On July 26, 1943, Los Angeles was blanketed by a thick gas that stung people's eyes and blocked out the Sun.
1943年7月26日，大量气体笼罩着洛杉矶，这气体不仅刺激人眼，还遮天蔽日。
Panicked residents believed their city had been attacked using chemical warfare.
恐慌的群众觉得洛杉矶市正在遭受一场化学武器袭击。
But the cloud wasn't an act of war. It was smog.
其实这个气体并不是战争的一种手段。这是雾霾。
A portmanteau of smoke and fog, the word "smog" was coined at the beginning of the 20th century
它是烟和雾的混合物，“雾霾”这个词在20世纪初被提出，
to describe the thick gray haze that covered cities such as London, Glasgow, and Edinburgh.
用来形容城市中飘散的厚厚灰霾，这在伦敦，格拉斯哥和爱丁堡等城市中都很常见。
This industrial smog was known to form when smoke from coal-burning home stoves and factories combined with moisture in the air.
工业雾霾由家庭火炉及工厂烧煤产生的烟，与空气中的水蒸气混合而成。
But the smog behind the LA panic was different. It was yellowish with a chemical odor.
但引起洛杉矶恐慌的雾霾却略有不同。它黄黄的，带有化学气味。
Since the city didn't burn much coal, its cause would remain a mystery
不过洛杉矶城市里已经很少烧煤，所以雾霾的形成一直是个谜，
until a chemist named Arie Haagen-Smit identified two culprits: volatile organic compounds, or VOCs, and nitrogen oxides.
直到某天一位叫阿里·哈根斯密特的化学家发现了两个元凶：挥发性有机化合物，简称VOCs，以及氮氧化物。
VOCs are compounds that easily become vapors and may contain elements, such as carbon, oxygen, hydrogen, chlorine, and sulfur.
VOCs是一种很容易变成蒸汽的化合物，也许包含很多化学元素，比如碳，氧，氢，氯，还有硫。
Some are naturally produced by plants and animals, but others come from manmade sources, like solvents, paints, glues, and petroleum.
其中有些由植物和动物产生，其它则来自工业产品，比如溶剂，油漆，胶水，还有石油。
Meanwhile, the incomplete combustion of gas in motor vehicles releases nitrogen oxide.
与此同时，机动车内未完全燃烧的汽油也会释放氮氧化物。
That's what gives this type of smog its yellowish color.
所以这种雾霾是黄色的。
VOCs and nitrogen oxide react with sunlight to produce secondary pollutants called PANs and tropospheric, or ground-level, ozone.
VOCs和氮氧化物与阳光结合会产生二次污染物，名为PANs，以及对流层或者说是地表层的臭氧。
PANs and ozone cause eye irritation and damage lung tissue.
PANs和臭氧会引起眼睛酸疼并造成肺部组织严重损伤。
Both are key ingredients in photochemical smog, which is what had been plaguing LA.
两者都是流窜整个洛杉矶城的光化学雾霾的关键组成部分。
So why does smog affect some cities but not others?
为什么其它城市没有遭到雾霾的袭击呢？
Both industrial and photochemical smog combine manmade pollution with local weather and geography.
工业和光化学雾霾与当地的天气、地理位置和人为污染有关。
London's high humidity made it a prime location for industrial smog.
伦敦的高湿度使它变成了工业雾霾的重灾区。
Photochemical smog is strongest in urban areas with calm winds and dry, warm, sunny weather.
光化学雾霾很容易出现在无风、干燥、温暖、阳光充足的城区。

The ultraviolet radiation from sunlight provides the energy necessary to breakdown molecules that contribute to smog formation.
阳光带来的紫外线辐射为打破形成雾霾的分子提供了必要的能量，
Cities surrounded by mountains, like LA, or lying in a basin, like Beijing,
如果城市被山环绕，比如洛杉矶，或建在盆地中，比如北京，
are also especially vulnerable to smog since there's nowhere for it to dissipate.
也非常容易生成雾霾，因为没办法扩散出去。
That's also partially due to a phenomenon known as temperature inversion, where instead of warm air continuously rising upward,
雾霾还与一种名为温度逆增的自然现象有关，它是指暖空气持续上升，
a pollution-filled layer of air remains trapped near the Earth's surface by a slightly warmer layer above.
充满污染物的气层被上层比较温暖的气层裹住，常年被困在地球表面。
Smog isn't just an aesthetic eyesore.
雾霾也不只是看上去让人不舒服。
Both forms of smog irritate the eyes, nose, and throat,
雾霾的两种形式都很刺激眼睛、鼻子和喉咙，
exacerbate conditions like asthma and emphysema, and increase the risk of respiratory infections like bronchitis.
也会加剧哮喘和肺气肿等疾病，还会增加呼吸道感染的可能性，比如支气管炎。
Smog can be especially harmful to young children and older people
雾霾对于小孩和老人特别有害。
and exposure in pregnant women has been linked to low birth weight and potential birth defects.
如果被孕妇吸入，可能会影响胎儿体重，或者引发潜在先天缺陷。
Secondary pollutants found in photochemical smog can damage and weaken crops and decrease yield, making them more susceptible to insects.
光化学烟雾中发现的二级污染物会损害作物并降低产量，让它们无法抵御昆虫的侵害。
Yet for decades, smog was seen as the inevitable price of civilization.
几十年来，雾霾一直被认为是人类文明前进的必然代价。
Londoners had become accustomed to the notorious pea soup fog swirling over their streets until 1952,
伦敦人民已经开始习惯浓稠的雾笼罩在街道上，直到1952年，
when the Great Smog of London shut down all transportation in the city for days and caused more than 4,000 respiratory deaths.
浓雾不仅让伦敦连续若干天关闭了城中所有交通运输服务，还造成了4000多起因呼吸疾病导致的死亡事件。
As a result, the Clean Air Act of 1956 banned burning coal in certain areas of the city, leading to a massive reduction in smog.
于是，1956年颁布的清洁空气法案开始禁止在城市中的某些区域燃烧煤炭，从此雾霾的影响显著消退。
Similarly, regulations on vehicle emissions and gas content in the US
与此同时，美国对汽车排放量和汽油成分做了明确规定，
reduced the volatile compounds in the air and smog levels along with them.
有效地减少了空气中有毒物质的浓度，从而减轻了雾霾现象。
Smog remains a major problem around the world.
如今，雾霾仍然是全世界面临的主要问题。
Countries like China and Poland that depend on coal for energy experience high levels of industrial smog.
像中国和波兰这样依赖煤炭能源的国家正在经历严重的工业雾霾的侵袭。
Photochemical smog and airborne particles from vehicle emissions affect many rapidly developing cities,
从汽车尾气中释放的光化学雾霾和大气颗粒影响了许多飞速发展的城市，
from Mexico City and Santiago to New Delhi and Tehran.
从墨西哥城到圣地亚哥，再到新德里和德黑兰。
Governments have tried many methods to tackle it, such as banning cars from driving for days at a time.
政府部门也尝试了很多方法解决雾霾问题，比如无车日等等。
As more than half of the world's population crowds into cities,
随着全世界超过一半的人口涌入城市生活，
considering a shift to mass transit and away from fossil fuels may allow us to breathe easier.
考虑使用公共交通，远离化石燃料，也许能使我们呼吸得轻松一些。