Klassen was born in Grand Rapids, Mich., and was graduated from the University of Michigan with a degree in civil engineering in 1925. In the same year he moved to Illinois and joined the state Department of Public Health. In 1935 he became the state's chief sanitary engineer, holding that post for 35 years until Gov. Richard Ogilvie selected him in 1970 to head the newly-formed Illinois EPA.

During his 20-month tenure, he laid the framework on which the Agency would grow; recruited the first staff, chiefly from the Department of Public Health which had previously housed many of the functions now delegated to the Illinois EPA; and selected the Churchill Road site that was the Agency's headquarters for more than a quarter century.

After leaving the Illinois EPA, Klassen became a self-employed environmental consultant.

Shortly after his 85th birthday he was appointed a consultant, and at 90 testified as an expert witness, in the lengthy litigation involving Love Canal in New York, the site that first focused widespread public attention on the emerging consequences of past unregulated environmental disregard.

For more than 20 years he served as a sanitary engineering consultant to the World Health Organization, and traveled the world establishing programs to protect both public health and the environment.

Klassen was a member of numerous state, national and international professional societies and organizations, including the International Executive Service Corp., American Planning Association, the Illinois EPA Policy Advisory Committee, and the Illinois Mapping Board. He served for some time as an assistant professor for the University of Illinois' College of Medicine in Chicago, and for 10 years was on the technical committee of the Indianapolis Speedway.

Survivors include his wife, Ida Wides Klassen; a son Jack of Houston, Tex.; a daughter, Jane Schafer of Springfield; six grandchildren and six great-grandchildren. Services were held in Springfield on April 11 and burial was in Camp Butler National Cemetery.

The Illinois EPA was about to turn 20 years old, and Clarence Klassen was about to turn 86, when he visited the Churchill Road headquarters and posed with the Agency's logo in 1990.