Industrial pollution is primarily the unwanted byproduct of industrial processes, for example, the burning of coal, oil and natural gas, and the manufacturing of certain chemical products which give off smoke and fumes. Because the burning fuels contain much sulfur, the principal ingredient of smoke and fumes is sulfur dioxide (SO 2 ) and sulfurous particulate matter. This type of pollutant occurs most severely near its source and during still, damp and cool weather; thus it is experienced worst at night in urban areas. Even in its less severe form, industrial pollution is not confined to cities alone. Because of air movements, pollutants may easily migrate for miles over surrounding countryside, leaving in its path a barren, or at least unhealthy, environment for all living things.
Since the early 1970's, a growing number of federal, state and locally mandated rules and regulations have begun to enforce the reduction and control of industrial pollution. By carefully monitoring emissions, industries have greatly reduced the amount of pollutants emitted from their plants and factories and are striving to obtain an acceptable level as regulation intensifies and penalties for non-compliance stiffen. Because of the mandated industrial emissions clean-up, many land areas and streams in and around the cities that were essentially devoid of vegetation and life, have now begun to move back towards the intended balance of nature.