NASA Should Update Astronaut Radiation Exposure Limits, Improve Communication of Cancer Risks
To protect astronauts from cancer-causing radiation in space, NASA should proceed with proposals to set a universal career-long radiation dose limit of ~600 millisieverts (mSv), says a new report from the National Academies of Sciences, Engineering, and Medicine. This dose limit should apply to all astronauts, regardless of gender and age, which would help promote equal opportunity for mission assignment and for participation in longer spaceflights. The report also recommends that NASA improve its systems for communicating cancer risk to astronauts.
Space travel exposes the body to higher levels of ionizing radiation than those typically experienced on Earth. At high doses, exposure to ionizing radiation can lead to long-term health effects including cancer, heart problems, and cataracts. Now that NASA is preparing for the Artemis lunar missions, as well as longer missions to the moon and Mars, it is reconsidering the approaches behind the development of the space radiation health standard for the permissible levels of radiation exposure for astronauts.
"NASA should continuously strive to base its standards on the best available science as it embarks on this new phase of space travel and exploration," said Hedvig Hricak, chair of the department of radiology at Memorial Sloan Kettering Cancer Center, and chair of the committee that wrote the report. "As science on radiation-related cancer risks is constantly evolving, NASA has an important opportunity to revisit its space radiation health standard. We hope this report will guide NASA in protecting the health of astronauts throughout their careers."
The report recommends that NASA proceed with its proposed single standard dose limit for all astronauts, which is based on "risk of exposure-induced death" (REID) calculations for a 35-year-old female (who is considered most susceptible to radiation-induced cancer risk). Currently, men and women astronauts have different allowable doses of radiation, based on their reported relative susceptibilities to different radiation-induced cancers. This means that women astronauts currently cannot fly as many days in space because they would reach the radiation limit sooner.
Applying the same dose limits to all astronauts would create equality of opportunity, but the agency should also consider the trade-offs. A single standard for men and women would mean some astronauts, primarily women, would be subject to greater risk by the time they reach the exposure limit. The single standard may also limit others — mainly older male astronauts — if the standard is below otherwise acceptable doses.
The report also recommends that NASA conduct an independent analysis of the validity of the acceptable risk limit. NASA currently uses a 3 percent REID to set the limit. However, the occupational hazards that initially informed the selection of this metric are relatively outdated. Upcoming planned missions into deep space present an important, near-term opportunity for NASA to reassess whether 3 percent REID is the most appropriate metric to use in setting the dose-based space radiation health standard.
"The risks of space travel are borne by a small group of astronauts, but all of society benefits — from the technological advances, new scientific knowledge, and the national and international pride and collaboration," said R. Julian Preston, committee vice chair and special government employee in the Radiation Protection Division of the U.S. Environmental Protection Agency. "But 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."
Communicating to Astronauts About Radiation-Induced Cancer Risks
To convey radiation-induced cancer risks to astronauts, NASA uses a "traffic light" color-coded system. However, this color-coded system may be misleading and inaccurately represent radiation risk, the report concludes. It says empirical evidence is needed to determine if the traffic light system is an effective tool for communicating risk. If NASA continues to use the traffic light system, the committee proposed several modifications.
The report also recommends that NASA provide all astronauts with an individual radiation risk assessment based on their age and sex. NASA should communicate a comprehensive picture of an astronaut's own cancer risk, including an explanation of the uncertainty associated with the risk calculation. NASA should continue to discuss any changes in radiation risks as part of routine health briefings. Astronauts should also get access to an up-to-date resource on radiation risks that they can view outside of formal meetings with NASA's Office of the Chief Health and Medical Officer.
To improve the agency's radiation risk communication processes, the report recommends that NASA develop a risk communication research agenda to better understand what information astronauts want, how astronauts process information about risk, and who/what are the most effective sources of information.
Communicating NASA's Waiver Process
Astronauts who travel on long-duration spaceflight missions, such as missions to Mars, are likely to be exposed to radiation levels that significantly exceed the proposed new standard. Unless technological advancements and engineering controls provide improved protections to astronauts for missions into deeper space, NASA would need to seek waivers to the radiation health standard to pursue these missions. While NASA may need this flexibility to pursue waivers, there is also a need for an explicit and public framework for how the agency will consider both mission and individual waivers.
The study — undertaken by the Committee on Assessment of Strategies for Managing Cancer Risk Associated with Radiation Exposure During Crewed Space Missions — was sponsored by NASA.
The National Academies are private, nonprofit institutions that provide independent, objective analysis and advice to the nation to solve complex problems and inform public policy decisions related to science, technology, and medicine. They operate under an 1863 congressional charter to the National Academy of Sciences, signed by President Lincoln.