WASHINGTON — A National Academies committee has endorsed a NASA proposal to change the radiation exposure limits the agency sets for its astronauts but cautioned that the revised limit is still insufficient for human Mars missions.
The June 24 report by a committee established by the National Academies and sponsored by NASA backs the agency’s proposal to set a single lifetime radiation exposure limit for astronauts, rather than different limits based on age and gender.
Currently, lifetime exposure limits range from 180 millisieverts for a 30-year-old woman to 700 millisieverts for a 60-year-old man. Those limits are based on models intended to set a limit of no more than a 3% risk of radiation exposure-induced death (REID) at the 95% confidence level.
NASA proposed changing that to a limit of about 600 millisieverts, regardless of age or gender. That limit is based on the mean 3% risk of REID for a 35-year-old woman, the most conservative case but measured to a different standard than the earlier calculation.
The change, the committee noted, will allow more opportunities for female astronauts given the higher radiation limits. “Taken together, the proposed standard creates equality of opportunity for spaceflight with the trade-offs of somewhat higher allowable exposure to radiation for a subset of astronauts (primarily women) and limiting exposures below otherwise acceptable doses for others (primarily older men),” the committee’s report stated.
The report, though, also recommended reconsidering the 3% REID risk metric for making that calculation. “Though not directly comparable, risk of fatal occupational injury is more than an order of magnitude lower than a 3 percent REID for hazardous occupations,” the committee noted in the report, concluding that “an important, near term opportunity exists for NASA to conduct an independent analysis of the validity of 3 percent REID.”
“As science on radiation-related cancer risks is constantly evolving, NASA has an important opportunity to revisit its space radiation health standard,” Hedvig Hricak, chair of the department of radiology at Memorial Sloan Kettering Cancer Center and chair of the committee, said in a National Academies statement.
Another recommendation of the committee was for NASA to change how it communicates radiation risks. The agency currently uses a “traffic light” color-coded system of red, yellow and green, but the committee said that approach can be misleading. It also recommended NASA provide individual radiation risk assessments for astronauts.
While the revised levels will increase flight opportunities for many NASA astronauts, the levels are still more conservative than many other space agencies. Roscosmos, the European Space Agency and the Canadian Space Agency all set lifetime exposure limits of 1,000 millisieverts for their astronauts and cosmonauts, without any age or gender differences. The Japanese space agency JAXA does have age and gender differences, varying between 500 and 1,000 millisieverts.
Even those higher levels fall short of projected radiation exposures for round-trip Mars missions, which the report noted would exceed 1,000 millisieverts. Any astronauts who fly on a Mars mission would need a waiver to NASA’s radiation exposure limits, which raises ethical questions. “NASA should develop a protocol for waiver of the proposed space radiation standard that is judicious, transparent, and informed by ethics,” the committee recommended.
“As missions go deeper into space, we need to communicate why astronauts are being asked to take on that risk and offer explicit ethical justifications. This report offers a framework for accomplishing that,” said Julian Preston of the U.S. Environmental Protection Agency, vice-chair of the committee, in a statement.