang博士指出：“大多数人防晒霜的用量不够，导致使用的防晒霜防晒效果只有实际SPF值得三分之一左右。因此当使用SPF值为60的防晒霜时，真正有用的SPF值仅仅为20或30。”虽然有很多教大家如何正确使用防晒霜的教程，包括防晒霜瓶子上也有，大多数人的涂抹量还是达不到SPF贴标检测过程的用量。除此之外，我们总是会漏涂，忘记补涂，游泳时很快跳下水，而且不知道使用别的防护措施来弥补这些过失。

　　So what does this all mean for your next trip down the sunscreen aisle? It boils down to this: The most important thing is to use a broad-spectrum sunscreen of at least SPF 30 — and to use it often and properly. But, given the fact that none of us is perfect (and certainly not when it comes to applying sunscreen), it just may be worth it to bump that SPF up a notch or two to help make up for that inevitable human error. Just remember that when you do, you should put higher SPF product in your shopping cart. From there on out, it’s (vigilant and careful sunscreen) business as usual.

　　所以这些对你下次涂抹防晒霜有什么启示呢?归根结底：最重要的是使用广谱防晒霜(SPF值大于等于30)，并且要学会合理且频繁的使用。但是，人无完人(尤其是谈到使用防晒霜时)，我们不妨每次多用一点点来弥补那些人为失误。记住尽可能购买高SPF值的防晒霜，这以后就是正常的防晒工作了。

（编辑：何莹莹）