A proposed change to astronaut exposures limits is a giant leap backwards for radiation protection

Abstract

Addressing the uncertainties in assessing health risks from cosmic ray heavy ions is a major scientific challenge recognized by many previous reports by the National Academy of Sciences (NAS) and the National Council on Radiation Protection and Measurements (NCRP) advising the National Aeronautics and Space Administration (NASA). These reports suggested a series of steps to pursue the scientific basis for space radiation protection, including the implementation of age and sex dependent risk assessments and exposure limits appropriate for a small population of radiation workers, the evaluation of uncertainties in risk projections, and developing a vigorous research program in heavy ion radiobiology to reduce uncertainties and discover effective countermeasures. The assessment of uncertainties in assessing risk provides protection against changing assessments of risk, reveals limitations in information used in space mission operations, and provides the impetus to reduce uncertainties and discover the true level of risk and possible effectiveness of countermeasures through research. However, recommendations of a recent NAS report, in an effort to minimize differences in age and sex on flight opportunities, suggest a 600 mSv career effective dose limit based on a median estimate to reach 3% cancer fatality for 35-year old females. The NAS report does not call out examples where females would be excluded from space missions planned in the current decade using the current radiation limits at NASA. In addition, there are minimal considerations of the level of risk to be encountered at this exposure level with respect to the uncertainties of heavy ion radiobiology, and risks of cancer, as well as cognitive detriments and circulatory diseases. Furthermore, their recommendation to limit Sieverts and not risk in conjunction with a waiver process is essentially a recommendation to remove radiation limits for astronauts. We discuss issues with several of the NAS recommendations with the conclusion that the recommendations could have negative impacts on crew health and safety, and violate the three principles of radiation protection (to prevent clinically significant deterministic effects, limit stochastic effects, and practice ALARA), which would be a giant leap backwards for radiation protection.

Keywords: Galactic cosmic rays (GCR); Heavy ions; High let radiobiology; Mars exploration; Radiation protection; Relative risk models; Risk assessment; Space radiation.