Following the end of World War II, an extensive study was performed on the survivors of the atomic bombs dropped on Hiroshima and Nagasaki. The study involved 91,230 people; 37,173 of whom were far enough from the explosion that they received virtually no radiation. This group served as the control group because their living environment was similar to those who were exposed.
This population has been carefully tracked and given periodic medical examinations. Some very interesting data have been released as a result of the study that give reassurance of the lack of danger from radiation doses below a certain level.
The exposed population as a whole has experienced a cancer mortality rate that is 6 percent higher than expected when compared to the control group. Many widely reported stories about the study do not provide any more details than this. There are more details about the study that are worth knowing.
There is some bad news. Children exposed to radiation while in their mother’s womb–particularly those who were exposed to more than a threshold level of 0.30-0.61 Gy during the period between 8 and 15 weeks after conception–have demonstrated a higher than average rate of mental retardation. Apparently radiation interfered with the rapid pace of brain cell division during that phase of development.
There is also good news. For the population whose doses were between 5 and 50 mSv–equivalent to about 1 to 15 years worth of background radiation–the actual number of cancers was 6 percent less than predicted. This group was large; it included over 28,000 people.
There has been no detectable increase in radiation-induced genetic mutations in the unexposed children or grandchildren of the A-bomb survivors. There is no increase in cancer rates for the children of bomb survivors who were not exposed to radiation themselves.
The survivors are now living longer than the average Japanese citizen, perhaps because of the subsidies available to survivors, or because they have paid closer attention to their health in the years since their exposure.
Information vs. MISinformation
One bit of information that is consistently used from the survivor data is the cancer rate of the people exposed to the very highest amount of radiation. The cancer risk for all radiation doses has been assumed to decrease in a linear fashion from the high exposure data and intersect a zero risk at a zero dose. The above data shows that this assumption is false. Despite strong evidence questioning its validity, the linear risk model is still the current basis for radiation regulation and accident calculations.
Using the data from the bomb survivors in the manner that it has been used has been compared to using the effects of alcohol on heavy drinkers to estimate the risk of cirrhosis for someone who has an occasional glass of wine. In both cases, there is ample evidence to show that there is not a linear relationship between the risk of large doses and that of small doses.