London used to be "foggy" for the same reason that cities like Beijing or Chongqing are "foggy" today. The "fog" was in fact smog, a mixture of smoke and fog. In other words, it was made by air pollution. In London, some of this pollution came from factories, but much of it came from the coal   that people burnt in their houses to keep warm during the winter. By the 1950s, London's smog problem had become so bad that the government decided to do something to clean the air. A new law was made and nobody could burn coal in any British city. Within a few years, the air became much cleaner. There were no more " _ ". Many Chinese cities now face the same sort of problem with air pollution that London faced 40 or 50 years ago. However, this problem is more difficult for Chinese cities to solve. One reason is that more of the pollution comes from the factories, rather than from coal burnt in people's houses. If these factories were closed, this would harm the economy and lots of people would lose their jobs. Another reason is that changing from coal to cleaner fuel  , like gas, is quite expensive. However, the air in many Chinese cities is becoming cleaner and cleaner, as the government and people pay more and more attention to cutting down pollution. As a result, there are fewer "pea-soupers" in Beijing than there used to be. How did the air in London become much cleaner?
There was not so much fog in the winter later. A law was made to keep people from burning coal in their houses. Many factories in the city were closed. People paid more attention to cleaning their city.