The atomic bomb blasts at Hiroshima and Nagasaki exposed those on the ground to substantially lower radiation doses within about a mile of ground zero than scientists had thought for the last 40 years, according to U.S participants in a major new Japanese-American study.
The results of the government-financed study are expected to revise upward the estimates of health risks from radiation doses, particularly from gamma rays, partly because it is now apparent that the cancers and other medical problems in the survivors of those blasts were caused by less radiation than had been assumed.
Dr. W.K. Ellett of the National Research Council said it is obvious that radiation will produce a higher incidence of cancer, although he said the study could not be used to determine the effect of industrial radiation on workers.
"The number of cancer cases per rad [the measure of radiation exposure to humans] increases," Ellett said of the findings, "but it won't change the risk to civilian [nuclear industry employes] very much" because the survivors within two kilometers (about 1 1/4 mile) of the atomic bomb explosions were exposed to much higher levels than permitted in U.S. industry.
"It still won't settle the problem of low-level [health] effects," Ellett added.
Health studies of Japanese atomic bomb survivors have been under way since 1947, two years after the bombs were dropped. The major effort, which began several years later and used 100,000 survivors, provides the primary information to scientists worldwide on the long-term health effects of radiation on humans.
This latest phase of the study focuses primarily on the 20,000 Japanese survivors who received significant radiation doses because they were between one kilometer and two kilometers from ground zero in the two cities. Almost all individuals closer than one kilometer were killed instantly by the heat or the blast of the bombs.
The newest findings stem primarily from revised calculations that:
*Neutron radiation from the Hiroshima bomb was roughly one-tenth of levels that were estimated by the last study done 20 years ago.
*Overall radiation exposure of survivors inside houses was 30 percent to 50 percent lower than that set by the earlier study because it underestimated shielding effects of roofs, floors and neighboring buildings, according to sources.
*Gamma ray exposure to individuals out in the open some two kilometers or more from ground zero was found to be somewhat higher than earlier estimates, but at those distances the doses were still relatively low.
Neutron radiation and gamma radiation, or gamma rays, are two different forms of ionizing radiation that can affect the human body. Gamma rays travel farther from the blast, but neutrons are more damaging to humans.
Results of this newest phase of the bomb victims study, which has been under way almost five years, are to be announced next March at an international conference in Hiroshima.
Past phases of the study included examinations every two years of the 20,000 close-in survivors along with attempts to perform autopsies on every victim who died. A tumor registry was also established.
When the new dose estimates are approved, they will be applied to each of the 20,000 victims currently being followed. Thereafter, with data on exactly where each victim was at the time of the explosion, including the direction he or she was facing, each file will contain dose estimates for bone marrow, organs and even tissues throughout their bodies.
This latest phase of the survivor study began by chance when a weapons scientist at the Los Alamos National Laboratory did a computerized calculation of the Hiroshima bomb to fill out a table on yields of weapons. He discovered that the neutron output for the bomb was sharply lower than previously assumed.
Other researchers, including scientists at the Lawrence Livermore Laboratory, came up with similar results.
Estimates of the radiation effects of the Hiroshima bomb have always been rough calculations since it was the only weapon of that design ever exploded. Data on its yield originally were developed using such tenuous methods as radio readings from devices dropped at the time the bomb exploded and samples of materials picked from the ground weeks after the explosion.
A recent study put the Hiroshima yield at 15 kilotons (equal to 15,000 tons of TNT) although estimates at the time of the explosion were placed at 14 kilotons to 17 kilotons and later at 12.5 kilotons.
The Nagasaki estimate has been much narrower since three Nagasaki-type bombs were tested in the South Pacific after the war. Its currently listed yield is 22 kilotons, with earlier estimates ranging only from 19 to 22 kilotons.
Another major category of research, undertaken by the Japanese in this current phase, involves the use of newly sensitive methods called thermoluminescence to determine gamma ray levels caused by the bomb in samples of building materials taken from the two cities.
In the 20 years since the last analysis, this technique has become substantially more sophisticated.
At Los Alamos, an actual Hiroshima-type bomb was recreated so that better calculations could be made of how it operated. One major finding was that the several tons of iron and other nonnuclear material in the bomb "served as an effective shield" to prevent wide distribution of neutrons.
With the neutrons all but eliminated from dose calculations, scientists have had to take a closer look at the health effects of gamma rays. That is because in most overall dose calculations, the neutrons were considered more of a cancer-causing health risk.
"The risk estimates could increase up to a factor of two," one scientist said last week.
The new findings show that knowledge of radiation effects from nuclear weapons continues to be limited despite experimentation in the 40 years since the first two bombs were dropped.
The first major finding of the survivor study, which took place within five to seven years after the bombings, was the sharp increase in leukemia among the victims. In the past 30 years, however, the excess of expected leukemias has slowly declined to where it is now close to normal. Overall, 190 excess leukemias have to date occurred within the population of the two cities -- 50 percent above what would normally have appeared.
Another 336 cancers in excess of the expected level have also appeared so far -- 3 percent above normal.
Other problems, including birth defects, miscarriages and sterility, have affected the survivors.