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Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks (2021)

Chapter: 3 NASA's Spaceflight Radiation Exposure Standard

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Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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3

NASA’s Spaceflight Radiation Exposure Standard

Health standards have multiple purposes. They are an occupational exposure limit, setting a ceiling for exposure in the course of a working lifetime. They are an expression of the maximum level of risk that is acceptable to the employer and that should be clearly understood by the employee—in this case the astronaut. Health standards also frame and direct the engineering and administrative controls of exposure that are needed to mitigate risk to the achievable level. In the case of the National Aeronautics and Space Administration (NASA), the risk to an individual astronaut that is reflected in the standard includes both a concern for long-term astronaut safety and, in the setting of long-duration missions beyond low Earth orbit (LEO), the consequences of any harm to an astronaut within the mission that could adversely affect the mission’s outcome. As a rule, health standards are established based on the best available science and are revised as the scientific information evolves.

NASA’s Space Permissible Exposure Limit for Space Flight 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, as well as mission operational profiles for human spaceflight missions (Polk, 2021). The standard currently states:

Planned career exposure to ionizing radiation shall not exceed 3 percent risk of exposure-induced death (REID) for cancer mortality at a 95 per-

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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cent confidence level1 to limit the cumulative effective dose (in units of sievert) received by an astronaut throughout his or her career. (NASA, 2014, pp. 21–22)

As described in Chapter 2, NASA’s current cancer risk assessment model, NASA Space Cancer Risk (NSCR) 2012, estimates REID from cancer for a set of mission-specific conditions. NASA is proposing to revise the standard’s subsection 4.2.10.1, such that the space permissible exposure limit (SPEL) will remain based on risk (REID) but expressed as a dose-based limit. Under the proposed standard, the maximum allowable effective dose of ionizing radiation for an astronaut’s career would apply equally to male and female astronauts and be independent of an astronaut’s age. NASA is also proposing setting the dose thresholds based on mean REID and risk of exposure-induced cancer (REIC) calculations for a 35-year-old female.

The committee has considered the changes that NASA is proposing for its space radiation exposure standard, how the current and proposed standards differ, and the benefits and drawbacks of the proposed standard vis-à-vis its intended purposes. The committee’s conclusions and recommendation concerning these matters are found in this chapter. In addition, because NASA asked the committee to compare NASA’s processes for assessing uncertainty in radiation-induced cancer risk with terrestrial methods used for clinical applications, this chapter discusses briefly how standards are used to manage terrestrial radiation exposure, as well as standards used by other space agencies.

RADIATION EXPOSURE STANDARDS USED BY OTHER AGENCIES

Standards Used to Manage Terrestrial Radiation Exposure

The objective for managing terrestrial radiation exposure is grounded on reducing the potential for the radiation detriment related to stochastic effects. The system of radiological protection as recommended by the International Commission on Radiological Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP) includes implementation of the principles of justification, optimization of protection, dose constraints, and dose limits (ICRP, 2007; NCRP, 1993, 2018a,b). Specifically, occupational dose limits are recommended based on an implied calculation of the risk of stochastic effects (primarily the probability of

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1 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.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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cancer and heritable effects) as a function of dose. In terrestrial radiation exposures, such doses can be reasonably well estimated or evaluated by direct measurements for individuals or groups.

Recommended dose limits for occupational planned exposure situations are based on the concept of effective dose and a linear-non-threshold model for dose response (ICRP, 2004, 2007; NCRP, 1993, 2018a,b). For occupational exposure, a judgment was made by ICRP and NCRP to control the lifetime effective dose to be on the order of 1 Sv, with a corresponding nominal radiation detriment-adjusted risk coefficient for cancer and heritable effects on the order of 5 percent Sv–1 (ICRP, 2007; NCRP, 2018b). ICRP sets the occupational effective-dose limit at 20 mSv per year, averaged over defined periods of 5 years (ICRP, 2007). NCRP recently updated its recommendations and has developed numeric protection criteria for managing the dose to an individual that are similar—but not identical—to those made previously by NCRP (1993) and ICRP (2007). NCRP now recommends that the annual effective dose to an individual from occupational exposure should not exceed 50 mSv and that the cumulative lifetime effective dose for an individual from occupational exposure should not exceed 10 mSv multiplied by the individual’s current age in years (NCRP, 2018b). NCRP further emphasizes that “optimization of protection together with the recommendations related to annual and lifetime management of effective dose provide flexibility and are expected to maintain the individual lifetime effective dose well below 1 Sv” (NCRP, 2018b, p. 49).

As currently implemented, NASA radiation limits differ substantially from radiation limits used for radiation workers on Earth in that they are specific risk limits (NCRP, 2014). NASA’s proposal to revise its space radiation standard in terms of effective dose based on a mean of less than 3 percent REID and applied universally for sex and age would be more consistent with the approaches of current standards used to manage terrestrial radiation exposure. However, the unique and complex nature of radiation exposures in space (see Chapter 2) introduces significant uncertainties in such a risk-to-dose transfer, which would require NASA’s continued review of the evolving scientific knowledge about the relationship between risks and dose and consideration of future modifications of the standard when appropriate.

Standards Used by Other Space Agencies

When considering deterministic effects, all space agencies set similar limits for acute radiation exposures, such as from solar particle events (SPEs). These limits are based on threshold doses and tissue tolerances as published in ICRP Publication 41 and NCRP Report 142 (ICRP, 1984; NCRP, 2002). However, for stochastic health risks such as development of

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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cancer, there is no harmonization across space agencies in setting radiation standards. ICRP Task Group 115 is currently working on risk and dose assessment for radiological protection of astronauts and cosmonauts and aims to establish a framework that could be applied uniformly by all of the space agencies during international crewed missions (Durante, 2021).

Like NASA, other space agencies set career limits for astronauts based on a level of acceptable cancer risk due to space radiation. The agencies use different approaches for establishing these career limits, and the limits themselves may differ (Durante, 2021; McKenna-Lawlor et al., 2014). The European, Canadian, and Russian space agencies use a dose-based standard and limit career exposures to an effective dose of 1 Sv (or 1,000 mSv) independent of age and sex (see Table 3-1). This dose limit corresponds to a 5 percent risk of cancer mortality (ICRP, 1991, 2007). The Japanese Space Agency (JAXA), similar to NASA’s current standard, is risk based and sets the career limit for astronauts at 3 percent lifetime-attributed cancer mortality and is sex and age dependent.

The committee did not conduct a comprehensive review of the models used to derive the different radiation health standards, but it recognizes that different space agencies may develop and adopt their own cancer risk model. For example, the Russian Space Agency’s (RSA’s) cancer risk

TABLE 3-1 Radiation Exposure Career Limits Summary: International Space Station Partner Agencies

Space Agency Career Limit Sex/Age Dependency
Canadian Space Agency 1,000 mSv No sex or age dependency
European Space Agency 1,000 mSv No sex or age dependency
Russian Space Agency 1,000 mSv No sex or age dependency
Japanese Aerospace Exploration Agency 3 percent REID @ the mean Yes
Lower limit: 500 mSv for 27- to 30-year-old female Upper limit: 1,000 mSv for > 46-year-old male
National Aeronautics and Space Administration (current) 3 percent REID @ the 95 percent confidence level Yes
Lower limit: ~180 mSv for 30-year-old female Upper limit: ~700 mSv for 60-year-old male
National Aeronautics and Space Administration (proposed update) 600 mSva No sex or age dependency

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

SOURCE: Adapted from Semones, 2021.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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projection model replaces ICRP’s effective dose concept with one called generalized dose, which, in addition to the dose and the radiation quality factor, incorporates a temporal factor that converts the effects of persistent radiation to a single acute exposure; a spatial factor that is analogous to tissue weighting factors; and a modification factor that accounts for the contributions that the space environment (e.g., microgravity) has on the equivalent dose. Another distinction of the RSA model is that it calculates total radiation risk as the sum of the radiation risk of cancer plus the radiation risk of other detrimental health effects (Shafirkin et al., 2002). In addition to the risk-based dose limit, RSA calculates years of life lost (YLL) because of radiation.

The European Space Agency (ESA) is developing a radiation-attributed decrease of survival (RADS) risk model that calculates the cumulative decrease in survival at attained age owing to previous radiation exposure (Walsh et al., 2019). While the RADS model is similar to the risk projection models used by JAXA and NASA, unlike those two, the RADS model uses all solid cancers (rather than organ-specific cancers) along with major organ models, such as lung and breast. The RADS model also uses different relative biological effectiveness (RBE) and dose and dose-rate effectiveness factor (DDREF) values, and it uses cancer incidence rather than mortality risk to account for the improved ability to cure people who have developed cancer (Ulanowski et al., 2019).

Space agencies recognize that a mission to Mars will result in most astronauts exceeding the agencies’ career limits for radiation exposure. To the committee’s knowledge, only NASA has a process for granting a waiver to an astronaut that would allow him or her to fly on a mission that exceeds the career limit. The other agencies acknowledge that a waiver process may be needed as they plan for long-duration missions.2

NASA’S PROPOSED SPACE RADIATION EXPOSURE HEALTH STANDARD

NASA provided the committee with details about the proposed changes to its space radiation exposure standard including draft language for section 4.2.10.1 of the standard (see Box 3-1). To summarize:

  • 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 for the most susceptible population but conveyed as a dose-based limit.

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2 Committee discussion with ICRP presenters. Open session of the Committee on Assessment of Strategies for Managing Cancer Risks Associated with Radiation Exposure During Crewed Space Missions webinar, April 14.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×
  • 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 mean REID and REIC. These mean values will be converted to effective-dose values.
  • NASA proposes a 3 percent mean 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 ~600 mSv3 that applies equally to male and female astronauts regardless of an astronaut’s age.

Before moving to a discussion of the committee’s analysis of the proposed changes to the space radiation standard, it is important to begin with an overview of the basis for NASA’s current space radiation standard.

THE BASIS FOR NASA’S CURRENT SPACE RADIATION EXPOSURE STANDARD

In 1970, the National Academies’ Space Studies Board made recommendations to NASA for guidelines for career doses for long-term

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3 NASA has indicated that the proposed limit of 600 mSv is an approximate value. The final standard will be +/– 10 percent of the 600 mSv estimate.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

mission design and manned operations (NRC, 1970). At that time, NASA employed only male astronauts and the typical age of astronauts was 30–40 years. A “primary reference risk” was proposed by the 1970 National Academies committee equal to the natural probability of cancer over a period of 20 years following the radiation exposure (using the period from 35 to 55 years of age) and was essentially a doubling dose. The estimated doubling dose of 382 roentgen equivalent man (rem) (3.82 Sv), which did not include a dose-rate reduction factor, was rounded to 400 rem (4 Sv). The 1970 National Academies recommendations were implemented by NASA as dose limits and used operationally for all missions until 1989 (Semones, 2021).

REID is a calculation that has been at the core of NASA’s risk management process for decades. REID estimates the probability that an individual will die from cancer associated with the radiation exposure (UNSCEAR, 2000). For example, in this report, 3 percent REID implies that within a cohort of 100 astronauts, 3 of them are likely to die of radiation-induced cancer at some point in their lifetime.

Following the recommendation of NCRP Report 98 (1989), which provided guidance to NASA concerning radiation protection in LEO, the NASA space radiation standard has been set at 3 percent REID for both sexes and all ages since 1995. The NCRP recommendation was based on an assessment of risks of fatal cancer of highly exposed terrestrial radiation workers and of lifetime risks of fatal accidents among workers in other occupations that were described as “less safe” and “most hazardous.” Comparison of space radiation risks with the “most hazardous” terrestrial occupations was found not to be reasonable because astronauts are exposed to many risks other than radiation. At the time of the NCRP analysis, occupational dose to radiation workers including those who work in fuel cycle facilities and nuclear power plants, industrial radiographers, and medical professionals was limited to 50 mSv per year and could reach 2.5 Sv throughout their career, assuming a 50-year career in the industry. This corresponded to a 5 percent risk of excess cancer mortality.

For “less safe” industries (e.g., agriculture and construction), the lifetime risks of fatal accidents at the time ranged from 2 to 5 percent. NCRP noted that “comparison of the radiation risks with the middle group of ‘less safe’ [within the range of safe, less safe, and most hazardous] occupations with lifetime risks of about three percent seems the most reasonable” (NCRP, 1989, p. 162). The appropriateness of using 3 percent risk of fatal cancer as the basis for the NASA space radiation standard has been reviewed and endorsed in subsequent NCRP reports (NCRP, 2000, 2014); NCRP Commentary 23 specifically recommended that “planned career exposure to ionizing radiation shall not exceed 3 percent REID for cancer mortality at a 95 percent confidence level” (NCRP, 2014).

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

As described earlier, setting the REID at 3 percent for both sexes and all ages has resulted in different permissible radiation dose limits for male and female astronauts under the current space radiation health standard. In the most recent review focused on age and sex differences in the standard, NCRP Report No. 132 (2000), stated that it

continues to recommend gender and age differences in dose limits … because the overall risks per unit dose for women appear higher than for men due to the greater probability of women developing some radiation induced cancers, such as stomach, thyroid and breast, the longer average lifespan of women, and the decrease in risk with age for both sexes.

In light of newer scientific data (see Table A-1 for a summary of the current evidence on sex-specific radiation risks) that show large sex-differences in lung cancer risk following exposure to radiation among the atomic bomb survivors from Japan, NASA requested in 2019 that NCRP (SC 1-27) examine whether similar sex differences in radiation-induced lung cancer exist among other populations exposed to chronic or fractionated radiation.4

The NASA Space Cancer Risk (NSCR) Model is used to calculate REID using available epidemiological data, physics-based transport, radiation quality and dose rate, U.S. site-specific cancer rates, and other information (see section on the NSCR model in Chapter 2 for more information about the components and uncertainties considered in the model). NASA employed conservative uncertainty criteria (97.5th percentile) on cancer mortality, in part to account for unknown non-cancer risks, such as cardiovascular risks that were not considered in the model. These risks are currently considered separately in other standards.

In practice, REID values approach 1 percent for many astronauts that have flown on the International Space Station or the Russian space station Mir (Cucinotta et al., 2008; see Figure 3-1).5 As currently calculated, the career exposure limit for a 55-year-old male astronaut is 400 mSv, and for a 35-year-old female astronaut it is 120 mSv over the course of her career.

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4 This NCRP report is in progress and the committee has not had access to the results. For more information, see https://ncrponline.org/program-areas/sc-1-27-evaluation-of-sex-specific-differences-in-lung-cancer-radiation-risks-and-recommendations-for-use-in-transfer-and-projection-models (accessed April 28, 2021).

5 Though the career exposure limit is 3 percent REID, NASA currently uses the “administrative limit” of 1 percent REID to meet the 3 percent standard at a 95 percent confidence interval (Semones, 2021).

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×
Image
FIGURE 3-1 Summary of mission personnel dosimetry for astronauts on all past NASA space missions through 2007, including Mercury, Gemini, Apollo, Skylab, Apollo-Soyuz, Space Shuttle, NASA-Mir, and the International Space Station (ISS), plus estimate of effective dose for an astronaut on a Mars mission, with radiation worker annual limit and average U.S. annual dose noted for context. Summary of results for doses comes from thermoluminescent dosimeters worn by astronauts, biodosimetry, and estimates of effective doses for all NASA missions to date and includes data from astronauts who have flown more than once in space, with the maximum being seven times by two astronauts.
SOURCES: Adapted from Cucinotta et al., 2008; Huff et al., 2016; and Semones, 2021. © 2021 Radiation Research Society.

Considering 3 Percent REID

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.

While 3 percent REID has been used by NASA since the 1989 NCRP report, 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, NCRP Report No. 132 (NCRP, 2000) noted that the use of comparisons to fatalities in the “less safe” industries, such as mining and agriculture, in the 1989 NCRP report was no longer viable due to the large improvements made in ground-based occupational safety.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

The 3 percent REID also exceeds the current level of risk in other high-hazard occupations in the United States such as mining or construction 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 for hazardous occupations than a 3 percent REID (BLS, 2019). As discussed earlier in this report, NASA is unique in its mission of space exploration and discovery. Another unique feature setting 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, 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.

In summary, the committee believes an important, near-term opportunity exists for NASA to conduct an independent analysis of the validity of a 3 percent REID.

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

NASA requested that this committee review and assess NASA’s proposed process and strategies for managing the risk of cancer due to exposure to space radiation (i.e., NASA’s proposed changes to the space radiation health standard). As described in Chapter 1, NASA requested that this committee consider the components of the standard proposed to be changed using a dose-based standard, applying the same dose-based standard to male and female astronauts, and basing the standard on the 35-year-old female reference base. The committee was not asked to create a new standard nor evaluate NASA’s stochastic cancer model underlying the space radiation health standard.

The committee’s main analysis appears in this section. It 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 components are interconnected but each raises ethics and policy issues separately and when combined into the proposed standard.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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  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. For example, applying a single standard to male and female astronauts does not directly result in an increased allowable exposure. It is the decisions about selection of the age and sex on which to base the standard and choices made in using the 3 percent REID to determine the level of acceptable risk that determine whether dose exposure limits would be increased or decreased compared to the current standard. The combination of choosing to calculate the exposure threshold based on 35-year-old females or other age/sex category, and using the mean, median, 95 percent, or 75 percent confidence level will in combination have the effect of changing the acceptable dose limit when calculated based on 3 percent REID. While each component of the standard needs to be individually justified, it is not possible to reach complete conclusion about the reasonableness or justifiability of the standard overall based exclusively on consideration of only the individual components. Thus, caution is warranted when basing policy decisions on each component in isolation from the resulting combination.

Commitment to a Single Standard for Male and Female Astronauts

In a 2014 report, the Institute of Medicine 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.

From an ethics perspective, by instituting a single standard, NASA would be changing policy in ways that reflect considerations of the principle

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

of justice and its application. First, and as noted above, a single standard would create equality of opportunity for the members of the NASA Astronaut Corps, reflecting justice as fairness. Second, with its consequence of creating greater inclusion of female astronauts, a single standard would respect compensatory justice by rectifying the past limits on women’s participation and underrepresentation in spaceflight. Third, by moving toward more balanced inclusion of males and females in spaceflight, a single standard would respect distributive justice through more equitable distribution of the risks of spaceflight, which were disproportionately borne by male astronauts as well as creating equitable distribution of the benefits of participating in crewed missions (IOM, 2014).

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. It is also the case that terrestrial occupational health radiation dose-limiting standards apply irrespective of gender. NASA moving to a single standard is consistent with standard occupational health practice, although there are notable differences between NASA’s other proposed changes to the radiation health standard and occupational health practice. For example, the standard for occupational dose limits for terrestrial workers includes both an annual dose limit and a lifetime dose limit. Historically, NASA’s dose limits for short-term missions within LEO are several times higher than annual occupational dose limits for terrestrial workers because NASA’s limits are intended to prevent acute risks that might disrupt missions, while annual dose limits for terrestrial workers are intended to control the accumulation of career doses (Cucinotta, 2010).

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

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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radiation exposure–based harms, acting on the ethics principle of non-maleficence (preventing or removing harm to others). It is also the case that the upcoming NCRP SC 1-276 report will provide more information on the differences in lung cancer between males and females based on the latest epidemiological data.

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, easier to understand 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 (Simonsen and Slaba, 2020).

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 provides this rationale in more detail in Chapters 2 and 4.

The committee notes that NASA’s proposal to set the permissible dose based on the mean allows for greater dose than the current standard while continuing to use 3 percent REID. This use of the mean results in acceptance of a higher level of risk under the revised standard, compared to use of the 97.5th percentile. 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. 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

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6 At the time of the publication of this report, the NCRP Committee SC 1-27 on Evaluation of Sex-Specific Differences in Lung Cancer Radiation Risks and Recommendations for Use in Transfer and Protection Models was working on a report with recommendations for NASA.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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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, or if a different REID cutoff is justified as more appropriate, or if NASA determines that the 3 percent REID is inadequately protective. See Table 3-2 for examples of the effect of modifying certain variables of the ensemble model on the dose limit output.

To reiterate, the three components, or decisions, embedded in NASA’s proposed updates to the radiation health standard do not exist in isolation—each component, or decision, interacts with the others such that changing one component results in changes to the overall standard. For example, if NASA had utilized a 35-year-old female as the basis for the new standard, but derived the dose from 3 percent REID by the current methodology of 97.5 percent confidence level, there would be no change to the allowable exposure for a 35-year-old female, but the allowable dose for a 55-year-old male would decrease. Alternatively, if NASA adopted a 1 percent mean REID, the allowable dose would be reduced for all astronauts.

Ethical Considerations

As described in Chapter 1, the committee reviewed documents from NASA presenting the proposed updates to the standard, discussed the

TABLE 3-2 Effect of Modifying Variables of the Ensemble Model on the Dose Limit Output

Modification Effect on Dose Limit
3 percent REID → 1 percent REID Decrease
35-year-old female → sex-averaged Increase
97.5 percent confidence level → mean Increase
Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

proposed updates with NASA leaders in public sessions, and received updated versions of the proposed standard following each of the two public sessions. The committee also considered the scientific literature and reports from NASA or other expert panels on the issue of space radiation. NASA did not provide the committee with a formal ethics analysis for each component of the proposed standard or for the overall standard but did, in public session, discuss the importance of the principles of fairness and autonomy. Furthermore, ethical analysis of the proposed standard does not appear in the Statement of Task for this study but consideration of bioethics issues was requested as part of a presentation by NASA officials during a public session meeting.

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.

Summary

A single radiation standard for male and female astronauts requires a single dose threshold to be applied. If a single standard is required by NASA, three options exist: (1) use the most protective threshold (e.g., based on risk to 35-year-old females), (2) use the least protective threshold, or (3) choose some value between the most and least protective standard. Under this logic, the most defensible approach is what NASA has proposed (i.e., to use, within the context of the 3 percent REID that this committee

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

has considered as a fixed starting point, the most protective threshold in setting a single, universal standard for male and female astronauts). If 3 percent REID remains the basis for NASA’s risk management process, the resulting standard increases the allowable exposure to radiation and risk of cancer for almost all astronauts. If NASA wanted to use the mean and reference base of a 35-year-old female to set the standard and ensure the standard was more protective than the current standard, the option would be to use a lower mean REID (e.g., 1 percent REID instead of 3 percent REID).

It is also the case that the space travel work environment has changed. A human has not traveled beyond LEO since the end of the Apollo program and it has been NASA’s intent to re-evaluate the standard prior to new long-duration missions further afield. Astronauts are also a unique population operating in a uniquely hazardous environment. Radiation carcinogenesis is one important risk but there are many others that NASA considers in the context of a mission.

The risks of space travel are borne by a small group (astronauts) but the benefits are for all of society. This imbalance imparts unique responsibilities for NASA to provide protection, as much as possible, for astronauts to limit mission disruption during flight, throughout their careers, and after leaving the agency.

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

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

validity of 3 percent REID and make explicit the agency’s justification for the metrics it chooses.

The committee notes that in the rationale section of the proposed radiation exposure standard (see Box 3-1), it says, “Any total exposure (which includes the past exposures plus projected exposure) that exceeds the limit would require a waiver by the agency prior to the mission.” Furthermore, the committee notes that NASA has published in numerous papers that astronauts on a Mars mission will be expected to exceed the career limit of ~600 mSv effective dose (see Figure 3-1 and Table 3-3). 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 reached the following conclusion regarding NASA’s proposed space radiation health standard:

Conclusion I: The committee concludes that astronauts who travel on long-duration spaceflight missions are likely to be exposed to radiation levels that exceed the proposed new space radiation standard of an effective dose of 600 mSv. For example, a mission to Mars is likely to exceed the exposure standard by up to

TABLE 3-3 Projected Radiation Risks for Astronauts on Lunar and Mars Missions

Absorbed Dose
(Gy)a
Effective Dose
(Sv)
Fatal risk, % (95% CI)
Men
(age 40 years)
Women
(age 40 years)
Lunar mission
(180 days)
0.06 0.17 0.68%
(0.20–2.4)
0.82%
(0.24–3.0)
Mars orbit
(600 days)
0.37 1.03 4.0%
(1.0–13.5)
4.9%
(1.4–16.2)
Mars exploration
(1,000 days)
0.42 1.07 4.2%
(1.3–13.6)
5.1%
(1.6–16.4)

NOTES: Calculations are at solar minimum, where GCR dose is highest behind a 5 g/cm2 aluminium shield. CI = confidence interval; GCR = galactic cosmic rays; Gy = grey; Sv = sievert. Intervals are obtained by varying the input parameters of the NSCR NASA risk model according to “parameter uncertainty distributions” determined by NASA based on expert judgment.

a Mean for tissues known to be sensitive to radiation and at risk of cancer, including lung, colon, stomach, bladder, bone marrow, and breast and ovaries in women. Competing causes of death are included in calculations because they decrease risk probabilities if high (i.e., >5 percent).

SOURCE: Adapted from Cucinotta and Durante, 2006.

Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

150 percent. Unless technological advancements and engineering controls provide improved radiation shielding or other protections to astronauts, for a mission to Mars to proceed, NASA would need to seek waivers to the radiation health standard both for the mission and for each astronaut.

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Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
×

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Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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Suggested Citation:"3 NASA's Spaceflight Radiation Exposure Standard." National Academies of Sciences, Engineering, and Medicine. 2021. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks. Washington, DC: The National Academies Press. doi: 10.17226/26155.
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Astronauts face unique health-related risks during crewed space missions, and longer-duration missions that extend to greater distances in our solar system (including to the Moon and Mars) will likely increase those risks. Cancer risks due to ionizing radiation exposure are one of these health-related risks. Assessing, managing, and communicating radiation-induced cancer risks associated with spaceflight are challenging because of incomplete knowledge of the radiation environment in space, limited data on radiation-induced cellular damage mechanisms, lack of direct observations from epidemiological studies, and the complexities of understanding radiation risk.

At the request of the National Aeronautics and Space Administration (NASA), an ad hoc committee of the National Academies of Sciences, Engineering, and Medicine convened to provide advice on NASA's proposed updates to their space radiation health standard, which sets the allowable limit of space radiation exposure throughout the course of an astronaut's career. Space Radiation and Astronaut Health: Managing and Communicating Cancer Risks provides the committee's recommendations and conclusions regarding the updated space radiation health standard, NASA's radiation risk communication strategies, and a process for developing an ethics-informed waiver protocol for long-duration spaceflight missions.

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