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, it 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 Beijing, 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.

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