Bonnie Landstrom, age 11, of New Bedford, Mass., for her question:
WHAT CAUSES AIR POLLUTION?
We have air pollution when clear, odorless air becomes hazy, smelly air. Air that is polluted harms health, kills plants and damages property. Most air pollution results from the burning or combustion process.
Man causes air pollution by pouring millions of tons of gases and particulates into the atmosphere each year. Particulates are tiny particles of solid or liquid matter.
One of the most common forms of air pollution is smog.
The burning of gasoline to power motor vehicles and the burning of coal to heat buildings and help manufacture products are examples of air pollution that comes as a result of combustion. Each time a fuel is burned in a combustion process, some type of pollutant is released into the air.
The pollutants can range from small amounts of colorless poison gas to clouds of thick, black smoke.
Weather conditions can help reduce the amount of pollutants in the air. Wind scatters pollutants and rain and snow wash them into the ground. But in many areas, pollutants are put into the air faster than weather conditions can dispose of them.
In crowded cities, thousands of automobiles, factories and furnaces can add tons of pollutants to a small area of the atmosphere each day.
Sometimes weather conditions cause pollutants to build up over an area instead of clearing them away. One such condition, called a thermal inversion, happens when a layer of warm air settles over a layer of cooler air that lies near the ground. The warm air holds down the cool air and prevents pollutants from rising and scattering.
Serious pollution problems result when thermal inversions occur over a city that is pouring tons of pollutants into the air.
Probably the most serious result of air pollution is its harmful effect on human health. Both gases and particulates burn people's eyes and irritate their lungs. Particulates can also settle in the lungs and worsen such respiratory diseases as asthma and bronchitis.
Air pollution can also harm plants. Poisonous gases in the air can restrict the growth of, and eventually kill, nearly all kinds of plants.
Most materials get dirty and wear out more quickly in polluted air than they do in clean air. Polluted air even harms such hard and strong materials as steel and concrete. In some cities, statues and other art objects that stood outdoors for hundreds of years have been moved indoors because today's air pollution threatened to destroy them.
Air pollutants may also affect the weather. Both gases and particulates can cause changes in the average temperatures of an area.