Summary¹

The National Aeronautics and Space Administration (NASA) works to realize the benefits of space exploration, and these benefits accrue to society through technological and scientific advances, as well as national and international pride and collaboration. Astronauts are in a unique class of employees, as they work for an agency whose mission is exploration. This implies both a high level of risk and uncertainty as astronauts explore space, as well as NASA’s duty to care for their safety during a mission and throughout an astronaut’s lifetime. NASA has long recognized that crewed space missions carry a range of unique hazards and challenges, including health-related risks. As NASA prepares for space exploration missions that extend to greater distances into our solar system and for longer durations, including missions to the Moon and Mars, these challenges are amplified.

Assessing, managing, and communicating radiation risks for space exploration is challenging because of incomplete knowledge of the complicated radiation environment in space, limited data on the cellular damage mechanisms resulting from radiation, absence of direct observations resulting from epidemiological studies, individual characteristics affecting susceptibility, and complex concepts associated with radiation risk ascertainment.

Beyond the protection of Earth’s magnetic field, astronauts are exposed to a complex radiation environment composed of galactic cosmic rays (GCR) and solar particle events (SPEs). When humans are exposed to ionizing radiation, in general terms, the risk of cancer increases with increasing dose of

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¹ This Summary does not include references. Citations for the discussion presented in the Summary appear in the subsequent report chapters.

radiation. Cancer risk projections have largely been based on the epidemiology data from the Life Span Study (LSS) of the Japanese atomic bomb survivors. There remain some uncertainties associated with the development of risk estimates from epidemiological studies, including the LSS data.

The NASA space radiation health standard sets a permissible limit for spaceflight radiation exposure, which functions to prevent in-flight risks that would jeopardize mission success and to limit chronic risks to acceptable levels based on legal, ethical or moral, and financial considerations. NASA notes that the standard is a quantifiable limit of exposure to a component of the environment during spaceflight over a given length of time, as in lifetime radiation exposure.

This report reviews and assesses NASA’s processes for long-term risk assessment and management for currently anticipated crewed missions with respect to radiation-induced cancer risk and specifically considers NASA’s proposed updates to the space radiation standard (see Box S-1).

To accomplish the task for this NASA-sponsored study, the National Academies of Sciences, Engineering, and Medicine (the National Academies) empaneled a committee of 18 members with expertise in the areas of radiation and cancer biology; biostatistics and mathematical modeling; risk communication, management, and uncertainty; medical genetics; clinical medicine; ethics; occupational health and safety; radiation dosimetry and physics; epidemiology; and two former astronauts with clinical medicine expertise (see Appendix B for the biographical sketches of the committee members and staff).

NASA provided additional context and clarity on what was in, and out, of scope for this study during public presentations and discussion with the committee (see Box S-2).

NASA’S CURRENT SPACE RADIATION HEALTH STANDARD

The NASA Space Permissible Exposure Limit for Spaceflight Radiation Exposure Standard 4.2.10 (“the standard”) informs crew mission assignments, crew health care (preflight, in-flight, and postflight), space vehicle design and layout, and mission operational profiles for human spaceflight missions. The standard currently states:

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² Based on the committee’s review of NASA’s document NASA/TP-2020-5008710, Section II.I, “95 percent confidence level” refers to the 97.5th percentile (also the upper limit of a 95 percent probability interval) of an uncertainty distribution of REID. This distribution is obtained by varying the input parameters of the NSCR NASA risk model according to “parameter uncertainty distributions” determined by NASA based on expert judgment.

NASA’s current cancer risk model, NASA Space Cancer Risk (NSCR) 2012, provides the output that NASA uses to set the 3 percent risk of exposure-induced death (REID) for cancer mortality within the standard. REID estimates the probability that an individual will die from cancer associated with the radiation exposure. For example, in this report, 3 percent REID implies that within a cohort of 100 astronauts, 3 are likely to die of radiation-induced cancer at some point in their lifetime. The current standard is intended to apply only to radiation exposure incurred during missions in low Earth orbit (LEO).

Why NASA Is Considering Revisions to the Current Radiation Exposure Standard

NASA has indicated that the primary reason for updating the space radiation exposure standard is because the current standard is for LEO missions exclusively. Now that the Artemis lunar mission, additional longer-duration lunar missions, and missions to Mars are in planning and development, NASA needs to define a radiation exposure standard that considers both missions in LEO and missions into deeper space. NASA is also aware that recent updates from epidemiological and radiobiological studies on sex differences in radiation-induced cancer risks may affect its cancer risk assessment model and what is an acceptable level of radiation exposure for astronauts. NASA is seeking advice on the longstanding concern that the current radiation standard results in an unequal work environment that limits female astronauts to shorter space careers because of scientific data indicating that females have an increased risk of cancer from exposure to ionizing radiation compared to men.

NASA’S PROPOSED SPACE RADIATION HEALTH STANDARD

NASA provided the committee with details about the possible changes to its space radiation exposure standard and the draft language of the proposed changes to section 4.2.10.1 of the standard (see Box S-3):

• NASA is proposing to move from a standard built on and conveyed as a risk limit to a standard that is still based on risk but conveyed as a dose-based limit.
• The proposed maximum allowable effective dose has been determined by applying the cancer risk model, NSCR 2012, to the most susceptible case—that of a 35-year-old female—to calculate her mean REID and risk of exposure-induced cancer (REIC). These acceptable mean REID values were converted to effective-dose values.
• NASA intends to use a mean 3 percent REID as the basis for the dose-based limit. Hence, for all astronauts, the maximum allowable space radiation exposure would be the effective-dose equivalent for a 35-year-old female astronaut whose mean REID is at 3 percent.
• The standard would delineate an effective-dose career limit of approximately 600 millisieverts (mSv)³ that applies equally to male and female astronauts, regardless of an astronaut’s age.

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³ NASA has indicated that the proposed limit of 600 mSv is an approximate value and will be verified prior to establishing a new standard. The final standard will be +/– 10 percent of the 600 mSv estimate.

Contemporary space exploration is an increasingly cooperative effort. Multiple companies are developing their own space vehicles and business plans, and international collaborations have propelled space exploration efforts. Compared to other international space agencies, both the current and proposed NASA standards are generally more restrictive with respect to career dose limits (see Table S-1).

Considering 3 Percent REID

While 3 percent REID has been used by NASA since the 1989 National Council on Radiation Protection and Measurements (NCRP) Report No. 98, the committee discussed that it may be time for NASA to reconsider the level of REID on which to base the standard. The initial occupation hazards that were used to decide on 3 percent have changed and are constantly evolving. Indeed, the 2000 NCRP Report No. 132 noted that the use of comparisons to fatalities in the “less safe” industries, such as mining and agriculture, in the 1989 NCRP Report No. 98 was no longer viable due to the large improvements made in ground-based occupational safety.

Three percent REID also exceeds the current level of risk in other high-hazard occupations in the United States and could be due for reconsideration by NASA and other external experts. 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. NASA is unique in its mission of space exploration and discovery. Another unique feature that sets the agency apart from traditional terrestrial employers subject to federal occupational safety regulations is that NASA is self-regulating and uses its own frameworks to set protective standards in order to minimize,

TABLE S-1 Radiation Exposure Career Limits Summary: International Space Station Partner Agencies

^a Proposed career dose limit. Could be exceeded with individual waiver.

manage, and effectively communicate risks of space travel to astronauts. Views also differ on the appropriateness of comparing NASA to terrestrial occupational standards given the different nature of work, the work environment, and relationship between employer and employee.

NASA’s limit of 3 percent REID was taken as a starting point for this committee’s work as it was not part of the study task to consider NASA’s underlying risk model or the use of any particular REID limit. However, the committee believes an important, near-term opportunity exists for NASA to conduct an independent analysis of the validity of 3 percent REID.

COMMITTEE’S ANALYSIS OF NASA’S PROPOSED SPACE RADIATION EXPOSURE HEALTH STANDARD

The committee’s analysis includes scientific and ethical considerations related to the components that make up the proposed revised standard as well as the implications of their relationship and combination as part of a new health standard.

Considering the Interconnected Components of the Proposed Standard

REID informs or serves as the basis for the three components of the proposed revised radiation standard. The three components are interconnected

but each raise ethics and policy issues separately and when combined into the proposed standard:

1. Commitment to a single standard for male and female astronauts;
2. Selection of the age and sex category on which to base the standard; and
3. Choices made in calculating dose threshold. That is, setting the permissible exposure standard based on the mean, median, 95 percent or 75 percent confidence level of REID.

Notably, a commitment to a single standard requires that standard to have a reference point and justification for that choice, so 1 and 2 are linked to each other more closely than to 3. All three components taken together determine the acceptable dose to adopt for the standard.

Commitment to a Single Standard for Male and Female Astronauts

In a 2014 report, the Institute of Medicine (IOM) recommended that NASA should implement an ethics framework and its concomitant responsibilities as part of the agency’s policies and procedures. The report included a recommendation to “provide equality of opportunity for participation in long duration and exploration spaceflights to the fullest extent possible.” For this 2020–2021 study committee’s consideration, NASA has proposed a revised radiation standard that is responsive to the 2014 committee’s recommendation by proposing a single radiation standard that applies to all astronauts independent of sex and age. Such a single standard would provide equality of opportunity, at least to the extent that it avoids radiation exposure standards that differ by sex and result in differential opportunities for participation in crewed spaceflights. Principles of compensatory justice and distributive justice are also served by a single standard.

The decision to apply a single dose-based limit to all astronauts, regardless of sex and age, also aligns NASA with the majority of its international space agency partners.

Selection of the Age and Sex Category on Which to Base the Standard

NASA is proposing that the universal dose-based standard be determined based on the mean REID using a 35-year-old female as the reference. NASA indicates this is the “most protective” approach because this age group is projected to be at the highest risk. Therefore, setting the standard based on the 35-year-old female would be the most protective for any given age and sex. Compared to the option of calculating the REID based on sex-averaged for non-sex organs or the average for lung and non-sex organs,

calculating the REID using a 35-year-old female is a better option because it is more straightforward and more protective based on current science. On one hand, this approach sets a single, clear, and consistent dose limit for all astronauts; but on the other hand it may result in a more restrictive limit than a more individualized approach would allow.

It is reasonable for NASA, in its role as a government agency asking astronauts to accept risk in the interest of society, to adopt an approach that provides the highest level of protection to those at greatest risk of radiation exposure-based harms, acting on the ethics principle of non-maleficence (preventing or removing harm to others).

Choices Made in Calculating Dose Threshold

NASA proposes to utilize the mean value for REID and resulting exposure threshold calculations. NASA’s decision to use the mean REID would be a change from its current standard, which is based on the 97.5th percentile of REID. Other options that NASA considered include using the median, 75 percent or 95 percent. Among the considerations that suggest the approach of evaluating the risk at the mean rather than out in the tails of the uncertainty distribution are that the mean, while still imperfect, is representative of expected exposures, more stable and consistent than a quantile, more understandable by a wider audience, and could provide a better basis for decision making.

As is well recognized by NASA, estimation of REID associated with exposure to space radiation involves multiple sources of uncertainty. The mean of the REID distribution generated from NASA ensemble modeling is estimated with lower uncertainty, compared with the currently used 97.5th percentile of this distribution.

Using the mean will warrant focused attention on communicating with astronauts about the uncertainties surrounding the exposure limit. Using the risk distribution (including description of the tails) and confidence levels in communicating with astronauts, policy makers, and the public is warranted.

The committee notes that NASA’s proposal to set the permissible dose based on the mean, while maintaining the 3 percent REID limit previously applied to the 97.5th percentile, results in a higher dose than the current standard. This higher probability of harm seems to conflict with an ethics commitment to protection from harm, minimization of risk, and NASA’s requirement to ensure astronaut safety by keeping exposures as low as reasonably achievable (ALARA). The committee recognizes that NASA is engaging in policy decisions and standard setting to protect crews to the greatest extent possible to limit mission risk as well as long-term risk to astronaut health and well-being as the agency considers long-duration

missions. Revised calculations for dose threshold within the limits imposed by 3 percent REID may be acceptable with appropriate justification.

Combined Implications of NASA’s Proposed Radiation Health Standard

In NASA’s proposed radiation health standard, career thresholds are driven by mean REID calculations for a 35-year-old female and would be applied to all astronauts, regardless of sex and age. The effective dose equivalent to 3 percent REID, for a 35-year-old female, is ~600 mSv (although NASA notes that values presented are approximate, +/– 10 percent, and will be verified prior to establishing a new standard). Compared with the existing standard, this proposed standard will increase the allowable exposure for a 35-year-old female by a factor of ~3 and for a 55-year-old male by a factor of ~1.5. Future modifications to this standard could be warranted if, for example, improved models suggest that 3 percent REID is associated with a different dose, if a different REID cutoff is justified as more appropriate, or if NASA determines that the 3 percent REID is inadequately protective.

Ethical Considerations

This committee notes that among the consequences of the proposed single standard are that (1) the revised standard creates equality of opportunity by applying the same dose limits to all astronauts without reference to age or sex; (2) some astronauts (primarily women) would be exposed to greater doses of radiation and therefore greater risk than would have been the case with current criteria-based standards adjusted for sex and age, creating a more risky work environment for some; and (3) a single standard with dose limits based on risk to 35-year-old females comes at the expense of potential greater allowable exposures for some older and male astronauts, which could be seen as an unfair restriction of opportunity for them. 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).

Such an approach can be defended on ethics grounds, but doing so requires weighting some ethics-related commitments more heavily than others in support of the revised standard—equality of opportunity over more individualized risk assessment, and equality of opportunity over commitments to limiting risk (at least for some astronauts). It will be important for NASA to offer explicit ethics justifications for the approach adopted and the resulting standard to be shared with astronauts and their families, as well as made publicly accessible.

The committee makes the following recommendations regarding NASA’s proposed space radiation health standard:

Recommendation 1: NASA should proceed with the proposed approaches to revising the space radiation health standard. As proposed by NASA, the agency should:

• Apply a single space radiation standard to all astronauts;
• Utilize the most protective approach in setting the space radiation standard;
• Set the standard as a dose limit; and
• Utilize the mean value of the risk distribution based on 3 percent risk of exposure-induced death.

In implementing this recommendation, NASA should make explicit the agency’s own ethical and policy analysis justifying the revisions to the proposed standard.

Recommendation 2: In the near future, NASA should re-examine whether to use risk of exposure-induced death (REID) or other metrics, or a combination of metrics, in setting the dose-based space radiation health standard. NASA should conduct an independent analysis of the validity of 3 percent REID and make explicit the agency’s justification for the metrics it chooses.

The committee notes astronauts on a Mars mission will be expected to exceed the career limit of a 600 mSv effective dose (see Figure S-1), which would require a waiver. The committee recognizes that to complete a crewed mission, especially long-duration missions to other planets, there are a multitude of risks that the astronauts and mission support staff have to address. The committee encourages NASA to continue using the principle of keeping ionizing radiation exposure ALARA to guide it in eliminating, minimizing, and mitigating risks, and to follow a transparent and ethics-based framework for deciding on the granting of waivers for a mission and for astronauts, as recommended in previous National Academies reports and endorsed by NASA.

The committee reached the following conclusion regarding NASA’s consideration of revisions to the current radiation exposure standard:

COMMUNICATING RADIATION-INDUCED CANCER RISKS

A key component of risk management is evidence-based, thorough, and effective communication of the risks. Achieving risk communication aims in the context of NASA’s space radiation standard requires an understanding of the risks and the standards themselves, and an understanding of how astronauts understand and interpret the risks, related standards, and both formal and informal communications about them.

NASA designed the system shown in Figure S-2 to communicate the proposed standard.

Identifying Risk Communication Recipients and Their Needs

The committee assumes that the primary audience for Figure S-2 is NASA astronauts. Several other communication recipients were proposed or discussed with NASA, including program managers, commercial spaceflight astronauts, international space agencies, policy makers, and other interested members of the public.

NASA astronauts have prior training and significant expertise that is likely to influence their risk communication needs, including training in probability and statistics, and on radiation and its biological effects. Even among astronauts, as the committee heard anecdotally, there may be variations in decision contexts, such as concerns about reproductive health, which could influence their decision needs. NASA noted anecdotally that the astronauts find REID confusing, and that dose is more familiar to astronauts than risk. Similarly, NASA suggested that the use of “traffic light” color bands to represent the three-stage risk profile would be familiar to astronauts, given the widespread use of color-coded risk matrices at NASA.

While NASA astronauts are the primary audience for communication of radiation exposure risks, other audiences will also be recipients of this communication. Audiences outside of NASA may not have the same technical background as NASA personnel and might interpret and react differently to figures, data, and information about radiation-induced cancer risks.

Considering NASA’s Risk Communication System for the Space Radiation Standard

The committee understood that Figure S-2 is intended to be NASA’s primary system for communicating the space permissible exposure limit (SPEL). To evaluate this figure and related materials, the committee adapted a framework for communicating epistemic uncertainty. This framework assesses the message’s source, content, format, and the anticipated effects of the communication.

Presentations and materials provided to the committee by NASA show that both formal and informal communications with astronauts about radiation risks come from multiple sources, including the Space Radiation Analysis Group at NASA, the Human System Risk Board, flight surgeons, and others. In addition to these communications, the committee notes that it could be valuable to allow astronauts to access information on cancer risks associated with space radiation on their own.

Tailored communications are generally found to be more effective than general communications and this would seem to be particularly the case in considering how best to communicate with individual astronauts.

Appropriately designed visual aids can improve both risk understanding and health-relevant decision making. Graphical presentations such as gradient bands, probability density functions, or cumulative distribution functions to represent the central tendency and associated uncertainty in estimated risk appear to help convey the actual estimated distribution of probability for the risk estimates, and thus may reduce misinterpretation.

Although matrices like the “traffic light” in Figure S-2 can be helpful in tracking and prioritizing risks, they pose communication challenges. The categorization of risk consequences is subjective, as it reflects a specific risk attitude, and therefore it is best practice to be transparent about how risk categorization decisions are made. Colors can be helpful in risk communication, because they represent relative magnitude of risk and inherently provide evaluative descriptions of each band as “low,” “medium,” and “high.” But, in the context of radiation, colors and evaluative descriptions may be misleading and inaccurately represent risk and any associated risk management decisions. Ideally, any proposed communication tool or message should be designed based on stated communication goals, the evidence from prior research regarding how best to achieve those goals with the targeted audience, and at least exploratory (i.e., formative) empirical evaluations of the message or tool with representatives from the targeted audience.

The committee reached the following conclusions:

Communicating an Individual Risk Assessment to an Astronaut

The committee makes the following recommendations regarding NASA’s strategies for communicating individual radiation-induced cancer risks.

Recommendation 3: To inform astronauts about their radiation risk, NASA should provide all astronauts with an individual radiation risk assessment and revise the risk communication system (i.e., the traffic light) for the updated space radiation standard to do the following:

• Assess and communicate the radiation risk at an individual level (as opposed to generic risk assessments) for all astronauts independent of the actual or projected radiation exposure and risk.
• Communicate the mean value of the risk estimate associated with an astronaut’s radiation exposure.
• Communicate the uncertainties for the risk distribution using both uncertainty intervals and limits, and visual representations of the risk distribution such as probability density curves, histograms, or heat maps.
• Address specific questions and concerns that individual astronauts may have regarding their overall health risks following communication of their actual or projected radiation dose, and help them place radiation risks into perspective compared to other mission risks and their baseline risk of developing cancer.

Recommendation 4: NASA should communicate a comprehensive picture of an individual astronaut’s cancer risks due to radiation exposure, beyond the information contained in the traffic light system. To do so, NASA should do the following:

• Respond to questions from astronauts regarding their total radiation exposure, and help astronauts put their radiation-induced cancer risk in context.
• Continue to discuss any changes in radiation risks as part of routine health briefings for the astronaut office, crews, and individual astronauts.
• Provide astronauts with an up-to-date resource on their radiation risks that they can access outside of formal meetings with NASA’s Office of the Chief Health and Medical Officer.
• Provide astronauts with easy access to summary information regarding what is known about the cancer risk factors that might interact with radiation exposures to influence long-term health outcomes for astronauts.

RISK COMMUNICATION AND NASA’S WAIVER PROCESS

NASA has acknowledged that with current technology, a mission to Mars would expose all astronauts to space radiation that exceeds

the SPEL, despite taking measures to keep radiation exposure as low as reasonably achievable (the ALARA principle). Therefore, it would be necessary for NASA to use a waiver process that evaluates any proposed mission and evaluates why individual astronauts would be asked to fly on such missions. As an employer, NASA has both the legal and ethical responsibility to assure the health, safety, and well-being of its employees. The fact that NASA serves as both the employer and the organization that establishes and assures compliance with its own health standards confers particular ethical responsibilities and opens NASA to a high level of public scrutiny.

Space exploration missions face challenges to risk mitigation that are not typically found in terrestrial high-hazard work. Terrestrial workers in high-risk jobs may choose to end their exposures by leaving their job. This is not the case for astronauts, particularly those on long-duration missions beyond LEO. There may be missions that NASA believes are so time sensitive and have sufficient importance and urgency that there is justification for exceeding the established standard for all astronauts. In this instance, unlike employers subject to Occupational Safety and Health Administration standards, NASA may seek to obtain permission for a mission waiver that would permit the agency to subject all volunteers for that mission to an unusual level of risk that would be unacceptable in less time-sensitive and critical missions. Waivers for specific missions and for individual participation in any given mission were considered in depth by the 2014 IOM Committee on Ethics Principles and Guidelines for Health Standards for Long Duration and Exploration Spaceflights.

That committee recommended that NASA follow a three-level, ethics-based decision framework when considering a waiver to an existing standard or standards. The threshold consideration would be to consider and explicitly make a determination as to whether “any missions that are unlikely to meet current health standards are ethically acceptable” and if so, what “specific conditions must be fulfilled” to approve the waiver. The committee expected NASA would make this general determination to establish and articulate criteria independent of any specific mission and that these criteria would be known both to the NASA Astronaut Corps and the general public. If NASA decides a specific contemplated mission meets these criteria, the agency would then be in a position to consider individual astronaut participation and crew composition. This consideration would include the skills and expertise needed for the mission, as well as astronauts’ individual health and risk considerations. Astronauts would be making these decisions alongside NASA at this stage.

The committee reached the following conclusion regarding NASA’s waiver process:

The committee makes the following recommendation regarding NASA’s waiver process:

Recommendation 5: NASA should develop a protocol for waiver of the proposed space radiation standard that is judicious, transparent, and informed by ethics. To avoid the perception that an exception to the standard is built into the space radiation standard itself, NASA should follow the ethics decision framework in developing a waiver protocol and it should provide supporting analysis and explanation justifying any waiver to the standard.

Risk Communication Research Agenda for NASA

Given the unique needs and characteristics of spaceflight programs and astronaut populations, NASA would benefit from engaging in risk communication evaluation and research to better understand and improve the effectiveness of their communication strategies.

The committee makes the following recommendation regarding a NASA risk communication research agenda:

Recommendation 6: NASA should conduct research to develop evidence-based risk communication and the agency should develop a radiation risk communication research agenda to fill knowledge gaps such as (1) what information astronauts want; (2) how astronauts process risk information; and (3) who/what are the most effective sources of information for astronauts. In addition, NASA should carry out research to examine and improve the effectiveness of its current and proposed risk communication strategies and materials.