Natural pollution has always been present on earth and is a real factor to be considered when discussing air pollution, although it causes only a small percentage of the overall pollution problem. Sources of natural pollution are the direct result of decaying organic matter and wind-born smoke as well as particulates from such natural events as plains and forest fires (ignited by heat or lightning), volcanic eruptions, sand and dust swept by the breeze; such pollution can spread over a large area of the countryside.
The phenomenon of natural pollution has been seen most dramtically in the example of volcanic eruptions, with their resulting plumes of smoke, steam and volcanic ash that can blot out the sun's rays as it spreads higher and wider into the atmosphere. Once high in the sky, upper air currents catch and carry the smoke and ash (fine material), while condensing the steam into water vapor. As the water vapor, smoke and ash travel on their wind-borne journeys, the smoke dissipates into the atmosphere while the ash and moisture settle back to earth in a trail up to hundreds of miles long. The result of this fallout is that human and animal health is affected, lives are lost, and millions of dollars in property is damaged or destroyed. And as evidence of his ultimate frailty, man can only stand by and watch it happen.