CAMBRIDGE, Massachusetts — It has been more than two and a half years since the Fukushima nuclear disaster began to unfold, and still the world watches events closely, fearfully. The drumbeat of danger seems never ending: Earlier this month, to take just one example, international news reports spread word that six workers at the plant had been accidentally doused with radioactive water.

Yet leading health scientists say the radiation from Fukushima has been relatively harmless, which is similar to results found after studying the health effects of Chernobyl. With all that evidence, why does our fear of all things nuclear persist? And what peril does that fear itself pose for society?

Our anxiety about nuclear radiation is rooted in our understandable fear of the terrible power of nuclear weapons. But in the 68 years since those weapons were first used in anger, we have learned, from the survivors of Hiroshima and Nagasaki themselves, that ionizing radiation — the type created by a nuclear reaction — is not nearly the powerful carcinogen or genetic mutagen that we thought it was.

Beginning shortly after World War II, epidemiologists and radiation biologists began tracking atomic bomb survivors. Researchers have followed roughly 112,600 Japanese: 86,611 who had been within 10 kilometers of the center of the explosions, and 26,000 who were not exposed.