夏日炙热的阳光不仅会让皮肤变黑变粗糙，经常暴晒还会导致皮肤癌。正确地使用防晒霜可以防止太阳紫外线对皮肤的伤害。一起来看看防晒霜的正确用法。

Wearing sunscreen is widely acknowledged as being one of the best things we can do for our health. More than 80% of melanoma skin cancer cases are caused by sunburn – and this number is rising year on year. There are an estimated 1.5 million new cases of skin cancer globally each year, with this figure expected to rise by 50% by 2040.

涂抹防晒霜被广泛认为是我们为健康所能做的最好的事情之一。超过80%的黑色素瘤皮肤癌病例是由晒伤引起的——这个数字每年都在上升。全球每年大约有150万例新发皮肤癌病例，预计到2040年这个数字将增加50%。

Despite these stark facts and repeated public health warnings about the risks of sun exposure, there's a lot of confusion around how, and when, to apply sunscreen.

尽管关于日晒风险有这些严酷的事实和反复的公共卫生警告，但人们关于如何以及何时涂抹防晒霜仍存在很多困惑。

Why do we need to protect our skin from the sun?

我们为什么需要保护皮肤免受阳光伤害？

When we're exposed to the sun, UV radiation damages the DNA, proteins and other molecules found within our skin cells, says Richard Gallo, professor of medicine at University of California.

加州大学医学教授理查德·加洛指出，当我们暴露在阳光下时，紫外线辐射会损害我们皮肤细胞内的DNA、蛋白质和其他分子。

In moderate amounts, exposure to UV radiation from sunlight helps our skin cells to generate vitamin D. But as we expose our skin to more sunlight, it tries to protect itself by tanning through the production of melanin, Gallo says. "If the exposure is too high, the skin can't protect itself and you get a burn," he says.

加洛指出，适量接触阳光中的紫外线辐射有助于我们的皮肤细胞生成维生素D。但当我们的皮肤暴露在更多的阳光下时，皮肤会通过产生黑色素来努力进行自我防护。他说：“如果被过度暴晒，皮肤就无法保护自己，你就会被晒伤。”

This can lead to DNA damage in our cells, which can cause premature ageing, and can increase the risk of developing skin cancer, Gallo says.

加洛说，这可能导致我们细胞中的DNA损伤，从而导致过早老化，并增加患皮肤癌的风险。

Exposure to UV radiation is the main cause of the most common forms of skin cancer.

接触紫外线辐射是引发最常见的皮肤癌的主要因素。

One 2018 study found that after sunscreen is applied, some UV protection kicks in immediately – though it takes around 10 minutes for this to become stable. However, experts usually recommend that people apply sunscreen 20-30 minutes before sun exposure to allow time for it to absorb into the skin. It might be wise to apply it twice, since studies show that most people tend to underapply sunscreen.

2018年的一项研究发现，在涂抹防晒霜后，会立刻产生一部分对紫外线的防护作用——尽管防护效果需要大约10分钟才能稳定。然而，专家通常建议人们在日晒前20-30分钟涂抹防晒霜，以便留出时间让它被皮肤吸收。可能明智的做法是涂抹两次，因为研究表明大多数人通常涂抹的量都不足。

In one study, researchers asked 31 participants to apply sunscreen in a lab under black light, and found that applying it twice reduced the surface area of skin that they missed the first time around.

在一项研究中，研究人员要求31名参与者在实验室的黑光灯下涂抹防晒霜，发现涂抹两次减少了他们第一次遗漏的皮肤表面积。

Scientists also advise reapplying sunscreen after sweating, being immersed in water, or if our skin has rubbed up against clothing or sand.

科学家还建议在出汗、浸泡在水中或皮肤与衣物或沙子摩擦后重新涂抹防晒霜。

It's also important to not mix sunscreen with other skin products, such as moisturiser, says Richard Blackburn, professor of sustainable materials at the University of Leeds' school of design. This is because many sunscreens that contain metal nanoparticles like zinc oxide may be less effective because of how these ingredients react with other ingredients.

利兹大学设计学院可持续材料学教授理查德·布莱克本说，不要将防晒霜与润肤露等其他皮肤产品混用，这一点也很重要。这是因为许多含有金属纳米颗粒（比如氧化锌）的防晒霜可能因为这些成分与其他护肤品成分的反应而导致效果降低。

Blackburn advises against mixing different brands of sunscreen, or mixing a sunscreen with other skin products, in case some of their ingredients are incompatible with each other.

布莱克本建议，不要混用不同品牌的防晒霜，或将防晒霜与其他护肤产品混用，以防它们的一些成分彼此不相容。