Perhaps the most intensely studied human population in history are the approxi­mately 90,000 survivors of the Japanese bombing who have been followed since the war in what is known as the Life Span Study. The United States National Academy of Sciences established the Atomic Bomb Casualty Commission in 1947 to study the health effects of the bomb survivors. This later morphed into the Radiation Effects Research Foundation (RERF),4 a joint US-Japan research effort to continue studies of the survivors in order to better understand the health effects of radia­tion. Of the nearly 200,000 survivors that were living in the cities of Hiroshima and Nagasaki at the time of the bombing, 120,321 were included in the Life Span Study. This number includes 93,741 who were within 10,000 meters of the hypocenter of the bomb and 26,580 people who were not in the city at the time of the bombing and constitute control populations (30). Individual dose estimates are available for 93% of the irradiated individuals. The incidence of cancer and all causes of death are followed rigorously in the people included in the Life Span Study. This database constitutes the primary basis for determining the risk of getting cancer from a cer­tain dose of radiation. Out of about 48,000 people who received doses of 5 mSv or more, 6,308 had cancer by 2003—the latest year of compilation—but only 525 of these could be attributed to radiation, judging from the spontaneous incidence of cancers from a similar size population who were not irradiated (32).

The United Nations Scientific Committee on the Effects of Radiation (UNSCEAR) and the US National Academy of Sciences Committee on the Biological Effects of Radiation (BEIR) periodically review the data from the Life Span Study, as well as fundamental research on carcinogenesis, and publish reports that form the scientific consensus on the risks of cancer from radiation. The latest BEIR VII report was published in 2006 (29). The risks are analyzed based on the specific site of a cancer, the age and sex of the individual, and the dose of radiation the individual received. The bottom line is that the risk of cancer is thought to be linearly related to dose, so that the larger the dose, the greater the risk.5 To put it into perspective, out of a group of 100 non-exposed people, 42 will normally get some type of cancer. If these same 100 people were exposed to a dose of 100 mSv (0.1 Sv), one additional person would get cancer (Figure 7.6).

That dose would be about 40 times the average natural background radiation that people are exposed to around the world.

There is an ongoing scientific discussion about whether a linear no-threshold (LNT) dose-risk model is the best model, or whether it overstates the risk. The sci­entific issues center on experimental results that show both an enhanced effect of radiation from what is known as the “bystander effect“ and a reduced effect from what is known as “hormesis" The bystander effect is the ability of cells that have been irradiated to cause effects on bystander cells that have not been irradiated, thus enhancing the action of a dose of radiation (33). Hormesis is an adaptation to a small dose of radiation or other toxic agents such that subsequent doses have less of an effect (34, 35). Both effects are supported by numerous scientific studies on cells, but it is still not clear what effects they have on radiation-induced cancer in humans. So the consensus is still to use an LNT dose risk model, and the latest results from the Life Span Study strongly support that hypothesis (32).

That is not quite the end of the story. The International Commission on Radiological Protection (ICRP) and the US National Council on Radiation Protection and Measurements (NCRP) are international and national organizations that recommend dose limits based on the scientific reports that are enforced by regulatory agencies. The scientific studies and these organizations recognize that the risks of radiation are based on the Japanese bomb survivors who were exposed to high doses given very rapidly, yet most people exposed to radiation from a nuclear accident will have relatively low exposures at a low dose rate. This reduces the actual risk of cancer by a factor of about two, which is known as the Dose and Dose Rate Effectiveness Factor (DDREF). Furthermore, the risk of cancer is lower for people exposed as adults, compared to being exposed as children. The ICRP concludes that the risk of death from cancer for doses below 200 mSv is 4% per Sv for adults and 5% per Sv for a population that includes children. These risks are doubled to 8% and 10% for doses higher than 200 mSv given at a high dose rate. So that is the bottom line. You can then predict the risk of dying from cancer from a given dose of radia­tion by simply multiplying this risk factor by the actual dose.