The Million Person Study relevance to space exploration and Mars

Understanding the health consequences of exposure to radiation received gradually over time is critically needed. The National Aeronautics and Space Administration (NASA) bases its safety standards on the acute exposures received by Japanese atomic bomb survivors. Such a brief exposure differs appreciably from the chronic radiation received during a two to three year mission to Mars. NASA also applies an individual risk-based system for radiation protection that accounts for age, sex, smoking history, and individual life styles. Because the Japanese life span study (LSS) reports women to be at 2 to 3 times greater lifetime risk of developing cancer than men, female astronauts are allowed less time in space. Another concern is the potential behavioral and cognitive impairments from galactic cosmic radiation (GCR) impinging on the nervous system that might jeopardize the mission, and, possibly, lead to dementia later in life. GCR are high-velocity heavy ions traveling through space. There are no human circumstances/analogs similar to GCR that can provide direct information on the possible effects of such high-LET exposure to brain tissue. The MPS provides a more representative group (healthy men and women) for risk estimates than the 1945 Japanese population exposed briefly to the atomic bombs. The permissible career exposure limit set by NASA for each astronaut is a 3% risk of exposure-induced death (REID) from cancer at a 95% confidence level to account for uncertainties in risk projections. Because the MPS is 10 times larger than the LSS, the 95% confidence levels will be narrower and thus allow more time in space, all things being equal. Sex-specific differences in radiation risk can be examined more fully in the MPS with over 250,000 women compared with about 32,000 women in the LSS. Non-cancer outcomes such as neurological disorders also can be evaluated following low-dose rate exposures to high-LET alpha particles. Workers at several nuclear facilities had intakes of radionuclides, such as plutonium, that exposed brain tissue to alpha particles (Helium nuclei) for life. Such workers are being evaluated for mortality from dementia and other motor neuron diseases; can be evaluated for clinically diagnosed incidences of these conditions; and, though challenging, could be interviewed and ask to take cognition tests. Ischemic heart disease is also under study. The MPS, thus, provides another line of human inquiry to assist in decision-making and policy guidance for space missions beyond earth orbit.