Even before there were people, there were cases of air pollution. There were dust storms. Volcanoes erupted, sending ash and poisonous gases into the atmosphere. When people appeared on the scene and began their conquest of nature, they also began to pollute the air. They cleared land, which made possible even larger dust storms. They built cities, and the soot  from their hearths filled the air. The Roman author Seneca wrote in A.D. 61 of the "stink, soot and heavy air "of the imperial city. In 1257, the Queen of England was forced to move away from the city of Nottingham because the heavy smoke was unbearable. The industrial revolution brought even worse air pollution. Coal was burned to power factories and to heat homes. Soot, smoke, and sulfur dioxide  fill the air. The good old days? Not in the factory towns. But there were large rural areas unaffected by air pollution. With increasing population, the entire world is becoming more urban. It is the huge cities that are most affected by air pollution. But rural areas are not unaffected. In the neighborhoods around smoky factories, there is evidence of increased rates of spontaneous abortion  and of poor wool quality in sheep, decreased egg production and a high death rate in chickens. Traffic police in Tokyo have to wear gas masks and take "oxygen breaks" --breathing occasionally from tanks of oxygen. Smog in Athena at times has forced factory closings and traffic restrictions. Acid rain in Canada is caused by air pollution in the United States, contributing to strained relationships between the two countries. Sydney, Rome, Tehran, Ankara, Mexico City, and most other major cities in the world have had frightening experiences of air pollution. One of the two major types of smog--consisting of smoke, fog, sulfur dioxide, sulfuring acid (H2SO4), ash and soot--is called London smog. Indeed, the word smog is thought to have originated in England in 1905 as a contraction of the words "smoke" and "fog." Probably the worst case of smog in history started in London on Thursday, 4 December, 1952. A large cold air mass moved into the valley of the Thames River. A temperature inversion placed a blanket of warm air over the cold air. With nightfall, a dense fog and below-freezing temperatures caused the people of London to put coal into their small stoves. Millions of these fires burned throughout the night, pouring sulfur dioxide and smoke into the air. The next day, Friday, the people continued to bum coal when the temperature remained below freezing. The factories added their smoke and chemical fumes to the atmosphere. Saturday was a day of darkness. For twenty miles around London, no light came through the smog. The air was cold and still. And the coal fires continued to bum throughout the weekend. On Monday, 8 December, more than one hundred people died of heart attacks while trying desperately to breathe. By the time a breeze cleared the air on Tuesday, 9 December, more than 4,000 deaths had been caused by the smog. Soot and ash can be removed by electrostatic precipitators . Unfortunately, they use large amounts of electricity, and the electrical energy, which is hardly affordable for most of us, has to come from somewhere. Fly ash removed from the air has to be put on the land or water, although it could be used in some way. The elimination  of sulfur dioxide is more difficult. Low-sulfur coal is rare and expensive. Although sulfur can be washed from finely pulverized  coal, the process is expensive. There are also processes for changing dirty coal into clean liquid and gaseous fuels. These processes may hold promise for the future, but they are too expensive to compete economically with other fuels at present. They also waste a part of the coal's energy. What is the passage mainly talking about?
The type and form of pollution. The cause and effect of pollution. The situation and elimination of pollution. The concept and examples of pollution.