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The findings came to light following a questionnaire conducted in February last year by a team of researchers including Megu Otaki, a professor emeritus of statistics at Hiroshima University, covering 142 former army cadets aged between 15 and 19 at the time of the atomic bombing.

The army cadets were gathered together outside Hiroshima on the day the bomb was dropped — Aug. 6, 1945 — before venturing into the city to assist with relief operations between noon and around 5 p.m. In the 2016 questionnaire, the former cadets were asked questions about operation content and locations, inhalation of dust particles, as well as their subsequent health conditions — eliciting responses from 64 of them in total.

In its decision on the effects of internal exposure from inhaling dust particles tainted with radioactive materials, the Japan-U.S. research organization Radiation Effects Research Foundation said that, “The amount in this case is low enough to be ignored.” This decision has been used by the Japanese government in recognizing A-bomb survivors as suffering from A-bomb related diseases.

However, Otaki states that, “It is very likely that the acute symptoms and the disorders that A-bomb victims later developed were mainly caused by internal exposure to radiation (from dust particles). The impact (of the dust particles) has been underestimated.”

The survey found that the frequency of acute symptoms such as hair loss and diarrhea was 11.7 times higher in the group (21 people) exposed to dust particles while operating within a 2-kilometer radius of the bomb’s hypocenter than those who weren’t exposed at locations 2 kilometers or more away (22 people, including some unknown). Similarly, the frequency of acute symptoms was also found to be 5.5 times higher among those who were exposed to dust particles more than 2 kilometers away from ground zero (9 people) than those who weren’t exposed. In addition, there were more cases of people developing cancer and leukemia among the groups exposed to the dust particles.

Commenting on these results, Otaki says, “Although the sample size is small, the conditions of the subjects such as age, health conditions, and the length of relief operation time are almost the same, meaning the data is very reliable.”

In addition, upon re-examining data released by the foundation in 2001 — which showed the relationship between estimated radiation dose and the frequency of chromosomal abnormalities in 3,042 atomic bomb victims — it has become clear that the radiation dose received by victims who were indoors is possibly 30 percent higher than initially thought. Based on this, the team of researchers has concluded that, “It is very likely that people developed chromosomal abnormalities after being exposed to radiation by inhaling dust particles upon going back into damaged buildings.”

With regard to residual radiation and internal exposure to radiation, the foundation has previously concluded that compared to the initial levels of radiation emitted at the time of the explosion, the residual radiation values are lower, making residual radiation “less of a threat to people’s health.” Based on this conclusion, the foundation devised a formula for calculating the estimated exposed dosage deriving only from the initial radiation, which the government has used to recognize “A-bomb related diseases.”

However, there has been a string of judicial rulings determining that the extent of internal exposure has been underestimated, based on examinations of symptoms and experiences of plaintiffs involved in “A-bomb related disease” certification lawsuits.

With this kind of reality in mind, Otaki says, “There are concerns that atomic bomb victims who should have been supported have actually been abandoned. We must reconsider the calculation method.”