OMAHA, Neb. — Dr. Denham Harman, a renowned scientist who developed a prominent theory on aging that’s now used to study cancer, Alzheimer’s disease and other illnesses, has died in Nebraska at age 98.

Harman, who worked into his mid-90s at the hospital in Omaha, died after a brief illness, medical center spokesman Tom O’Connor said.

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 utilization is adverse chemical reactions in cells, which results in aging and, ultimately, death.

The medical community initially scoffed at the theory proposed by Harman, who also contributed to nearly three dozen patents between earning his doctorate in chemistry at the University of California in Berkeley in 1943 and his medical degree from Stanford in 1954.

But by the 1980s, free radicals had 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 Harman’s research inspired thousands of young scientists, including himself, to work on aging research.

“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 Tuesday, adding that the late physician’s theory “is a cornerstone of the aging field.”

Harman believed that the aging process could be slowed by reducing the production of free radicals with a healthy diet, regular exercise and taking certain vitamins.

He also recommended not smoking and limiting drinking alcohol.

Although he officially retired in 1986, but he worked for more than another decade on his research at the University of Nebraska Medical Center – where he worked for 52 years – before again retiring in 2010.

The director of the National Institute on Aging, Dr. Richard Hodes, said Tuesday that Harman was 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.”

Harman developed his theory of free radicals while working as a research fellow at the University of California at Berkeley, where he had also earned his undergraduate degree in chemistry.

Harman contributed to 35 patents while working for Shell Oil’s research facility in northern California. He also helped create the American Aging Association trade group in 1970, to promote scientific research on aging.

Harman’s theory remains influential in the field even though later research has identified other contributors to aging, said Steven Austad, who leads the biology department at the University of Alabama-Birmingham and focuses on aging research.

“That one big idea he had basically stimulated 60 years of research,” Austad said.