Decades ago, studies started bubbling up, reporting that not only was the use of sunscreen not found to be protective against skin cancer, but it was associated with significantly elevated risk.

幾十年前，就有研究報告指出，使用防晒霜不僅不能預防皮膚癌，反而會使患癌風險大大增加。

But subsequent studies offered the obvious explanation of this paradox.

但隨後的研究為這一悖論提供了顯而易見的解釋。

People who use sunscreen are more likely to be sunbathers.

使用防晒霜的人更喜歡晒太陽。

That's like finding those who use motorcycle helmets are more likely to die on the highway.

這就好比發現使用摩托車頭盔的人更容易死在高速公路上一樣。

It's not that sunscreen and helmets aren't protective, it's just that they're associated with engagement in risky activities.

這並不是說防晒霜和頭盔沒有保護作用，只是它們與從事危險活動有關。

But you don't know until you put it to the test.

但是，只有經過試驗才能知道。

As I noted in my last video, randomized controlled trials have shown that regular use of sunscreen can arrest visible signs of skin aging, including biopsy-proven reductions in UV-related skin damage.

正如我在上一段視頻中提到的，隨機對照試驗表明，定期使用防晒霜可以抑制皮膚老化的明顯跡象，包括活檢證實的紫外線相關皮膚損傷的減少。

But are their interventional trials proving sunscreen can prevent cancer?

但是，是否有干預性試驗證明防晒霜可以預防癌症呢？

Yes. Organ transplant recipients are highly susceptible to skin cancers because they have to be given immune suppressants to prevent organ rejection.

是的。器官移植受者極易患皮膚癌，因為他們必須服用免疫抑制劑來防止器官排斥。

A group of 120 organ recipients were equally informed about sun protection measures, but half were given free, broad-spectrum SPF 50 plus sunscreen for daily application to their head, neck, forearms, and hands.

一組 120 名器官接受者同樣被告知了防晒措施，但有一半人免費獲得了 SPF 50 以上的廣譜防晒霜，每天塗抹在頭部、頸部、前臂和手部。

After two years, there were 9 new basal carcinomas in the control group versus only 2 in the sunscreen group.

兩年後，對照組出現了 9 個新的基底癌，而防晒霜組只有 2 個。

That sounds good, but it may have just been a fluke.

聽起來不錯，但可能只是僥倖而已。

In contrast, there was a highly significant difference in the proportion of patients who got new invasive squamous cell carcinoma, 8 new cases in the control group versus 0 in the sunscreen group.

相比之下，新患浸潤性鱗狀細胞癌的患者比例差異非常明顯，對照組新患 8 例，而防晒霜組為 0 例。

But wait until you hear about what happened to their actinic keratoses, the precancerous growths that can turn into skin cancer.

但是，等你聽說了他們的光化性角化病（癌前病變，可轉化為皮膚癌）的情況再說吧。

They all started out covered in them, a total of 191 detected in each group.

他們一開始都渾身是傷，每組共有 191 人被檢測到。

In the subsequent 24 months, the control group developed 82 new ones.

在隨後的 24 個月中，對照組新增了 82 例。

How many did the sunscreen group develop? Negative 102.

防晒霜組研製了多少？負 102.

More than 100 precancerous growths in the sunscreen group spontaneously regressed and vanished completely, compared to 82 new growths developing in the control group without a single one disappearing.

防晒霜組有 100 多個癌前病變生長點自發消退並完全消失，而對照組則有 82 個新生長點，但沒有一個消失。

Their bodies, even in their immunocompromised state, could heal itself once it just stopped being bombarded with so many cancer-causing rays.

他們的身體，即使在免疫力低下的狀態下，只要不再受到這麼多致癌射線的照射，也能自我痊癒。

Other randomized control trials have shown similar findings, but generalizability is limited.

其他隨機對照試驗也得出了類似的結果，但推廣性有限。

The studies all used high-risk populations, for example, those living in subtropical climates or with a personal history of precancerous lesions.

這些研究都使用了高危人群，例如生活在亞熱帶氣候地區或個人有癌前病變史的人群。

So cancer prevention efficacy in the general population would be expected to be less.

是以，預計普通人群的防癌效果會比較差。

On the other hand, the studies could also have underestimated the impact since they were relatively short-term, not exceeding four years, and ethically had to allow the control group participants to slather on their own sunscreen, which would dilute the difference between groups.

另一方面，這些研究也有可能低估了其影響，因為這些研究的時間相對較短，不超過四年，而且從道德上講，必須允許對照組的參與者塗抹自己的防晒霜，這樣會沖淡組間的差異。

For maximum effectiveness, sunscreen needs to be applied properly, which apparently rarely happens in the real world.

要發揮防晒霜的最大功效，就必須正確塗抹防晒霜，而在現實生活中，這種情況顯然很少發生。

In a study of nearly 5,000 skiers and snowboarders, almost no one, only about 4%, were fully compliant with sunscreen recommendations.

在一項對近 5000 名滑雪者和滑雪板愛好者進行的研究中，幾乎沒有人（只有約 4%）完全遵守防晒建議。

First, there's the amount.

首先是金額。

The FDA standard is 2 mg per square centimeter.

美國食品和藥物管理局的標準是每平方釐米 2 毫克。

What does that mean?

這是什麼意思？

Use the teaspoon rule.

使用茶匙規則。

One teaspoon for your face, head, and neck, one for the front of your torso, another for your back, one teaspoon for each arm, and two teaspoons for each leg.

臉部、頭部和頸部一茶匙，軀幹前部一茶匙，背部一茶匙，手臂各一茶匙，腿部各兩茶匙。

That's a total of 9 teaspoons.

總共是 9 茶匙。

That's about the total volume equivalent of a golf ball or a shot glass to help you visualize it.

這相當於一個高爾夫球或一個玻璃杯的總體積，可以幫助你直觀地理解。

Unfortunately, the average sunscreen user may only apply a quarter of the recommended amount.

遺憾的是，普通防晒霜使用者可能只使用了建議用量的四分之一。

This is why high SPF sunscreens are often recommended, like 50 plus SPF.

這就是為什麼通常建議使用 SPF 值較高的防晒霜，如 SPF 值 50 以上的防晒霜。

Although the FDA recommends a minimum SPF of only 15 to prevent skin cancer, under normal consumer conditions, even an SPF of 50 may effectively give you only an SPF of 12.5 because most people don't put on enough.

雖然美國食品和藥物管理局建議預防皮膚癌的 SPF 值最低為 15，但在正常消費條件下，即使 SPF 值為 50，由於大多數人塗得不夠多，是以有效的 SPF 值也可能只有 12.5。

Randomized, double-blind, head-to-head, split-face experiments, where you're randomized to apply one sunscreen to one side of your face and another sunscreen to the other side, show that in real-life use SPF 100 plus sunscreen works significantly better than SPF 50 plus sunscreen.

隨機、雙盲、頭對頭、分臉實驗（即隨機在臉的一邊塗一種防晒霜，在另一邊塗另一種防晒霜）表明，在實際使用中，SPF 100 以上的防晒霜比 SPF 50 以上的防晒霜效果要好得多。

More than 50% of the participants were sunburned more on their SPF 50 plus side, compared to only about 5% on their SPF 100 plus side.

超過 50% 的參與者在 SPF 值為 50 以上的一側被晒傷得更嚴重，而在 SPF 值為 100 以上的一側被晒傷得更嚴重的只有約 5%。

Now, a potential downside of higher SPF sunscreens is that they could provide a false sense of security.

現在，SPF 值較高的防晒霜的一個潛在缺點是，它們可能會給人一種虛假的安全感。

Those randomized to an SPF 30 sunscreen ended up spending more cumulative time in the sun than those receiving an SPF 10 sunscreen, as much as five times longer in some cases.

與使用 SPF 10 防晒霜的人相比，被隨機分配使用 SPF 30 防晒霜的人在陽光下的累計時間更長，有些人甚至長達五倍。

What else?

還有什麼？

Well, people tend to remember to use sunscreen on a sunny day at the beach, but sun protection is needed even on cloudy days, since the UV rays are not dampened as much as visible light.

人們往往會記得在海灘的晴天使用防晒霜，但即使在陰天也需要防晒，因為紫外線不會像可見光那樣被削弱。

There's even a phenomenon known as cloud enhancement, where overcast skies can sometimes result in even more UV reaching the Earth's surface compared to clear skies.

甚至還有一種被稱為 "雲增強 "的現象，與晴朗的天空相比，陰霾的天空有時會導致更多的紫外線到達地球表面。

Sunscreen labels often suggest waiting at least 15 minutes after application before going outdoors, but when put to the test, sunscreen was found to start working immediately, with a full effect apparent by minute 10.

防晒霜標籤通常建議在塗抹後至少等待 15 分鐘再到戶外活動，但在測試中發現，防晒霜會立即開始發揮作用，在第 10 分鐘時就會完全顯效。

However, if water resistance is required, it may be prudent to wait the full 15 to 30 minutes after application before taking a dip.

不過，如果需要防水，最好在塗抹後等待 15 至 30 分鐘再浸泡。

The terms waterproof or sweatproof appear to be meaningless marketing, as no difference in retention was noted between quote-unquote waterproof products and those that were merely labeled water-resistant.

防水或防汗這些術語似乎是毫無意義的營銷，因為我們注意到，防水產品和僅標有防水標籤的產品在保持力方面沒有區別。

Both were better than non-water-resistant products, though, which lost nearly all their protective effects within 20 minutes of water immersion.

不過，這兩種產品都優於非防水產品，後者在浸水 20 分鐘後就幾乎完全失去了保護作用。

The suggestion that sunscreen should be reapplied every two to three hours only applies under conditions in which it is rinsed off by water or sweat or rubbed off by friction from clothing or sand.

防晒霜應每隔兩三個小時補塗一次的建議，只適用於防晒霜被水或汗水沖洗掉，或被衣服或沙子摩擦掉的情況。

Even after allowing sunscreen to dry first for 20 minutes, between 15 and 60% of its protective effect can be lost after contact with sand.

即使讓防晒霜先乾燥 20 分鐘，在與沙子接觸後，其保護效果也會損失 15% 到 60%。

Otherwise, if the recommended amount is applied and the sunscreen layer is not disturbed,

否則，如果使用了建議的用量，並且防晒層沒有被破壞，就可以使用、

SPF can be maintained for as long as 8 hours.

防晒指數可維持 8 小時之久。