updated 4/24/2006 2:58:29 PM ET 2006-04-24T18:58:29

Scientists trying to predict the long-term health effects of the Chernobyl explosion look to Hiroshima and Nagasaki for clues, but drawing parallels is difficult.

Chernobyl released about 100 times more radiation than the U.S. atom bomb dropped over Hiroshima, according to radiation experts. But the death toll attributed to Chernobyl is far lower, around 50 according to the Chernobyl Forum, a group comprising several U.N. agencies; at least 140,000 died in Hiroshima.

A bomb versus slow exposure
That’s partly because atomic bombs deliver a huge burst of heat and energy that kills instantly, flattens buildings and sparks fires.

“What killed people at the moment of the explosion was the blow and the heat more than radioactivity,” said Didier Louvat, a radiation waste expert with the International Atomic Energy Agency.

The bomb leveled some 90 percent of Hiroshima. In contrast, the Ukrainian city of Pripyat, where Chernobyl’s workers lived, suffered no physical damage; residents slept through the explosion.

But radioactive material that had been accumulating in the reactor over its three years of operation streamed into the atmosphere for 10 days — a bombardment of radiation far more severe than in Japan.

Pripyat is still so radioactive that it remains deserted 20 years after the blast. Nagasaki and Hiroshima are large, bustling cities.

“An atomic bomb ... bursts into an instantaneous flash of gamma radiation and neutrons that disperses and is then gone,” said Dr. Burton Bennett, who has researched Chernobyl as part of the Chernobyl Forum and the atom bombings through the Japan-based Radiation Effects Research Foundation.

“Once the energy has been absorbed, there is very little residual radioactivity,” he said. Also, gamma rays do not cause ground materials to become radioactive, whereas Chernobyl’s fallout has been absorbed into the soil and plants of the contaminated regions.

Japan offered ominous indicators of future
But despite the differences, Japan’s experience offered some ominous indicators for the regions of the former Soviet Union most heavily hit by fallout.

Leukemia cases among atomic bomb survivors in 1950-1990 spiked well above the usual incidence rate of the disease and 51 percent of the cases were attributed to bomb radiation, according to the research foundation. The leukemia cases, however, largely occurred within the first 10 years.

That suggests that with the passage of 20 years since the Chernobyl disaster, the threat may have passed, Bennett said. Among Japanese survivors, only seven percent of other cancer deaths — stomach, lung, breast, colon, for instance — were found to be radiation-linked.

Bennett said those results have helped predict some of the health problems to expect from Chernobyl. But Chernobyl also presented its own problems. Since the doses were acquired over a longer period of time — not instantaneously as in Hiroshima — researchers had to take into account many other factors, such as diet and lifestyle, Bennett said.

The Chernobyl Forum predicts that among the 600,000 people who received higher radiation doses because of Chernobyl, an additional 4,000 radiation-related cancer deaths could be expected.

“Studies of the long-term effects in Japan have given us the best basis,” Bennett said.

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