Space agency-specific standards for crew dose and risk assessment of ionising radiation exposures for the International Space Station

Abstract

The Partner Agencies of the International Space Station (ISS) maintain separate career exposure limits and shared Flight Rules that control the ionising radiation exposures that crewmembers can experience due to ambient environments throughout their space missions. In low Earth orbit as well as further out in space, energetic ions referred to as galactic cosmic radiation (GCR) easily penetrate spacecraft and spacecraft contents and consequently are always present at low dose rates. Protons and electrons that are trapped in the Earth’s geomagnetic field are encountered intermittently, and a rare energetic solar particle event (SPE) may expose crew to (mostly) energetic protons. Space radiation protection goals are to optimize radiation exposures to maintain deleterious late effects at known and acceptable levels and to prevent any early effects that might compromise crew health and mission success. The conventional radiation protection metric effective dose provides a basic framework for limiting exposures associated with human spaceflight and can be communicated to all stakeholders. Additional metrics and uncertainty analyses are required to understand more completely and to convey nuanced information about potential impacts to an individual astronaut or to a space mission. Missions to remote destinations well beyond low Earth orbit (BLEO) are upcoming and bestow additional challenges that shape design and radiation protection needs. NASA has recently adopted a more permissive career exposure limit based upon effective dose and new restrictions on mission exposures imposed by nuclear technologies. This manuscript reviews the exposure limits that apply to the ISS crewmembers. This work was performed in collaboration with the advisory and guidance efforts of International Commission on Radiological Protection (ICRP) Task Group 115 and will be summarized in an upcoming ICRP Report.

Introduction

Over the past 22 years, sixty-seven crewed “Expeditions” aboard the International Space Station (ISS) were completed by over 250 crew members. Mission durations varied from fewer than two months to nearly a year. Individual crew members often complete two or three missions. Five international space agencies are the leading partners in the ISS Program: Canadian Space Agency (CSA); European Space Agency (ESA); Federal Space Agency, Russian Federation (RSA); Japan Aerospace Exploration Agency (JAXA); and National Aeronautics and Space Administration (NASA), USA. Each Space Agency uses unique procedures to analyse and limit radiation exposures, and to communicate the associated risks to their crew members. It is noted that different terms are used in the literature when discussing space crew: astronauts (America, Europe, and allies), cosmonauts (Russian Federation, former Soviet Union). Additional crew name alternates are taikonauts (China), or more rarely spationaut (France), vyomanaut (India), angkawasan (Malaysia), etc. (Wikipedia, 2020). For this publication, it was agreed to use the term “astronaut.”

On-orbit exposures must be managed for the various natural sources of ionising radiation, mainly galactic cosmic radiation (GCR) that includes energetic protons and heavy ions, protons and electrons trapped in the Earth’s geomagnetic field, and (predominantly) protons from solar particle events (SPEs). Additional ionising radiation exposures occur during all phases of a mission in support of medical certification for flight, health care maintenance, and crew participation in biomedical research protocols. Some agencies consider some of the mission-associated exposures that occur on the ground (e.g., medical diagnostic, biomedical research studies) to be occupational exposures and thus count toward the career limit “budget.” Nevertheless, many of those exposures are routinely reported.

Over the past 20 years, on ISS, the personal badges worn by crewmembers usually record absorbed dose rates below 0.3 mGy/day but have exceeded 0.4 mGy/day. This is about two orders of magnitude higher than the world average background exposure of 0.0014 mGy/day outdoors [1]. NASA-badged ISS crewmembers typically accumulate well under 100 mGy absorbed dose during a single ISS mission; long duration and multiple missions accumulate more. Effective doses usually accumulate at a rate of ∼0.2 to ∼0.5 mSv or more each day of a mission. A separate paper in this Special issue compares expected rates for an example exploratory mission beyond Earth’s influence where GCR rates are higher than simultaneous rates in low Earth orbit (LEO) [2].

Compliance with exposure limits occurs through screened crew assignment, design and evaluation of radiation shielding, environment monitoring and duration or timing of mission activities, restrictions on onboard sources of emissions, etc. All anticipated exposures to ionising radiation during the missions are maintained as low as reasonably achievable (ALARA). Each Space Agency performs a pre-flight medical certification for each astronaut who is assigned to a new long-duration mission. Each personal history of occupational exposure to ionising radiation is compiled and combined with a projection of the exposure for the proposed mission to confirm that the cumulative exposure will not exceed their Agency’s occupational limit for career effective dose. Aeromedical Flight Rules manage ionising and non-ionising radiation sources (e.g., light, laser, radiofrequency) during the mission. Contingency conditions and responses are laid out in those Flight Rules. These measures include consensus protocols for response to contingency events such as transient high ionising radiation events and apply to all astronauts. Electromagnetic/radiofrequency and light sources including sunlight and lasers are controlled by engineering safety design of ISS systems and payloads hardware, protective equipment, and Aeromedical Flight Rules that define “keep-out zones.”

Canadian Space Agency (CSA)

The current practice is for the NASA radiation health officer (RHO) to provide reports of CSA astronaut mission exposures and cancer risk estimates. During their missions, the CSA astronauts wear powered NASA crew personal dosimeters called Crew Active Dosimeters that store and self-report time-resolved exposure information. This is in accordance with the ISS Medical Operations Requirements Document (SSP 50260, 2021), which states, “The Radiation Health Officer (RHO) of each IP (International Partner Agency) maintains, at a minimum, the crew personal dosimeter exposure records for each of the Agency’s crewmembers. The NASA RHO provides a report of mission exposure and cancer risk assessments to the IP whose crewmembers are participating in that mission” [3].

The CSA utilizes the career limit of 1 Sv effective dose (higher than NASA’s career limit) and the same NASA approach for short-term limits (30-day and annual) (discussed below) to protect crew during missions in LEO aboard the ISS. These are stipulated in the Medical Evaluation Documents (MED), Volume A: Medical Standards for ISS, Appendix A, Rev 3.3. (2014) and are shown in Table 1 [4].

Table 1.

Canadian Space Agency CSA Radiation Protection Requirements (crew exposures shall not exceed the specified exposure limits). BFO – blood forming organs.

Short-term limits for CSA crewmembers are the same as those for NASA crewmembers
Career limits
Organ	BFO (5.0 cm)	Eye (0.3 cm)	Skin (0.01 cm)
Organ-specific exposure limit (Sv)	1.0	4.0	6.0

European Space Agency (ESA)

The ESA's standards of practice for radiation protection of astronauts’ address the complex and dynamic radiation environment of space. ESA applies––as do all ISS Partner Agencies––the general and specific rules and regulations established for human spaceflight and ISS. The Medical Operations Requirements Document (MORD) [5], the Medical Evaluations Document (ISS MED Vol. A and B) [6], [7] and the Flight Rules [8] of International Space Station Program are key documents that provide guidance and regulations of Human Spaceflight for the ISS Partner Agencies. Monitoring the health of the crew, including their exposure to ionising radiation in space, is essential and obligatory.

Operational radiation limits used by the ISS partners are listed in the Medical Evaluations Documents (ISS MED Volume A) [6]. For Low Earth Orbit (LEO) activities, a multilateral consensus on organ-specific equivalent dose limits for the Blood Forming Units (BFO) has been established by the ISS Partners. BFO refers to bone marrow, spleen, and lymphoid tissue. Active “red” bone marrow is considered a surrogate for all BFO. Table 2 (ISS MED, Volume A, 4.18.2) [6] lists these consensus dose limits from ISS partner agencies. These concordant limits are used to guide mission assignment and the development of in-flight recommendations for the crew, including the final steps toward mission termination. Although the unit in Table 3 is sievert (Sv), it indicates the upper limits to avoid (deterministic) tissue effects.

Table 2.

Consensus dose limits for BFO adopted by the Multilateral Medical Operations Panel applied for ISS operations.

Ionising Radiation consensus dose limits
Organ Specific Equivalent Dose Limits: Exposure Interval	Blood Forming Units (BFO) dose (Sv)
30 days	0.25

Table 3.

The additional limits for eye and skin are ESA-specific limits based on threshold for deterministic radiation effects in organs as given by ICRP 60.

Organ specific equivalent dose limits (Sv) ESA*
Exposure Interval	BFO	Eye	Skin
30 days	0.25	0.5	1.5
Annual	0.50	1.0	3.0

^*ISS limits for stochastic effects adopted by ESA are under revision.

For non-deterministic (stochastic) effects, ESA as in Table 3, (MED, Volume A, Appendix) [6], has adopted a career limit of 1 Sv for the radiation protection as dose limit, irrespective of age and sex [9]. The ICRP recommendations for the astronaut career limit are essentially based on the 50 mSv/year limit for exposed workers, assuming a maximum astronaut career of 20 years. This results in an occupational limit of 1 Sv, independent of age and gender. The choice of this limit was advocated by the Multilateral Radiation Health Working (RHWG) for the ISS and accordingly adopted by the Multilateral Medical Operations Panel (MMOP). *All limits are subject to ongoing review by ISS agencies. For some ISS agencies, however, all limits have remained unchanged over the past several decades, although the ICRP has now lowered the maximum annual dose for terrestrial workers to 20 mSv/year ICRP Pub. 103 2007 [10].

ESA radiation protection includes the following core elements: information, education, medical monitoring and care including radiation monitoring, dose management, evaluation and documentation of the dosimetric data obtained, radiation protection briefings [9]. There is a sequence of activities that begins with the selection of the astronaut and continues during basic training, which provides an introduction to space radiation and protective measures. Once an astronaut is assigned to a specific space mission further briefings are provided by the Space Medicine Team (HRE-OM) and the NASA Space Radiation Analysis Group (SRAG) to complete the mission specific radiation protection training. The in-flight exposure assessment is based on the crew personnel dosimetry of each astronaut as part of the medical monitoring. Analysis of the ambient exposure situation onboard by means of active- and passive onboard dosimetry add necessary environmental data. Upon return, exposure monitoring will continue aboard the spacecraft until landing. Dosimeters are retrieved as so-called “Late Stowage, Early Retrieval Items,” which allows for faster turnaround of equipment. The dosimeters are returned to the origin laboratory for post-processing, analysis, and dose determination. Personal dosimetry is mandatory for the entire ISS crew and is the responsibility of the astronaut's home agency [5]. ESA's current standard for ISS astronaut’s dose assessment is based on passive dosimetry that does not require power. To date, ESA provides each ESA astronaut with a set of three technically identical personal dosimeters for their mission. The European Crew Personal Dosimeters (EuCPD) are to measure inside the spacecraft/space station (Intravehicular Activities IVA), outside the ISS (Extravehicular Activities EVA) or as a reference. Each EuCPD consists of different sensor types that respond to low and high linear energy transfer (LET) radiation using thermoluminescence. Estimates of the contribution of GCR and neutrons are derived from CR-39, PADC (polyallyl diglycol carbonate) components that complete the detector to determine the total dose. Analysis and evaluation of exposure take place post flight [9].

Active dosimetry provides real-time measurements with enabling for display, providing storage- and download- capabilities of crew personal- and environmental- dose data. The ESA Active Dosimetry system (EAD) was successfully launched, installed, and operated as a proof of concept and technology demonstration onboard the ISS [11]. In the past, biodosimetric studies have been conducted on selected long-term missions. Chromosomal changes were examined in lymphocytes in vitro as part of the applied medical operational testing [12]. Based on the long experience accumulated in the NASA biodosimetry program [13]; it became clear that, even if chromosomal aberration dosimetry elegantly proved that risk estimates are solid and consistent with biological observables, experimental uncertainties are too high and will cover any significant inter-individual variability in response. The quest for accurate, molecular biomarkers able to predict individual risk in astronauts is still ongoing [14], [15]. Collection of all data for the individual astronauts, plus environmental measurements assessing the radiation field onboard the ISS are conducted to date. Moreover, mobile units had been placed in selected locations of ISS modules as standalone active detectors capturing exposure data of the ambient environment over time. Data derived from active detectors of NASA feed into the ISS crew's onboard warning system to provide complete monitoring and control of ionising radiation aboard ISS.

Beyond ISS operations, ESA flew five modified mobile unit active dosimeters configured as stand-alone detectors to autonomously measure throughout the ARTEMIS I flight into space as part of advanced mission testing for space exploration outside LEO [16].

ESA radiation risk assessment

Understanding and minimizing the risks from exposure to ionising radiation in space is critical to crew protection [17], [18]. Cancer risk assessment is the leading stochastic effects used for late mortality risk estimates [19], [20]. However, the effects on the central nervous system, the cardiovascular system and the connective tissue [21], [22], [23] are increasingly coming into focus as they may affect crew health and/or mission success, particularly on long duration and deep space missions. To date, also the discussions about the influence of age and gender as those about the importance of genetic predisposition on radiation effects are by no means concluded [24]. ESA roadmap for space radiation research includes spaceflight and ground-based experiments, as well as risk modelling studies [25], [26]. Ground-based studies in space radiation protection in Europe are based at the GSI synchrotron in Darmstadt (Germany), able to accelerated heavy ions up to 1 GeV/n [27]. The ongoing upgrade of the facility (FAIR project) will bring the energy to a world record maximum of 10 GeV/n, making it the most accurate ground-based simulator of cosmic rays in deep space [28]. The IBPER (Investigation on Biological and Physical Effects of Radiation) is a sister program of the Space Radiation Health Program run by NASA at the Brookhaven National Laboratory in the USA [29].

Progress has been made toward possible options for the development of a European radiation risk model [18]. ESA HRE-OM is looking in a risk metrics particularly suitable for the radiation health risk assessment (HRA) of astronauts. Different metrics are used by different agencies – e.g. NASA uses the Risk of Exposure-Induced Death (REID) while Canada is considering adopting the Disability-Adjusted Life Years (DALY) approach [30]. The metrics considered in ESA, Radiation Attributed Decrease of Survival (RADS) [31], [32], [33], represents the cumulative decrease in the unknown survival curve at a certain attained age due to the radiation exposure at an earlier age. A health risk assessment methodology based on these metrics has been recently published [27]. RADS is historically known as the cumulative risk. Unlike other risk metrics, RADS is independent of survival function. It is interpreted as the relative reduction in survival probability at a certain age due to radiation exposure. In REID, the baseline rates are the same for exposed and unexposed populations, and this creates a bias at high doses, relevant for long-term exploration missions [34]. This bias is removed in RADS. However, the independence of RADS from survival function does not mean it is free from demographic health statistics, which can be very variable. Information about baseline cancer risk is required when a relative risk model is used to estimate the excess risk due to radiation exposure. Moreover, RADS does not consider competing risks, which play a role in modifying the chance that the event of interest occurs. As a result, RADS continues to increase with age as long as the excess risk remains positive, and this feature makes it difficult to assess its significance based on a comparison with the baseline risk.

Apart from the metrics, it is known since many years that most of the uncertainty on risk estimate is associated to the RBE of the radiation in space [20]. This justifies the efforts of NASA and ESA in ground-based space programs and in the quest of biomarkers of radiation risk.

Japan Aerospace Exploration Agency (JAXA)

JAXA implements exposure management for Japanese ISS astronauts. The exposure management starts from the astronaut selection phase and continues until retirement of the astronaut. Though dose during space flight is the most significant and characteristic parameter in the lifetime exposure management, JAXA also considers mission-related exposures for astronauts that occur on the ground. JAXA formed its own career dose limits and procedures to predict doses before a mission assignment, to estimate doses during inflight mission and to assess individual doses after return of flight. A procedure is also in place to assess risks from radiation exposure.

Career dose limits for Japanese ISS astronauts are shown in Table 4. The limits for effective dose are aimed at limiting the probability of the occurrence of stochastic effects to an acceptable level. JAXA used mainly lifetime attributed cancer mortality for the probability and around 3% for the acceptable level in order to form the limits. The limit for organ dose equivalent is aimed at avoiding the occurrence of tissue reactions induced by radiation exposure. JAXA used threshold doses recommended by the ICRP publications.

Table 4.

Career dose limits for Japanese ISS astronauts.

	Effective Dose (Sv)			Organ Dose Equivalent (Sv)
	Age at the first spaceflight	Male	Female	Skin
Career dose limits	27–30	0.6	0.5	20
	31–35	0.7	0.6
	36–40	0.8	0.65
	41–45	0.95	0.75
	>45	1	0.8

*If cumulative dose for lens of eye of astronaut would exceed 0.5 Sv, JAXA would provide him/her with opportunities of medical examinations as long as necessary.

JAXA applies the career dose limits to the sum of individual doses from space missions, medical examinations for medical certification, astronaut training and participation in biomedical research protocols throughout the career of each astronaut. Once a year, JAXA confirms that cumulative individual doses of each astronaut are within the dose limits and that cumulative doses combined with predicted mission doses for a proposed mission are less than the limits before crew assignment to a specific mission.

Dose assessment for Japanese ISS astronauts

JAXA has developed a dose calculation system, which consists of several space radiation models, data tables from some radiation transport codes and data tables from fluence-dose conversion coefficients for predicting, estimating, and assessing inflight doses. Fig. 1 shows a sketch of the dose calculation system. JAXA uses CREME86 [35] for GCR model, AP8 [36] for trapped proton model, AE8 [37] for trapped electron model, HERMES [38] for transportation of proton and neutron, EGS4 [39] for transportation of electron and photon, JINC [40], [41] for transportation of heavy ions, ICRP116 [42] for fluence-dose conversion coefficients of electron and photon, ICRP123 [43] for fluence-dose conversion coefficients of proton, neutron and heavy ions and CADrays 3-D mass models [44] for ISS Geometry. Before a mission assignment, only this system is used to predict spaceflight effective dose and organ doses. In contrast, during spaceflight, effective dose and organ doses are estimated by combining doses calculated by this system and readings of the RAD, which is NASA’s onboard instrument, by means of Equations (1), (2).

$$E_{\mathit{inflight}}=E_{\mathit{cal},crew}\times \frac{D_{\mathit{RAD}}}{D_{\mathit{cal},RAD}}$$ (1)

$$H_{T,inflight}=H_{T,cal.crew}\times \frac{D_{\mathit{RAD}}}{D_{\mathit{cal},RAD}}$$ (2)

Figure 1.

Schematic view of JAXA space radiation dose calculation system

Here, $E_{\mathit{inflight}}$ and $H_{T,\mathit{inflight}}$ are effective dose and dose equivalent of organ $T$ estimated in flight, respectively. $E_{\mathit{cal},crew}$ and $H_{T,cal,crew}$ are effective dose and dose equivalent of organ $T$ at crew position in ISS calculated by the system. $D_{\mathit{cal},RAD}$ shows skin absorbed dose at the RAD position in ISS calculated by the system and $D_{\mathit{RAD}}$ shows absorbed dose measured by the onboard RAD instrument. All of the JAXA effective doses and dose equivalents are calculated using quality factors as a function of LET [Q (L) functions from ICRP60].

After the return of the crew to the Earth, individual effective dose and organ doses are assessed using LET distributions from the JAXA personal dosimeter, Crew-PADLES, according to Equations (3), (4).

$$E_{\mathit{crew}}=\sum _L{E}_{\mathit{inflight}}(L)\times \frac{D_{\mathit{PADLES}}(L)}{D_{\mathit{skin},inflight}(L)}$$ (3)

$$H_{T,crew}=\sum _L{H}_{T,inflight}(L)\times \frac{D_{\mathit{PADLES}}(L)}{D_{\mathit{skin},inflight}(L)}$$ (4)

Here, $E_{\mathit{crew}}$ and $H_{T,crew}$ are individual effective dose and dose equivalent of organ $T$ assessed after flight, respectively; $L$ denotes LET; and $D_{\mathit{PADLES}}$ and $D_{\mathit{skin},inflight}(L)$ show absorbed dose from LET distribution measured by Crew-PADLES and absorbed dose of skin estimated in flight, respectively. $E_{\mathit{inflight}}$ and $H_{T,influght}$ are from Equations (1), (2).

Risk assessment for Japanese ISS astronauts

JAXA presents calculated risks of radiation exposure to astronauts once a year and after the return to the earth, based on his/her cumulative dose throughout his/her career. At that time, JAXA uses lifetime attributed cancer mortality (LACM) for risk of stochastic effects. The LACM is calculated in Equation (5).

$$\mathit{LACM}=\underset{A_0}{\stackrel{\infty }{\int }}\prod _{i=A_0}^{a-1}\left\{1-p_0\left(i\right)-p_{D,A_0}(i)\right\}\times p_{D,A_0}\left(a\right)da$$ (5)

Here, $p_0(a)$ is the baseline mortality at age $a$ for all deaths in Japan, in 2007; $p_{D,A_0}(a)$ is the cancer mortality at age $a$ attributed to radiation exposure with effective dose $D$ at age $A_0$.

The $p_{D,A_0}(a)$ can be calculated as in Equations (6), (7), (8) for solid cancers and in Equations (9), (10) for leukaemia. Mean values of cancer mortality from ERR and EAR models is used for calculating $p_{D,A_0}(a)$ of solid cancers. On the other hand, cancer mortality from the EAR model is used for calculating that of leukaemia.

$$p_{D,A_0}(a)=\left\{p_{D,A_0}^{\mathit{ERR}}(a)+p_{D,A_0}^{\mathit{EAR}}(a)\right\}\times \frac{1}{2}$$ (6)

$$p_{D,A_0}^{\mathit{ERR}}(a)={\lambda }_M\left(a\right)\times \left\{1+{\beta }_s\times D\times \mathrm{e}\mathrm{x}\mathrm{p}(\log \left(1+\theta \right)/10\times (A_0-30)+\gamma \times \log \left(a/70\right))\right\}$$ (7)

$$p_{D,A_0}^{\mathit{EAR}}(a)={\lambda }_M\left(a\right)+{\beta }_s\times D\times \mathrm{e}\mathrm{x}\mathrm{p}(\log \left(1+\theta \right)/10\times (A_0-30)+\gamma \times \log \left(a/70\right))$$ (8)

Here, $p_{D,A_0}^{\mathit{ERR}}(a)$ and $p_{D,A_0}^{\mathit{EAR}}(a)$ are cancer mortalities obtained with the ERR and EAR models, respectively; ${\lambda }_M\left(a\right)$ represents the baseline cancer mortality of Japanese in 2005; ${\beta }_s$, $\mathit{\theta }$ and $\gamma$ are coefficients for all solid cancer from Tables A 4.6, A 4.7, A 4.8 and A 4.9 in ICRP Publication 103 [45]; In addition, a dose and dose rate effectiveness factor (DDREF) of 2 and a lag time of 5 years are applied for calculating the lifetime attributed cancer mortality of solid cancers

$${p_{D,A_0}\left(a\right)=s}_{D,A_0}\left(a\right)\times q$$ (9)

$$s_{D,A_0}\left(a\right)={\lambda }_m\left(a\right)+{\beta }_s\times (D+0.79D^2)\times \mathrm{e}\mathrm{x}\mathrm{p}\left\{\mathit{\theta }\times (a-A_0-25)\right\}$$ (10)

Here, $s_{D,A_0}(a)$ is the leukaemia morbidity at age $a$ attributed to radiation exposure with effective dose $D$ at age $A_0$; $q$ is the fatality rate of leukaemia which is from Table A 4.5 (bone marrow) in ICRP Publication 103 [45],${\lambda }_m\left(a\right)$ is the baseline leukaemia morbidity of Japanese in 2005; ${\beta }_s$ and $\mathit{\theta }$ are coefficients for leukaemia from Preston et al. [46]; For calculating the lifetime attributed cancer mortality of leukaemia, a DDREF of 1 and a lag time of 2 years are applied.

National Aeronautics and Space Administration (NASA)

Exposure management and dose assessment

NASA manages radiation exposures to ISS crew members and ensures compliance with exposure limits for individual astronauts through all phases of space flight. Pre-flight practices include development of vehicle/payload design specifications and definition of limits on acceptable exposure levels, vehicle shielding analysis, crew selection, and projections of space radiation environments and mission exposures. After being selected into the Astronaut Corps, Astronaut Candidates receive a detailed briefing on the space radiation environments and their potential early- and late- biological hazards. Before flight, an additional “refresher” briefing, and training covers the personal badging and operational radiation instruments on ISS. NASA does not use a formal informed consent process for government employees, in contrast to the Federal Aviation Administration’s requirement of private and commercial operators.

In-flight practices include adherence to ISS (and visiting vehicle) Flight Rules that monitor and limit cumulative exposures during nominal and transient space weather conditions during all intravehicular and extravehicular (“spacewalk”) mission segments. On orbit, astronauts wear an “active” crew personal dosimeter that can be read by the crew and telemetered to ground support personnel. Numerous operational instruments throughout the ISS act as area monitors to provide time-resolved absorbed dose measurements.

Modelled radiation environments external to ISS are transported using the one-dimensional model HZETRN [high nuclear charge and high kinetic energy (HZE) particle transport (TRN)] [47], rather than using pre-determined dose conversion factors. The modelled geomagnetically trapped proton environment is dose-normalized using various area monitoring instruments. Post-flight activities include exposure and cancer risk analyses, recordkeeping, and reporting to crew and flight surgeons. All exposures associated with health evaluations and medical care and voluntary biomedical research studies are optimised; those dosimetry results are recorded but not counted toward NASA astronauts’ occupational exposure limits. An annual report is provided to each active astronaut of their career exposure history for the time that they were in the astronaut corps. Mission dose and risk, biomedical research exposures, and medical diagnostic exposures are included in the reports. Astronauts who have retired from the astronaut corps are invited back for annual physical exams, however, post-career radiation exposure history from medical or their post-astronaut occupational activities is not tracked by NASA for longitudinal studies. To enforce Flight Rules and to manage communications of space weather conditions with the crew, NASA serves as the lead Agency that provides daily space weather monitoring as part of the Space Radiation Analysis Group’s ground support at the Mission Control Centre, Johnson Space Center, in Houston, Texas, USA.

Risk assessment, risk management and space permissible exposure limits (SPELs)

Recently, NASA revised the Agency Standard that limits radiation exposures to astronauts for all NASA programs for human spaceflight (NASA STD 3001, 2022) [48]. In addition to a new career exposure limit based upon effective dose, additional limits address specific sources of mission exposures – SPEs and nuclear technologies. Further guidance specific to galactic cosmic radiation exposures is currently under review. Three specific requirements are introduced…

…astronaut total career exposure limits, short-term acute exposure limits and nuclear technology exposure limits.

It is important to further minimize exposure from all sources of radiation below the following limits using the as low as reasonably achievable (ALARA) principle.

The first NASA requirement was defined to work with other management controls to limit the life-long radiation related risk of cancer incidence and mortality and to prevent or ensure acceptably low levels of risk of other adverse clinical health outcomes...

Career space permissible exposure limit for space flight radiation

An individual astronaut’s total career effective radiation dose due to space flight radiation exposure shall be less than 600 mSv. This limit is universal for all ages and sexes.

The NASA effective dose for determining the standard threshold limit is calculated using the NASA Q (based on the NASA cancer model of 2012), 35-year-old female model parameters (tissue weighting factors, phantom, etc.) for both males and females. Individual astronaut risk of exposure-induced death (REID) calculations are calculated using the appropriate NASA Q (based on the NASA cancer model of 2012) sex and age model parameters.

Rationale: The total career dose limit is based on ensuring all astronauts (inclusive of all ages and sexes) remain below 3% mean risk of cancer mortality (REID) above the non-exposed baseline mean. Individual astronaut career dose includes all past space flight radiation exposures, plus the projected exposure for an upcoming mission. Medical and biomedical research exposures are not included in the dose limit but are tracked for overall crew exposure history. This standard protects the career limits for all organs [listed below for non-cancer effects…]

The Career Space Permissible Exposure Limit (SPEL) requirement was recently redefined to limit all crew to the NASA-defined metric effective dose in females, replacing REID as the limit metric [48]. The newly defined limit targets a 3% increase in cancer REID for a female exposed at age 35 years to 600 mSv effective dose. The NASA formulism for effective dose departs from the prescriptive ICRP methodology and guidance on limits for occupational and public exposures (ICRP Publication 103, 2007, Table 6) [45] in several respects. First, no contribution of hereditary effects to the health detriment is included. Second, NASA applies pre-calculated cancer-mortality-based tissue/organ weights whereas ICRP considers cancer incidence, as well. Third, the weights are different for male and female adults, not averaged over both males and females and all ages (ages 30 to 60 y; Section 6.3 and Table 6.3; NSCR 2012). The sex-specific tissue/organ weights are derived from REID analysis of ISS missions and reflect the lack of smoking-attributable cancers seen in the average U.S. population. Appendix A shows the tissue weights used for an astronaut of a specific age (40y), the weights currently used for mission analyses (footnote b), and those specified by ICRP [45], [49]. The mortality risk-based tissue weights are environment dependent and thus differ between GCR or GCR-insignificant environments; this results from using a different quality factor for leukaemia than for solid tumours. In place of the ICRP-defined radiation weighting factor or quality factor, the NASA quality factor weights absorbed dose and is dependent upon the effective nuclear charge, Z_eff, and kinetic energy, E, of ions in body organs, with Q_NASA(Z_eff,E) = 1 for sparsely ionising portions of each particle track and Q_NASA(Z_eff,E) > 1 for more dense energy distributions along the track. When applied to leukaemias instead of “solid tumours,” Q_NASA(Z_eff,E) is reduced (by a factor of 4 at peak effectiveness) to reflect the evidence base of lower RBEs for leukaemias and other blood disorders. The relative contribution from each tissue or organ to effective dose is shown for an example mission in Appendix A, as well.

Table 6.

RBE for non-cancer effects^a of the lens, skin, BFO, and circulatory system.

Radiation Type	Recommended RBEb	Range
1 to 5 MeV neutrons	6.0	4–8
5 to 50 MeV neutrons	3.5	2–5
Heavy Ions	2.5c	1–4
Protons > 2 MeV	1.5	-

^aRBE values for late deterministic effects are higher than for early effects in some tissues and are influenced by the doses used to determine the RBE.

^bThere are not sufficient data on which to base RBE values for early or late effects by neutrons of energies <1 MeV or greater than about 25 MeV.

^cThere are few data for the tissue effects of ions with a Z > 18, but the RBE values for iron ions (Z = 26) are comparable to those of argon (Z = 18). One possible exception is cataract of the lens of the eye because high RBE values for cataracts in mice have been reported.

Permissible Mission Duration (PMD) is a metric used by NASA to indicate the projected number of days an astronaut would be able to fly a specific mission at a specified time without exceeding the career SPEL. PMD is reduced by exposures that an individual astronaut incurred on previous space missions. For NASA astronauts aboard ISS, the PMD allows for careers over 2.5 or even 3 years in total duration before reaching 600 mSv mean effective dose in females. Missions to Mars are expected to be possible, as well, during the more active part of the solar activity cycle but without a substantial solar particle event, with appropriate shielding for GCR, and for crews with low previous occupational exposures. A career limit exception process, when justified through a prescribed ethics-based framework, may be invoked for long duration and exploration-class missions [50].

NASA’s previous career cancer risk limit was defined as 3% REID evaluated at the 95% confidence interval, and in addition to exposures that occurred during space missions, the exposures from biomedical research studies were considered to be occupational and thus counted toward the career limit. That limit was more restrictive than the current limit, which is informed by the mean of the REID probability, evaluated for a female who is exposed at age 35-years. Considerable quantitative uncertainty remains in modelled REID and risk of exposure-induced incidence of cancer (REIC) for multiple, dynamic, and mixed field exposures in space. The sources of uncertainty exist in the environment projections and detector and biological response model (Q_NASA and DDREF) updates that can substantially revise mission analyses. In comparison, the 600 mSv female effective dose limit, evaluated at the mean, is a more stable and suitable metric for radiation protection purposes. When applying the reference NSCR-2012 tissue weighting factors that are based on the contribution of individual organs/tissues to REID, the sex-dependence of mean effective dose on ISS missions is subtle, with a ratio ∼1.08 for females:males in LEO and a slightly higher ratio for BLEO missions. Limiting effective dose at the mean of a calculated uncertain probability distribution is more restrictive than using a deterministic or “point value” and thus creates additional conservatism in the SPEL. This foreshortens PMD because the mean of the skewed distribution calculated by NSCR-2012 is ∼20% higher than the point estimate, thus approaching the “targeted” 3% mean REID level at lower doses. The age- and sex-dependence of the NSCR-2012 calculation of REID and REIC assures that all astronauts above age 35 when exposed to 600 mSv effective dose will have lower calculated excess lifetime cancer risk than a female who is similarly exposed at age 35y. REID and REIC statistics at the upper level of the 95% confidence interval continue to be reported to the astronauts for individual space missions and for all missions, and is reported, as well, for biomedical research study exposures.

Protection against non-cancer effects. The rationale of the first NASA requirement states that this limit protects against non-cancer effects for which short-term limits are defined for the lenses of the eyes, skin, “blood forming organs” and the central nervous system. A series of SPEs that occurred in October 1989, is used as a single reference event to demonstrate that inherent or enhanced vehicle and mission design protects against this potential threat.

These limits are imposed to prevent clinically significant non-cancer health effects including performance degradation, sickness, or death in-flight. Higher career dose limits for eye lenses, circulatory system, and the central nervous system (CNS) are imposed to minimize or prevent risk of late effects of degenerative tissue diseases, e.g., cataracts, stroke, coronary heart disease, etc.

A table is provided (reproduced as Table 5, below) in the NASA Standard 3001 with those limits for 30-day, 1-year, and career time periods using the metric Gray-Equivalent, which is defined as the product of the deterministic absorbed dose averaged over the specified organ or tissue volume and relative biological effectiveness (RBE) values for both early- and late-developing non-cancer effects, G_T = RBE(Z;E) × D_T. RBE is greater than unity for neutrons, protons, and heavy ions [51]. Averaging the RBE over, for example, protons from a solar particle event, typically results in an RBE that is lower than the average Q_NASA for that field. The exception is that the RBE for “blood forming organs” may be slightly higher than Q_NASA for high-energy protons. In operational practice, incomplete information about the RBE of external neutrons is circumvented by transporting neutrons to the organ/tissue dose points of interest and calculate there the Gray-Equivalent of the directly ionising components of the field. The non-cancer parameter, G_T, recognizes that early and late effects other than cancer may have a substantially different–usually lower–RBE than the cancer-based radiation quality factor. Hence, averaged over a transient event such as passage through intense proton or electron radiation belts or during an energetic SPE, the average quality factor may be conservatively high for representing the impact of the densely ionising components of the radiation field.

Table 5.

Dose Limits for Short-term or Career Non-cancer Effects (in mGy-Eq. or mGy). Note: RBEs for specific risks are distinct (NASA STD-3001, 2022) [48].

Organ	30-Day Limit (mGy-eq)	1-Year Limit (mGy-eq)	Career Non-Cancer Limit (mGy-eq)
Lens of the eye*	1,000	2,000	4,000
Skin	1,500	3,000	6,000
Blood-forming organs	250	600	n/a
Circulatory system**	250	500	1,000
Central nervous system***	500 mGy	1,000 mGy	1,500 mGy
Central nervous system (heavy particles, Z ≥ 10)***	n/a	100 mGy	250 mGy

^*Lens limits are intended to prevent early (<5 years) severe cataracts, e.g., from a solar particle event. An additional cataract risk exists at lower doses from cosmic rays for sub-clinical cataracts, which may progress to severe types after long latency (>5 years) and are not preventable by existing mitigation measures; however, they are deemed an acceptable risk to the program.

^**Circulatory system doses calculated as average over heart muscle and adjacent arteries.

^***Central Nervous System limits should be calculated at the hippocampus.

NASA Space Cancer Risk Model (NSCR-2012). Excess risk of cancer incidence and mortality is determined with NSCR-2012 for an individual of a given age at exposure, sex, and tobacco-smoking status [52]. The HZETRN transport model provides cumulative mission fluences of individual particles, f(Z;E), at multiple points in each organ or tissue which are coupled with the NSCR-2012 models of carcinogenic response. Modelled cancer risks for multiple exposures of individual organs and tissues are reported as a probability distribution where subjective uncertainty distributions represent the various individual contributions of correlated and uncorrelated exposure and risk. Radiation quality, dose rate response, transfer of risk to individual astronauts from other human cohorts are the dominant sources of quantified uncertainty. Output metrics include Risk of Exposure-induced Death (REID) and Risk of Exposure-induced Cancer-incidence (REIC).

As implemented for human spaceflight operations, NSCR-2012 calculates cancer REID probability distributions utilizing evidence from epidemiological data, primarily U.S. (organ-specific cancer rates for incidence and mortality for tobacco never-smokers) coupled with the dosimetry and follow-up solid-cancer mortality analysis of Japanese atomic bomb survivors and other cohorts to provide age- and sex-specific excess cancer rates.

A mixture of excess additive risk and multiplicative excess risk models is used to tie epidemiological evidence for baseline human cancer incidence rates to an evidence base of cancer response from humans/animals/cells exposed to sparsely ionising radiation. For typical exposure to GCR and trapped protons on ISS and missions, NSCR-2012 finds that lung cancer mortality contributes 33% to 40% of the total REID for females, and a lower amount (but more than 25%) for males [52]. The dependences of exposure rate and “radiation quality” for densely ionising ions are informed by human, animal, and cell data. Since space mission exposures are typically chronic and low dose, REID and REIC risk metrics are reduced relative to a comparable acute exposure that is sampled over a skewed probability distribution of the DDREF centered on 1.5; the central value was suggested by the BEIR VII Report [53].

The REID and REIC probability distributions for ISS missions are skewed to lower REID (and REIC) with an upper 95^th percentile that is nearly 3 times higher than the mean. Typically, the modelled REIC statistic is ∼2 to 2.5 times higher than the corresponding REID for ISS missions. The REIC:REID ratio will be similar for missions beyond Low Earth Orbit unless the dose distribution or level of healthcare required changes substantially. Effective dose on ISS missions is less than 10% higher for females than for males with slightly larger differences likely for exploration class missions, although that difference is inversely related to the amount of available shielding.

The second NASA requirement:

Short-term radiation limits – Solar Particle Events

The program shall protect crewmembers from exposure to the design reference SPE environment proton energy spectrum (sum of the October 1989 events) to less than a NASA effective dose of 250 mSv.

Rationale: …For solar particle event shielding designs, an iterative approach should be taken for determining shielding designs that continue to iterate the design until less than a 10 mSv (reduction) is achieved from the previous iteration.

The rationale for this limit describes the intent to minimize “early” (in-mission) biological effects and control cumulative exposures that can lead to long-term cancer and non-cancer health effects. This limit reinforces the role of ALARA in all stages of a mission including vehicle and mission design.

The third NASA requirement:

Crew radiation limits for nuclear technologies

Radiological exposure from nuclear technologies emitting ionising radiation to crewmembers (e.g., radioisotope power systems, fission reactors, etc.) shall be less than an effective dose of 20 mSv per mission year (prorated/extrapolated to mission durations) and utilizing the ALARA principle.

The Rationale notes that this limit “…is based on not adding more than 10% radiation exposure beyond the space environment radiation of the mission… This standard is applied to both surface and free-space missions regardless of mission solar cycle. Twenty (20) mSv was also based on the occupational workers’ limit guideline from the ICRP” [48].

A waiver of this limit may be obtained for circumstances where overall risks from exposure to ionising radiation and perhaps other operational risks inherent to the mission are reduced by application of nuclear technologies. For example, nuclear propulsion could reduce the mission duration and overall radiation risks by substantially decreasing transit time to and from Mars.

NASA limits for non-cancer effects

The NASA exposure and design limits outlined above, along with ALARA practices, are intended to protect crews from anticipated sources of exposure to space radiation environments. Transient exposures may be encountered due to SPEs and extended (if unplanned) passage through intense trapped proton belts, or even from technological sources. “Non-cancer Limits” were defined (Table 5) to protect crew and mission success by preventing clinical manifestations of acute injury that might present during a mission [48]. These also protect against chronic health effects that may appear much later [54]. The non-cancer effects metric Gray-Equivalent was defined as $G_T=RBE\times D_T$ , where RBE values are defined in Table 6.

NASA guidance on galactic cosmic radiation

The uncertainty associated with the biological responses to the densely ionising component of GCR are a potential concern not only for professional astronauts but also for passengers on private missions, as well. Outside of NASA missions launched from the U.S.A., the Federal Aviation Administration (FAA) regulates the safety of “non-government astronauts” and they have not defined exposure limits. In contrast to NASA (but similar to RSA), the FAA does require a formal consent that requires flight crew and spaceflight participants (non-crew, clients, and private “space tourists”) to be fully briefed on known hazards and the potential for unknown hazards [55]. The chronic and difficult to shield GCR environment will dose limit space travellers on long duration missions. NASA is studying the effectiveness of shielding with the intent to provide guidance for designers of private and commercial space vehicles.

Russian Federal Space Agency (RSA)

Russian Federal Space Agency (RSA) implements exposure management for Russian ISS cosmonauts and space tourists. Historically, the exposure management was delegated to the Institute of Biomedical Problems of the Russian Academy of Sciences (IBMP RAS). Radiation exposure management starts before the planned space flight, when a pre-flight radiation exposure value, H_{before flight}, is estimated. This value includes all doses received during pre-flight medical examinations and in previous space flights, if any. Then the dose for the forthcoming space flight, H_in-flight, is roughly estimated based on current space radiation environment models, space weather forecast, radiation transport codes, and previous experience. The decision-making procedure for space flight approval is simple and based on the following conditions [56]:

$$H_{\mathit{beforeflight}}+H_{\mathit{in}-flight}<H_{\mathit{careerlimit}}=1Sv$$

$$H_{duringprevious2years}<0.2Sv$$

The effective dose limit, H_{career limit}, is set to 1 Sv independent on sex and age. In addition, if the dose received during the previous 2 years was higher than 0.2 Sv, then the expected in-flight dose should be below 0.02 Sv. The latter restriction is implemented to prohibit frequent flights of cosmonauts, namely, after a full-scale mission of about one year the dose in the next mission should be below 0.02 Sv. In all cases, an informed consent approach is used for the approval of a crew candidate for a planned space mission.

Dose assessment for RSA ISS cosmonauts

Individual in-flight doses are controlled and estimated daily by onboard stationary dosimeters located both in the ISS Russian Segment (RS) and in the United States Orbital Segment (USOS). In case of radiation contingency, the doses are estimated more frequently when needed. Individual dosimeters (passive detectors) are supposed to be worn by each crew member during the flight, extravehicular activity included, and are used for obtaining the individual dose of each crewmember after the flight completion. The typical approach to the assessment of the space radiation dose is shown in Fig. 2.

Figure 2.

Typical dose calculation approach as applied by RSA

RSA dose calculation method and tools include the GCR model CREME86 [57], SPENVIS [58], ISO 15390 [59]; Trapped radiation models for protons and electrons: AP8 (1987) and AE8 [60], [61]; Model for Solar energetic particles (SEP) events [62]; Depth-dose curves for all radiation sources (COSRAD code [63]; Radiation transport codes model [64], GEANT4 [65]; and ISS Geometry: ray tracing method [66], [67].

Risk assessment for RSA ISS cosmonauts

The RSA approach to radiation risk estimation of the ISS crew is based on the total radiation risk concept as a main quantitative criterion for the space radiation-induced hazard for human health due to late effects. More specifically, it is necessary to consider not only the risk of in-flight (short-term) radiobiological effects but also late-effects for the organism as a whole and for different organs. In-flight radiation-related effects can in principle influence the work efficiency of crew members. The restriction of late effects provides an acceptable level of health and working capacity for later periods of life.

The dependence of short-term and long-term health effects on dose and dose rate has been studied in detail for standard irradiation, namely, acute uniform exposure with gamma radiation with an energy of 250 keV. This knowledge can be used to estimate the probability of short-term (in-flight) and long-term radiation-induced health effects and levels of damage to different tissues/organs of cosmonauts, based on the developed principles of conversion of the reference (standard) radiation fields on the Earth to the mixed radiation field in space. More specifically, in the RSA standards, the generalized dose H is presented as the standard radiation dose inducing the same short-term and long-term radiobiological effects as the same dose from the radiation field in space, which is considerably more complex than that on Earth. The generalized dose is calculated using Equation (11) [68]

$$H[Sv]=\overline{D}[Gy]·QF·TF·SF·MF$$ (11)

Where, H is a generalized dose; $\overline{D}$ is mean tissue absorbed dose; QF is the quality factor of the radiation of interest, accounting for the different biological effectiveness of cosmic rays (HZE particles included) and 250 keV gamma radiation; TF is a factor to account for any difference between acute, protracted and chronic exposures; SF is a factor to account for any differences in biological effect between uniform and non-uniform spatial body exposure; and MF is a factor taking into account non-radiation biological effects due to hypomagnetic environment, hypergravity, microgravity, leading to hypodynamia, long-term psychological stress and so on.

In principle, different values of QF, TF, and SF should be used for short-term (in-flight) and long-term radiation-induced effects. For cosmic radiation, QF in Equation (11) may be obtained from the mean quality factors inferred from the data of the onboard radiation monitoring system dosimeters, which measure LET spectra. The mean QF values are calculated by the formulas recommended by ICRP Publication 60 [49].

Total radiation risk is considered in the RSA approach as the main quantitative criterion of radiation hazard for a crew member [69]. By definition, the total radiation risk is the additional risk of mortality for the whole life period that was caused by all pathological effects induced by radiation. To consider all possible hazards, it is necessary to take into account not only the risk of cancer incidence but also all radiation-induced effects that can influence the lifetime risk, as calculated using Equation (12):

$$R_{\mathit{total}}=R_{\mathit{cancer}}+R_{\mathit{otherradiationinducedeffects}}$$ (12)

The survival value V(T) (fraction of individuals that survive) as a function of age for natural conditions (without irradiation) was considered as follows using Equation (13) and Equation (14):

$$V\left(T\right)=exp\{-{\int }_{t_0}^T\mu \left(t\right)dt\}$$ (13)

$$\mathrm{M}\left(t\right)=\mu \left(0\right)exp({\lambda }_{o}t)$$ (14)

where t₀ is initial moment of time (or time of first exposure to radiation); T is the age at the time of exposure; and μ(t) is the baseline mortality rate; and λ₀ is a parameter defining the rate of decreasing compensatory reserves (i.e., rate of decrement in health) and increasing age-specific death rate under natural conditions (without irradiation).

From demography data for T > 25 years [70], μ(0) = 9.0·10^-4 y^-1, λ_o = 0.062 y^-1. The survival value after irradiation is calculated as follows using Equation (15):

$$V^{\mathit{rad}}\left(T\right)=exp\{-{\int }_{t_o}^T{\mu }^{\mathit{rad}}\left(t\right)dt\}$$ (15)

where μ^rad(t) is the mortality rate after irradiation: µ^rad(t) = µ(t) exp(B·H).

From radiobiology data [70], B = 0.36 Sv^-1. Then the additional mortality risk R^rad(T) at age T induced by the irradiation is expressed as Equation (16):

$$R^{\mathit{rad}}\left(T\right)=V\left(T\right)-V^{\mathit{rad}}\left(T\right)$$ (16)

Corresponding values of the duration of future life without and after irradiation are determined by using Equation (17) and Equation (18):

$$\overline{T}-{\int }_{t_0}^{\infty }V\left(t\right)dt$$ (17)

and

$${\overline{T}}^{\mathit{rad}}-{\int }_{t_0}^{\infty }{V}^{\mathit{rad}}\left(t\right)dt$$ (18)

Mean lifetime reduction due to radiation exposure, ΔT^rad can then be calculated by using Equation (19):

$$\mathrm{\Delta }{\overline{T}}^{\mathit{rad}}=\overline{T}-{\overline{T}}^{\mathit{rad}}$$ (19)

Numerous experimental data obtained with laboratory animals (mice, rats, and dogs) allowed development of a method to calculate the total radiation risk and related values. The estimation of total radiation risk and mean lifetime reduction for the whole lifetime period are presented in Table 7.

Table 7.

Calculated values of generalized dose (Equation (11)), total radiation risk, risk of cancer incidence, and lifetime reduction.

Calculated values	Age of a cosmonaut (years)	Mean tissue equivalent dose (mSv)**
		1000	1250	1500
Generalized dose*, mSv	–	538	656	775
Total radiation risk, %	–	7.00	8.53	10.0
Radiation risk of cancer, %	30	3.60	4.50	5.40
	40	2.36	2.95	3.54
	50	1.83	2.29	2.74
Mean lifetime reduction, years	30	2.42	2.95	3.49
	40	2.16	2.63	3.10
	50	1.89	2.30	2.71

^*GCR contribution was taken to be 30% of the total dose.

^**E effective equivalent dose from pre-flight medical procedures was taken to be 250 mSv.

The total radiation risk (TRR) and the mean lifetime reduction (MLR) are considered by RSA as suitable criteria to develop new dose limits for space flights. Such criteria consider the maximum hazard for cosmonauts and practically do not depend on cosmonauts’ age. The risk of cancer incidence depends on age and is significantly smaller than the total radiation risk (see Table 7). So, the risk of cancer incidence alone is not adequate for estimating long-term radiation-induced effects since cardiovascular and other non-cancer effects must be considered.

It should also be mentioned that the estimated values of total radiation risk are significantly higher than those considered for the risk of cancer. The RSA estimations include a total radiation risk value of about 10% if the career equivalent dose is equal to the US career limit for 25 years old cosmonauts, namely, 1,500 mSv. The corresponding mean lifetime reduction will be about 3.5 years for such a case. If the career equivalent dose is equal to 4,000 mSv (maximum career dose limit for astronauts), total radiation risk value will exceed 16-19% and the corresponding mean lifetime reduction will be about 5 years.

As a result of such analysis, the career limit of 1 Sv for the mean tissue dose for a cosmonaut was taken by RSA. It is emphasized that any value chosen for a career dose limit depends on the socially acceptable risk level for human activity in space.

Discussion

Career exposure limits for each of the ISS Partner Agencies were described along with summarized operational analysis methods and some context of the radiation protection management practices in place. NASA’s recent revision of their career SPEL from the REID metric to NASA effective dose means each Agency now employs the metric effective dose. However, the quantitative values calculated with that metric are Agency-specific. Agencies differ in the formulation of radiation quality factor and tissue and organ weights and differ in methods used to calculate organ/tissue doses. The exposure limits (Table 8) represent limits on disparate endpoints––for NASA it represents solely cancer mortality while for the Russian Federation it represents all exposure-related pathological causes of mortality. The ICRP-defined formalism, strictly adopted, considers non-cancer mortality and and hereditary effects, as well.

Table 8.

Astronaut career exposure limits for each ISS Space Agency.

Space Agency	CSA	ESA	JAXA*	NASA@	RSA
Career Exposure Limit [Sv]	1	1	0.5–1	0.6	1

^*JAXA’s career limit is age- and sex-specific, ranging from 0.5 Sv for a female 27y of age at first exposure to 1 Sv for males older than 45 at first exposure (see Table 4).

^@The NASA SPEL limits each astronaut to 0.6 Sv, calculated as the mean effective dose to a female.

Harmonization of career limits between ISS Partner Agencies was sought early in the ISS Program but those efforts concluded without a consensus agreement. Technical factors that make consensus elusive include the differences in perceived/modelled environments, the scope of deleterious effects considered, and quantitative risks and uncertain practical thresholds for clinical endpoints. Ambient environments are, in practice, evaluated from instruments that incompletely measured the radiation fields in space and thus require further analysis to fully characterize the radiation environments. That environmental models are in relatively close agreement provides confidence in the initial source terms, and environment normalization methods can improve on that and differences in modelled fields that arise with transport through shielding and the human body. Some uncertainty remains in the source term and modelling of the physical interactions and transport of these environments into the body’s critical tissues and organs. Still, the dominant cause of uncertainty arises from incomplete knowledge of the biological responses of humans to the chronic, low dose rate, mixed radiation environments present along with other stressors in space.

Non-technical factors confound consensus on exposure limits, as well. Cultural differences inherently bring differences in the cost/benefit evaluation of a mission or national interest and drive institutional differences in the amount of acceptable risk to an individual or mission. The Agencies differ in the decision of which non-spaceflight exposures constitute an occupational exposure that should be limited. Consensus agreements on risk acceptance must consider medical exposures (from which a presumed benefit occurs to the individual), atmospheric exposures during flight training, commercial, and personal air travel, voluntary exposures that occur from astronaut participation in biomedical research experiments, and previous occupational exposures. Governmental policy and regulations introduce further complexity into stakeholder interests in setting exposure limits. For example, since 2004 the United States maintains a nearly total legislative moratorium on new safety regulations for commercial and private human spaceflight (the U.S. Congress renewed the moratorium through September 2023).

NASA’s redefinition of the astronauts’ career limit metric in terms of the female effective dose metric was a recent (NASA STD 3001, 2022) [48] and significant step toward a common limitation framework. NASA’s REID and REIC analyses are useful quantities to inform astronauts of risk and to communicate with stakeholders who balance radiation exposure with other mission risks. Additional metrics are calculated by space agencies to convey potential health or mission impacts. RSA calculates age-dependent reduction of expected lifetime based on dose and age at exposure. The TRR parameter includes the total risk of cancer (Table 7), and that modelled cancer risk is age dependent, dropping by a factor of ∼2 with increasing age between 30y and 50y. In comparison, the JAXA effective dose limits drop nearly the same amount. The NASA cancer REID calculation does not reflect such a strong age dependence. Space agencies are investigating other metrics for limitation or communication of risks for cardiovascular effects (RSA) and non-cancer incidence and mortality [34].

Looking beyond Earth-orbit, circumstances might require a BLEO mission to be substantially extended or an unpredictable solar particle event may suddenly increase cumulative exposures above pre-mission projections. While ICRP outlines categories for emergency conditions, the ISS missions are very unlikely to encounter extreme transient environments. Partner Agencies do not specify acceptable levels of risk for a transient situation such as an extremely large SPE or multiple SPEs during a short time frame. “Early” or “late” tissue effects, such as to the lens of the eye and circulatory system are unlikely to cause functional impairment below 0.1 Gy [71]. However, in the absence of mitigation, such effects are more likely to occur at doses above 0.5 Gy. Uncertainty about individual responses (probability and severity) may impose an upper ceiling on acceptable health risks. For mission planning, a metric similar to Permissible Mission Duration used by NASA for ISS missions may be a useful operational quantity for defining an “allowable duration” of a specific segment of an exploration-class mission. For example, a metric such as permissible segment duration (PSD) could inform prospective crew and mission planners pre-flight, and Flight Directors or other decisionmakers in-flight, of the maximum duration that a specific phase of a specific mission could be extended before each crewmember’s cumulative exposure exceeds the career exposure limit. Similarly, the impact of a potential SPE (or multiple SPEs) on a BLEO mission could be expressed in terms of the reduction in PMD from a reference particle event.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

M. Shavers was supported by the NASA Human Health and Performance Contract with NASA Johnson Space Center. V. Shurshakov was supported by the Program for Basic Research of the Russian Academy of Sciences, project no. 65.2.

Appendix A. ICRP and NASA NSCR-2012 tissue/organ weights used for estimation of effective dose

For ISS operations, NASA’s Space Radiation Analysis Group (SRAG) implements the NSCR-2012 cancer risk analysis using the Radiation Analysis Environment (RAE). The analysis uses sex-specific, environment-specific, and tobacco-smoking dependent tissue weights that were generated from organ-specific REIDs for uneven exposure conditions. Two sets of weights are used for environments that include significant GCR exposure (as is present on ISS and in free space) and environments where GCR does not contribute significantly to the response, such as that generated in the Earth’s radiation belts or by a solar particle event (NSCR-2012; 2013). Table A.1 shows the tissue/organ weights specified by ICRP-103 and by NASA for environments that included substantial amount of GCR exposure, and otherwise. When GCR contributes a substantial amount of dose, NASA’s lower quality factor for leukaemia mortality necessitates the redistribution of tissue weights.

Table A1.

ICRP and NASA NSCR-2012 tissue/organ weights used for estimation of effective dose for ISS missions.

Effective dose is calculated as the sum of the products of tissue/organ weight and organ dose equivalent. An example for a moderately shielded BLEO mission is shown in Fig. A.1.

Figure A1.

The relative contribution of each tissue/organ to effective dose is shown for an example 30-day long BLEO mission. Uneven dose equivalent, H_T, for tissues and organs during a BLEO mission are amplified by NASA’s male or female w_T.