On the night of December 1, 1930, a dense fog moved over the Meuse Valley, in Belgium. Many factories in the valley poured smoke and fumes into the foggy air. This created a dark smog of smoke and fog combined. People in the valley began to cough and train for breath. The smog remained for four days. During that time, thousands of people became ill. The hospitals were filled with patients. Sixty people died. Most of them were older persons with heart and lung problems. Finally, a heavy rain washed away the smog. Scientists studied the causes of the disaster. They concluded that the illnesses and deaths were caused by chemicals in the smog.

The first reported event of this kind in the United States happened in Donora, a factory town in a valley near Pittsburgh. In 1948, a killer smog made half of the population sick, there were 17 deaths. Again, older people with lung or heart diseases were hit hardest.

London, England, has always been known for its "black fogs." In the winter of 1952, a milky white fog rolled into the city. It soon turned into black smog as the smoke of the city poured into the air. It was so hard to see that people had to walk in front of the buses to guide them. In this way, the most serious air pollution disaster in history began. When it was over, more than 4,000 people had been killed by the thick black smog.

New York City has had several London-type smogs since 1950. Each time, there were from 100 to 400 deaths caused by the smog. Although these smogs were not as deadly as London's, New York City has the worst air pollution problem in the United States.

In all the killer smogs, factories and homes poured smoke and fumes into the air from the furnaces. The chemical fumes combined with the water droplets in the fog to form harmful substances. These substances caused the illness of those who breathed the polluted air.

Usually, such harmful fumes rise into the upper air and are blown away by the wind. But sometimes there is an unusual weather condition called a temperature inversion. A layer of cold air remains near the ground as smoke and fumes pour into it. This is covered by an upper layer of warm air that acts like a lid. It prevents the polluted cooler air from rising. The harmful fumes pile up and make people ill. The smog may be so thick that airports are closed and chains of collisions occur on the highways.

Another type of smog occurs in Los Angeles. Here the weather may be clear and sunny. But stinging eyes and dry coughs show that harmful chemicals fill the air. The smog is due to invisible gases, mostly from automobile exhaust. Because these chemicals are changed by the sun high up in the air, Los Angeles smog is called photochemical smog. It contains automobile exhaust fumes and nitrogen oxides changed by the sun's rays. Added to these are sulfur dioxide and other fumes from factories and oil refineries. Photochemical smog is found in many large cities all over the world.

Killer smogs don't happen very often, fortunately. But in many large cities, a combination of automobile exhaust fumes, home furnace smoke, and factory waste gases pours into the air. This may also happen in the suburbs, or out in the country, where large factories have been built. A number of harmful substances have been found in the air there. When these substances are breathed in day after day, the health of the population is affected.

在1930年12月1日夜，一场浓雾漂移到了比利时的默兹山谷地区。在这山谷中的许多工厂都把烟尘和各种有害的微粒排放到这一场大雾当中了，于是就形成了烟与雾结合在一起的一大片浓黑的烟雾。这一山谷里的人们开始咳嗽，呼吸紧张。这场烟雾在这山谷里足足延续了四天，在这四天里成千上万的人病倒了。各家医院里都挤满了病人。结果60人死亡。死者中绝大多数都是患有心脏病和肺病的老人。最后，是一场大雨把这场烟雾冲洗掉了。科学家倦研究了这场灾难之后得出结论说，这场烟雾中的有害化学物质导致了人伞兵生病和死亡。

据报导，在美国的第一起这类事件发生在靠近匹兹堡的多诺拉。多诺拉是一个城镇，那里全是工厂。在1948年，一场烟雾导致了该镇有壮大数人口生了病，17人死亡。这里也还是患有肺病或心脏病的老年人受害最重。

英国伦敦一向以其"黑雾"称著于世。在1952年冬季，一厚层奶白色的雾滚滚漂进伦敦的空气当中。这场雾浓得令人什么也看不见，以致人们不得不步行在公共汽车的前头引导着司机慢慢开车前进。就这样，开始了历史上最严重的一场空气污染所造成的灾难。灾难结束时有四千多人被这场浓黑的烟雾夺去了生命。

自从1950年以来，美国纽约市发生了数起类似伦敦型的烟雾杀人事件。每次都因为有毒烟雾导致了一百到四百人的死亡。虽然这些烟雾令人致死的程度没有伦敦烟雾那样厉害，但是纽约市的空气污染问题在全美国是最坏的，最严重的。

在所有这一切烟雾杀人的事件当中，都是由于各大小工厂和各个家庭把他们炉火中的烟尘和微粒排放到空气中所造成的。这些化学微粒同雾中的极微小的水珠结合起来就形成了各种有害的物质，这些有害物质是那些呼吸这些被污染了的空气的人致病的原因。

一般情况下，这些有害的微粒漂浮到空气的上层以后，会被风吹走。但有时会有一种被称作"温度逆增"的不平常的天气状态。当烟尘和微粒拜谢到空气中以后，有一层冷空气仍然在贴近地面。这层冷空气就阻挡了这些被污染的冷空气，使之不能上升的高空中。这些有害的微粒越聚越多，于是就使人生病了。这些烟专利号有时很浓以致各个机场被迫关闭，公路上发生了一串一串的汽车相撞的事件。

在美国洛杉矶出现过另一种烟雾。这里天气晴朗阳光灿烂，但是人们感到眼睛刺痛，出现干咳嗽。这些症状表明空气中充满了有害的化学物质。这种烟雾是由看不见的无色气体所组成，这种气体主要是汽车尾气中的微粒和被阳光照射后起了化学变化的氮氧化物，此外还有从工厂和炼油厂排放出来的二氧化碳和其他的悬浮微粒。在全世界很多大城市中都发现有光化学烟雾。

幸好，这种烟雾杀人事件并不是经常发生的。但在许多大城市中，都把汽车尾气中的微粒、家庭火炉冒出来的烟尘及工厂的废气汇合在一起排放到空气中去了。这种做法也会出现在郊区或城市外围的农村，因为这些地方都盖起了大型的各类工厂。在这些的空气中发现有很多的有害化学物质，人们日复一日地呼吸了这些有害物质，身体健康就会大受影响。