Denham Harman, a scientific researcher who developed a prominent theory on aging that is used to study cancer, Alzheimer’s disease and other illnesses, died Nov. 25 at a hospital in Omaha. He was 98.

The death was announced by Tom O’Connor, a spokesman for the University of Nebraska Medical Center, where Dr. Harman worked as a researcher since 1958. The cause was not disclosed.

Dr. Harman developed the free-radical theory of aging in 1954, though it took years for additional research to prove its importance. The theory holds that one of the byproducts of oxygen use is adverse chemical reactions in cells, which results in aging and, ultimately, death.

The medical community initially scoffed at the theory proposed by Dr. Harman, but by the 1980s, free radicals had increasingly become part of research into cancer, cardiovascular disease and strokes. Free radicals have since been linked to Alzheimer’s disease.

Harvard Medical School professor David Sinclair said that Dr. Harman’s research inspired thousands of young scientists, including himself, to work on aging research.

Dr. Denham Harman, shown here in 2006, has died at 98. (Nati Harnik/Associated Press)

“Dr. Harman is one of the most influential scientists of the past 50 years, bringing world-class science to what was once a backwater of biology,” Sinclair said in an interview, adding that Dr. Harman’s theory “is a cornerstone of the aging field.”

Dr. Harman thought that with a healthy diet, regular exercise and certain vitamins, particularly vitamins C and E, aging could be slowed by reducing the production of free radicals. He also recommended limiting alcohol consumption and not smoking.

Richard Hodes, the director of the National Institute on Aging, called Dr. Harman a pioneer of aging research.

“The free-radical hypothesis has been a central element of the field ever since Dr. Harman’s landmark paper,” Hodes said. “Beyond his own work and continued exploration of the free-radical hypothesis, Dr. Harman’s contribution to science has helped lay the foundation for important, related areas of inquiry such as the mitochondrial and DNA-damage hypotheses.”

Denham Harman was born Feb. 14, 1916, in San Francisco. He graduated in 1940 from the University of California at Berkeley, where he also received a doctorate in chemistry in 1943.

He began his career as a research chemist for Shell Oil, where his work contributed to 35 patents, including the Shell No Pest Strip.

Dr. Harman received a medical degree from Stanford University in 1954 and joined the faculty of the Nebraska Medical Center in 1958.

In the 1970s, he played a major role in developing one of the country’s first academic centers for gerontology research at Nebraska.

He also helped found the American Aging Association trade group, which promotes scientific research on aging.

Although he officially retired in 1986, Dr. Harman continued to conduct research at the University of Nebraska Medical Center until 2010.

Survivors include his wife of 71 years, Helen Harman; four children; and four grandchildren.