|Fig. 1: Atomic cloud over Nagasaki, 9 August 1945. (Source: Wikimedia Commons)|
During World War 22 (1939-1945), an American bomber dropped the worlds first deployed atomic bomb over the Japanese city of Hiroshima. A visual of this massive explosion from above can be seen in Fig. 1. The bomb was known as "Little Boy", a uranium gun-type bomb that exploded with about thirteen kilotons of energy.  The bomb's explosion wiped out 90% of the city and immediately killed 80,000 of Hiroshima's citizens. Three days later, the United States dropped a second atomic bomb on Nagasaki, killing another 40,000 people. 
Of those who survive the heat and blast impact of the dropping of a nuclear bomb, many will experience radiation sickness in the time that follows the explosion. These consequences affect people located outside the perimeter of the immediate explosion, as those within the perimeter die from fatal burns and blast injuries. Additionally, radioactive fallout can be carried downwind, hence exposing a much larger population to radiation than those directly impacted by the blast. Those with potentially lethal radiation dose may not be aware of their exposure until days or weeks after the explosion. At this point, the damage to their blood system would be apparent from uncontrolled infections or bleeding from the gums. 
Radiation can also have health effects that happen on a longer time scale, such as cancer, by radiation-induced mutations in the DNA of living cells. Those who survive the immediate dangers of intense radiation exposure face an increased risk of developing certain cancers such as leukemia and thyroid cancer. With this, over time, many more lives are lost to nuclear war. 
Of the cancers found in Hiroshima survivors, chronic myelogenous leukemia (CML), was the most prominent. CML is a type of cancer that starts in certain blood-forming cells of the bone marrow. In CML, a change occurs in an immature version of myeloid cells which turns the cell into a CML cell. The leukemia cells grow and divide, building up in the bone marrow and spilling over into the blood. In time, the cells can also settle in other parts of the body, including the spleen. 
Studies have demonstrated an increase in incidence of CML in the victims of Hiroshima six years post the bombing.  This increase was observed for groups who were exposed to below and above 1 Gy of radiation. Researchers studied three separate cohorts of exposed individuals: individuals exposed to 0.5 Gy or more, 0.1-.49 Gy, and below 0.09 Gy.  The data revealed that those exposed to 0.5 Gy and 0.1-.49 Gy had similar increased rates of CML while those exposed to below 0.09 Gy of radiation had significantly lower rates of CML.  When researchers collected data on all types of Leukemia in both Hiroshima and Nagasaki survivors, CML was present in %17.6 of individuals exposed to 0 Gy, 36.5% of individuals exposed to 0.01-0.99 Gy, and 29.8% of individuals exposed to 1Gy or more.  This data suggests that a there exists a threshold for CML development somewhere between 0.5 Gy and 0.09 Gy of radiation exposure. While there is speculation around the direct effects of atomic radiation on stem cells, the direct reason for increased CML development in victims of Hiroshima has yet to be discovered.
© Maya Kuppermann. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Ham, Hiroshima Nagasaki (Thomas Dunne, 2014).
 "The Effects of Nuclear Weapons on Human Health," International Committee of the Red Cross, February 2013.
 "E. B. Douple et al., "Long-Term Radiation-Related Health Effects in a Unique Human Population: Lessons Learned from the Atomic Bomb Survivors of Hiroshima and Nagasaki," Disaster Med. Public Health Prep. 5, S122 (2011).
 M. Ichimaru et al., "Atomic Bomb and Leukemia," J. Rad. Res. 32, Suppl. 2, 14 (1991)