Research determines that “black rain” may have raised the risk of cancer for A-bomb survivors

by Tomomitsu Miyazaki, Senior Staff Writer

Research conducted by Hiroshima University’s Research Institute for Radiation Biology and Medicine (RIRBM), located Minami Ward, Hiroshima, has determined that “induced radioactivity” and “black rain,” which contained radioactive fallout, could have raised the risk of cancer for A-bomb survivors. In the case of the atomic bombing, “induced radioactivity” involves the transformation of dust and other debris blown into the air by the A-bomb blast into radioactive material. These new findings may make it necessary to fundamentally redefine the current medical risk assessment for survivors, which is based on the dose of radiation received in the immediate aftermath of the explosion.

A team of researchers, including Megu Ohtaki of RIRBM, looked at data involving over 37,000 A-bomb survivors who were directly exposed to the bombing and had lived until the year 1970, then analyzed this data using a new approach. Traditionally, the medical risks for A-bomb survivors have been calculated after estimating the level of their radiation exposure based on such information as the location where they experienced the bombing and their distance from the hypocenter.

In this research, however, the medical risks faced by the survivors are calculated based on the correlation between the location of exposure to the bombing and the risk of death from solid cancer. The result of this research indicates that the risk of death, using this new approach to assessment, is higher than it has been using the traditional approach. In addition, it was learned that people in locations to the west and northwest of the hypocenter, where the black rain fell in the aftermath of the bombing, suffered a slightly higher rate of risk from this factor alone. (For these calculations, deviations involving age or gender at the time of exposure and subsequent cancers were fixed.)

The research team believes that other factors, which have raised the survivors’ medical risks, were surely present apart from the radiation they were exposed to at the time of the explosion. The researchers suspect that the A-bomb survivors also suffered internal exposure to radiation by inhaling the “induced radioactivity” of dust particles and drinking contaminated water. The team has therefore concluded that the black rain has been another factor affecting the health of survivors.

Commentary: New research reveals that current method, disregarding internal exposure, is flawed

The new research conducted by RIRBM reveals that the current approach taken to assess medical risk in A-bomb survivors is flawed. At present, only neutron rays and gamma rays that were released at the point of explosion are used in the calculations to estimate risk.

Criticism over this method was voiced in the past, arguing that an accurate assessment could not be made if internal exposure from the radioactive black rain and induced radioactivity was disregarded. The reality, however, is that this contention was swept aside and the belief persisted that “internal exposure had little impact on the human body compared to direct exposure.”

The new research provides the first indication that factors involving internal exposure may raise the medical risks of A-bomb survivors. And as the results have been obtained from data on as many as 37,000 people, these can be considered significant findings.

However, there are further challenges to be addressed, such as clarifying the extent to which the risks were raised as well as the scientific mechanism that occurs in cases of internal exposure.

The research being undertaken on the effects to human health caused by internal exposure to radiation can lead to countermeasures to aid those who have been newly-exposed to radiation as a consequence of the accident at the Fukushima No. 1 (Daiichi) nuclear power plant. For the sake of these people, too, the research must be carried out as swiftly as possible.

The risk assessment for A-bomb survivors from Hiroshima and Nagasaki serves as the benchmark for the radiation protection standard set by the International Commission on Radiological Protection (ICRP). The A-bombed cities therefore have a critical role to play in helping to clarify the extent to which these medical risks were raised due to internal exposure. Toward this end, amassing research to assess the risks more accurately is a vital task.

(Originally published on May 3, 2012)