Epidemiological Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...

studies of the health effects of low levels of ionizing radiation, in particular the incidence and mortality from various forms of

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

, have been carried out in different population groups exposed to such radiation. These have included survivors of the atomic bombings of Hiroshima and Nagasaki in 1945, workers at

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

s, and medical patients treated with

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

Life span studies of atomic bomb survivors

Survivors of the atomic bomb explosions at Hiroshima and Nagasaki, Japan have been the subjects of a Life Span Study (LSS), which has provided valuable epidemiological data. The LSS population went through several changes: * 1945 – There were some 93,000 individuals, either living in Hiroshima or Nagasaki, Japan. * 1950 – An additional 37,000 were registered by this time, for a total of 130,000 LSS members. However, some 44,000 individuals were censured or excluded from the LSS project, so there remained about 86,000 people who were followed through the study. There is a gap in knowledge of the earliest cancer that developed in the first few years after the war, which impacts the assessment of

leukemia Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ...

to an important extent and for solid cancers to a minor extent. Table 1 shows summary statistics of the number of persons and deaths for different dose groups. These comparisons show that the doses that were received by the LSS population overlap strongly with the doses that are of concern to NASA Exploration mission (i.e., 50 to 2,000 milliSieverts (mSv)). Figure 1 shows the dose response for the excess relative risk (ERR) for all solid cancers from Preston et al. Tables 2 and 3 show several summary parameters for tissue-specific cancer mortality risks for females and males, respectively, including estimates of ERR, excess

absolute risk Absolute risk (or AR) is the probability or chance of an event. It is usually used for the number of events (such as a disease) that occurred in a group, divided by the number of people in that group. Absolute risk is one of the most understandable ...

(EAR), and percentage

attributable risk In epidemiology, attributable risk or excess risk is a term synonymous to risk difference, that has also been used to denote attributable fraction among the exposed and attributable fraction for the population. See also * Population Impact Measu ...

s. Cancer incidence risks from low- LET radiation are about 60% higher than cancer mortality risks.

Other human studies

The BEIR VII Report contains an extensive review of data sets from human populations, including nuclear reactor workers and patients who were treated with radiation. The recent report from Cardis et al. describes a

meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting me ...

for reactor workers from several countries. A meta-analysis at specific cancer sites, including

breast The breast is one of two prominences located on the upper ventral region of a primate's torso. Both females and males develop breasts from the same embryological tissues. In females, it serves as the mammary gland, which produces and sec ...

, lung, and leukemia, has also been performed. These studies require adjustments for photon energy, dose-rate, and country of origin as well as adjustments made in single population studies. Table 4 shows the results that are derived from Preston et al. for a meta-analysis of breast cancer risks in eight populations, including the atomic-bomb survivors. The median ERR varies by slightly more than a factor of two, but

confidence level In frequentist statistics, a confidence interval (CI) is a range of estimates for an unknown parameter. A confidence interval is computed at a designated ''confidence level''; the 95% confidence level is most common, but other levels, such as 9 ...

s significantly overlap. Adjustments for photon energy or dose-rate and fractionation have not been made. These types of analysis lend confidence to risk assessments as well as showing the limitations of such data sets. Of special interest to NASA is the dependence on age at exposure of low-LET cancer risk projections. The BEIR VII report prefers models that show less than a 25% reduction in risk over the range from 35 to 55 years, while NCRP Report No. 132 shows about a two-fold reduction over this range.

References

{{Space medicine Epidemiology Occupational safety and health Atomic bombings of Hiroshima and Nagasaki Radiation health effects Space medicine