Air pollutants are a fact of modern life.  The best guess is that between 100 and 200 million tons of man-made air pollutants are released each year into the atmosphere in the United States.  According to U.S. EPA?s ?Latest Findings on National Air Quality: 2000 Status and Trends?, over 160 million tons of pollution are emitted into the air each year in the United States.  As society has become more industrialized, laws regulating air pollution have become more intense. Before 1955 only smoke laws existed.  The U.S. Clean Air Amendments of 1967 and 1970 established maximum limits, or standards, for several major air pollutants.  Two standards are applied to each pollutant: The primary standard, which protects human health, and the secondary standard, which protects human welfare–principally plants and human structures.

The clean air legislation also established federal and state environmental protection agencies (EPAs) and pollution control boards to monitor pollutant concentrations and enforce the air quality standards.  For example, the Illinois EPA operates monitors for ozone and sulfur dioxide, the two most common plant-damaging pollutants, in 23 of the state?s 102 counties.  As a result, aggregate emissions of six principal pollutants tracked nationally have been cut 29 percent since 1970.  Despite this progress, ground-level ozone levels in the southern and north central states have actually worsened in the past 10 years.