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Air Pollution
   A great deal of energy is needed to run the factories of modern industrial nations. Automobiles, trains, planes, and buses need energy, too. Nearly all of this energy is produced in the same way—by burning fuels. The burning produces wastes. Some of the wastes get into the air, causing air pollution.
   Government officials in the United States estimate that 200, 000, 000 tons of these wastes enter the air each year—1 ton for each person in the country!
   A curtain of smog often hangs over big cities. It irritates the eyes and chests. The word "smog" is a combination of the words "smoke" and "fog", but "smog" itself is a mixture of many more ingredients. It begins with some of the pollution from burning: carbon monoxide, and oxides of nitrogen and sulfur are among them. Some of the pollutants react with one another to form new irritating substances. Energy is needed for the reactions, and it is supplied by the light of the sun. The resulting mixture is photochemical smog. ("Photo" means light.) It can be deadly.
   In London, Tokyo, New York, and other cities, a weather condition called a temperature inversion allows smog to hang over the city for several days at a time. Many people become ill, and the death rate among elderly people and people with lung disorders climbs rapidly.
   At least half of the pollutants in the air come from the engines of motor vehicles. As they burn fuel, they give off carbon monoxide as a waste. Carbon monoxide is a colorless, odorless gas, and a deadly poison. The amount of carbon monoxide that an engine gives off can be reduced by special devices designed to make the engine burn the fuel more efficienfiy.
   Automobile manufacturers are working on experimental cars run by electricity or other means that will reduce pollution. City governments in various parts of the world have begun to close certain streets to automobile traffic, hoping to lower pollution levels. Many city planners believe that cities, or at least their central areas should be kept free of automobiles.
   Motor vehicles are not the only air polluters. Coal and oil, used to heat homes and factories and to generate electricity, contain small amounts of sulfur. When the fuels are burned, sulfur dioxide, a poisonous gas, is produced. It is irritating to the lungs. Sonic cities have passed laws that allow coal and oil to be burned only if their sulfur content is low.
   Most electricity is generated by steam turbines. About half of the sulfur ,dioxide in the air comes from burning fuel to make steam. Nuclear power plants do not burn fuel, so there is no air pollution of the ordinary kind. But the radioactive materials in these plants could present a danger in an accident. Also, there is a problem in disposing of the radioactive wastes in a way that will not endanger the environment.
   Another type of pollution, called thermal (heat) pollution, is caused by both the fuel-burning and nuclear plants. Both need huge amounts of cold waters which is warmed as it cools the steam. When it is returned to the river, the warm water may stimulate the growth of weeds. It may also kill fish and their eggs, or interfere with their growth.
   Physicists are studying new ways of generating electricity that may be tess damaging to the environmeni. In the meantime, many power plants are being modernized to give off less polluting material. Also, engineers try to design and locate new power plants to do minimum damage to the environment.