System of radiological protection: Towards a consistent framework on Earth and in space

Introduction

Radiological protection of astronauts in space requires a comprehensive approach involving numerous scientific disciplines. Depending on the length of a space mission, both stochastic effects and harmful tissue reactions (deterministic effects) may pose risks and should be considered. For short- and medium-term missions lasting up to about one year, such as traveling to and staying at space stations, organ equivalent doses are well below the dose thresholds usually assumed for tissue reactions if the galactic cosmic radiation (GCR) and trapped radiation in the magnetosphere (i.e., in Van Allen Belts) are the only radiation sources. For example, for a mission involving 30 days in space between the Earth and the Moon, effective doses from GCR within a space vehicle with light shielding would be between 20 and 30 mSv (depending on shielding and calculation method). The equivalent dose to any organ would not exceed the threshold for tissue reaction [1]. In contrast, should a Solar Particle Event (SPE) occur, radiation exposure can be considerably higher with effective doses exceeding 500 mSv; equivalent doses to specific organs may be even higher [1]. Besides radiation-related health effects typically considered in current radiological protection on Earth, additional aspects may be relevant for space due to the specific and complicated tasks astronauts have to fulfill and the specific workplace they encounter. These aspects include, for example, space-related health effects resulting from demanding job requirements, such as the existence of multiple stressors (e.g., isolation, microgravity), and logistical and administrative hurdles, such as limited health care (with no immediate hospitalization possible and limited specialized medical treatment available), and no possibility of replacement. For these and other reasons, various health effects are of concern, including those that occur on a relatively short time scale and may affect mission success, but also those that occur in the long term and may affect the quality of life for astronauts upon return and retirement. Consequently, health effects of interest include, for example, acute and chronic radiation syndromes, diseases of the circulatory system, effects on the lens of the eye, effects on the central nervous system, and effects on the skin, as well as stochastic effects such as solid cancer and leukemia. Female astronauts of child-bearing age may have concerns about reproductive health issues.

As a further complication, the mixed-radiation field outside and within a space vehicle is of particular complexity involving not only low-linear energy transfer (LET) radiation such as gamma radiation, electrons, and positrons, but also high-LET radiation such as neutrons and heavy ions. The components of this mixed radiation field include a wide span of particle energies from very low energies (e.g., thermalized neutrons with energies in the range of several tens of meV) to very high energies (beyond GeV). Furthermore, compared to MeV gamma radiation, these particles may show very different relative biological effectiveness (RBE) depending on particle type and energy. The quantitative and even qualitative risks of exposure to the combined impact of a complex radiation environment, microgravity, and other stressors remain unclear. There are concerns that more aggressive or less treatable tumors could be initiated, or that oxidative stress may lead to cardiovascular or central nervous system effects.

Finally, even if all of the scientific aspects related to risk and dose assessment for astronauts were known, incorporating them into a consistent radiological protection framework would be challenging. For example, the following questions need to be answered:

• •Which radiation-induced health effects should be considered?
• •What dose quantities are the best for the radiological protection of astronauts?
• •Which metrics should be used to quantify radiation-related health risks?
• •How to address sex and age differences in radiation risk?
• •What kind of protection criteria should be applied?
• •How to decide on the tolerability of radiation-induced risks, given that astronauts are exposed to many other occupation-related risks?
• •How to deal with the fact that increased health risks due to radiation exposure may persist after an astronaut's career ends?
• •How to communicate radiation risk and make a comparison with other health hazards in a meaningful way?
• •How to harmonize national radiological protection guidelines, given that there might be different subpopulations with different levels of risk tolerance?

For these and other reasons, it is evident that the development of a framework of radiological protection is needed, which builds on current knowledge and experience of radiological protection on Earth, takes into account the specific circumstances of exposure to cosmic radiation in space, and contributes towards an approach of radiological protection of astronauts that is harmonized across space agency practices. This is of particular importance given that space agencies are currently using essentially different approaches, for example, in calculating radiation-related risks and dose limits in space [2], [3], [4], [5], and “non-occupational” exposures to space tourists are looming.

Consequently, in 2018, the International Systems Maturation Team (ISMT-Radiation), including space agencies operating the International Space Station (ISS), asked the International Commission on Radiological Protection (ICRP) to assess the potential role of relevant radiation-related health effects, including solid cancer and leukemia, as well as effects on the central nervous system, circulatory system, and lens of the eye. These effects have been reviewed by various space agencies in several papers, e.g. [6], [7]. For some of these effects, the role of ionizing radiation is still not yet fully understood, and it is unclear in which category of health effects they fall: “tissue reactions” or “stochastic effects.” Questions relating to this are whether there is a dose threshold for such health effects and, if not, what the dose-response curve at doses relevant to the radiological protection of astronauts is. In response to the ISMT-Radiation request, the ICRP established Task Group 115 (TG115), “Risk and Dose Assessment for Radiological Protection of Astronauts”, in 2019. Required expertise included, among others, radiobiology, radiation physics, medicine, and space flight missions.

With the goal in mind of maturing––if not harmonizing––ICRP guidance to provide an appropriate level of protection, this paper briefly summarizes the System of Radiological Protection and its ethical basis, which has been continuously developed by the ICRP. It describes how crosstalk between currently active ICRP Task Groups may provide input to the development of a radiological protection system for human space activities (for more information, see the ICRP website (www.icrp.org)). We anticipate commonality in radiation protection needs for human spaceflight and terrestrial settings that can improve radiation protection practices in either or both directions.

Specifically, this contribution also discusses issues that at first glance might be considered specific to radiological protection in space, but at second glance may also contribute to improving radiological protection for specific settings on Earth, such as individualized medical care for patients or workers, based on sex, age, and other factors. In reverse, terrestrial practices can inform space radiation protection practices in future scenarios, for example, how high transient exposures are managed. In space, the potential for brief “contingency exposure” to an SPE is managed through mission design, configured shielding, and restricted activities. In contrast, on Earth, there are events, such as designated occupational settings, where controlled and monitored “high dose rate” work is carried out. Such a way of managing special exposure situations could inform best radiation practices in space. In both settings, the judgment of justification may require considerations that are specific to an individual or that are broadly crucial to a nation or even more broadly to humanity.

While this paper focuses on the basic concepts of the ICRP System of Radiological Protection, which might be of relevance for radiological protection in space, three accompanying papers in this issue describe the work of TG115 that focuses more specifically on space-related issues, including a summary of space agency-specific standards for crew dose and risk assessment of ionizing radiation exposures for the International Space Station [2], a comparison of doses and risks estimated for example missions beyond low-Earth orbits [1], and time-integrated radiation risk metrics that can be used for evaluation of radiation-related cancer risk in space [3].

The system of radiological protection

The ICRP System of Radiological Protection (the “System”) has been developed for radiological protection on Earth [8]. So far, the radiological protection of astronauts in space is not explicitly addressed by the System. Exposures due to human activities in space are different in many ways as compared to those on Earth. Nevertheless, many critical elements of the System are relevant for radiological protection in space, although the specificities of the space environment must be considered. These key elements are briefly described below. On the other hand, regulations yet to be developed for the radiological protection of astronauts may also inform those implemented on Earth.

Three pillars – science, ethics, experience

Science

The System is informed by scientific evidence on the biological effects of ionizing radiation on biomolecules such as DNA, cells, tissues, and organisms, including plants, animals, and humans. Relevant endpoints include, for example, tissue reactions such as opacification of the lens of the eye or hematopoietic syndrome, and stochastic health effects such as solid cancer, leukemia, and heritable effects on offspring. More recently, non-cancer diseases, such as diseases of the circulatory system, have attracted additional attention. In parallel, techniques have been developed to simulate radiation transport and dosimetry of various radiation qualities. Furthermore, the scientific basis of the System also includes, for example, biokinetics, environmental transport of radionuclides, and contributions from social sciences.

Ethics

While natural sciences may answer questions about phenomena, ethics helps to inform judgments on what should be done based on an understanding of the phenomena. Natural sciences and ethics are necessary to recommend the best protection for humans and the environment. The ethical values that underpin the System are based on doing good / avoiding harm, prudence, justice, and dignity. These core values form the System's foundation, and their universality across cultures, genders, religions, and political regimes helps make the System relevant to everyone, everywhere.

Experience

To be useful, the System needs to learn from experience to be widely accepted in society, translated into national legislation and applied in everyday practice by radiation protection professionals. It must also be comprehensive and flexible enough to address all situations where protective measures against radiation exposures are necessary.

Three key principles – justification, optimization of protection, and application of dose limits

Justification

The principle of “Justification” comes in the first place. Whenever a decision is made that alters the radiation exposure situation, it should do more good than harm.

Optimization of protection

Once exposure to ionizing radiation has been considered “justified”, the principle of “Optimization of Protection” comes into play. This principle requires that the likelihood of incurring exposure, the number of people exposed, and the magnitude of their doses should all be kept As Low As Reasonably Achievable (ALARA), considering economic and societal factors. The principle of “Optimization of Protection” should not be misinterpreted as minimization of doses.

Application of dose limits

The principle of “Application of Dose Limits” applies when exposures to workers or members of the public are planned in advance and under control. Specifically, the total dose to any individual from regulated sources in planned exposure situations other than medical exposure of patients should not exceed the appropriate limits specified by ICRP. It is important to note that even if exposure is below a dose limit, the principle of “Optimization of Protection” should still apply, i.e., the doses should be kept ALARA.

Three exposure situations – planned, emergency, existing

Planned exposure situation

“Planned exposure situations” have in common that there is a plan to introduce radiation sources. For example, the operation of a nuclear reactor may be associated with planned exposures of the workers in some sort of maintenance.

Emergency exposure situation

If loss of control of a radiation source or a nuclear facility or a malicious event involving radioactive material(s) occurs, an “emergency exposure situation” may result.

Existing exposure situation

An “existing exposure situation” is defined as a situation already existing when a decision on control of exposure must be made. This may include, for example, exposure to radon in houses or exposure of aircrew to cosmic radiation at altitude.

Three categories of exposure – public, occupational, medical

The three categories of exposure introduced by ICRP (public exposure, occupational exposure, and medical exposure) can be experienced by a single individual.

Public exposure

All members of the public are exposed in one way or another to natural sources of ionizing radiation (e.g., from radon, cosmic radiation, terrestrial radiation). The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) suggests that most exposures are likely to be in the range of 1–13 mSv per year [9]. Members of the public may also be exposed to radiation from nearby industrial activities, visiting medical facilities, etc.

Occupational exposure

In its 2020/2021 report to the General Assembly, UNSCEAR estimated radiation exposure to approximately 24 million workers from 2010–2014. About 52% of those were employed in sectors that involve exposure to natural sources of radiation, and about 48% were employed in sectors that involve exposure to human-made sources of radiation. The worldwide average annual effective dose for all workers was estimated to be around 1.2 mSv. The annual effective dose was estimated to be around 2.0 mSv for workers exposed to natural sources and 0.5 mSv for workers exposed to human-made sources [10].

Medical exposure

Medical exposure remains by far the largest human-made source of radiation exposure of the population. According to UNSCEAR, in the period 2009–2018, about 4.2 billion medical radiological examinations were performed annually. The collective effective dose was estimated to be 4.2 million person sieverts for the global population of 7.3 billion people, resulting in an annual effective dose per caput of 0.57 mSv (excluding radiotherapy) [11].

Conceptually, effective doses from various radiation sources are additive. It may, therefore, well be that an aircrew member (pilot or cabin crew member) may be occupationally exposed to an average of 2 mSv per year from cosmic radiation as a worker, another 2 mSv per year from natural sources such as radon as a member of the public, and another 2 mSv from medical exposures as a patient. Consequently, this hypothetical individual would be exposed to an annual effective dose of about 6 mSv.

Three radiation dose quantities – absorbed dose, equivalent dose, effective dose

Radiation dose quantities are, for example, required for setting dose limits to prevent tissue reactions or setting dose limits, dose constraints, or reference levels to protect against the occurrence of stochastic effects. Absorbed, equivalent, and effective doses are the primary dose quantities used in radiological protection.

Absorbed dose

Absorbed dose is measurable and defined as the mean energy imparted in matter with the SI unit of J/kg, which has the special designation Gray (Gy). When the absorbed dose is used in practical protection applications, doses are averaged over tissue volume.

Equivalent dose

Equivalent dose represents an intermediate step in calculating the effective dose from the absorbed dose. It is defined as the product of the average absorbed dose in the volume of a specified organ or tissue and the dimensionless radiation weighting factor of the incident radiation. The radiation weighting factor reflects the higher biological effectiveness of high-LET radiations compared with low-LET radiations. The unit for equivalent dose is J/kg, and it has the special name sievert (Sv).

Effective dose

Effective dose is the protection quantity that is most often used in radiological protection. Effective dose is calculated for the whole body. It is the sum of equivalent doses to all organs, multiplied by dimensionless tissue weighting factors, which represent the sensitivity of each organ to radiation. The unit for effective dose is J/kg, with the special name sievert (Sv).

Three major radiological protection concepts – effective dose, LNT, detriment

Over the decades, ICRP has developed several concepts or tools to support the system of radiological protection.

Effective dose

The concept of “effective dose” developed by ICRP involves a risk-adjusted dosimetric quantity used to manage protection against stochastic effects, mainly cancer. It provides single values that relate to stochastic risk averaged over all organs and tissues as sites of radiation-induced cancer for both sexes, all ages, and different populations worldwide. It is widely used in radiological protection practices enabling comparison of estimated doses with dose limits, dose constraints, and reference levels expressed in the same quantity and set by regulatory authorities and standards organizations. The use of effective dose allows all radiation exposures from external and internal sources to be considered together and summed, relying on the assumptions of a linear non-threshold (LNT) dose-response relationship, the equivalence of acute and chronic exposures at low doses or low dose rates, and equivalence of external and internal exposures [12].

The LNT concept

The linear no-threshold (LNT) concept assumes that the risk of excess cancer and/or heritable disease will increase with radiation dose in a proportionate manner. The LNT dose-response is not universally accepted as biological truth. However, since it is not actually known what level of risk is associated with very low-dose exposure, it is considered by ICRP to be a reasonable concept or tool to be used for the practical system of radiological protection. For effective doses below about 100 mSv, ICRP recommends that a given dose increment will produce a proportionate increment in the probability of incurring cancer or heritable effects attributable to radiation [8].

Detriment

Radiation detriment is a concept developed by ICRP for radiological protection purposes. It is determined from sex-averaged and age-at-exposure averaged lifetime cancer risk estimates for a set of organs and tissues and the risk of heritable effects, taking into account the severity of the consequences of cancers (lethality, years of life lost, and compromised quality of life under non-lethal conditions) [13]. Total radiation detriment is the sum of the detriment for cancers and heritable effects. The values of radiation detriment should not be considered as projections of the absolute number of cancer or heritable disease cases in a population, but as inferences based on reasonable assumptions for radiological protection.

The ethical values behind the system of radiological protection

In its Publication 138 [14], ICRP described many ethical core and procedural values that underpin the System. These ethical values are considered fundamental across cultures. which is particularly important because the System should be broadly acceptable and applicable worldwide. In the context of this paper, ethical considerations may be helpful because quite a number of ethical questions arise regarding the radiation exposures of astronauts (see further below).

Four core values – beneficence/non-maleficence, prudence, justice, dignity

The core values identified by ICRP are beneficence/non-maleficence, prudence, justice, and dignity. Beneficence means promoting or doing good, while non-maleficence means avoiding the causation of harm. Prudence is the ability to make informed and carefully considered choices without fully knowing the scope and consequences of actions. Justice is usually defined as fairness in the distribution of advantages and disadvantages among groups of people (distributive justice), fairness in compensation for losses (restorative justice), and fairness in the rules and procedures in the processes of decision-making (procedural justice). Finally, dignity is an attribute of the human condition. Personal autonomy is a corollary of human dignity.

Three procedural values – accountability, transparency, inclusiveness

The procedural values identified by ICRP are accountability, transparency, and inclusiveness. Accountability means that those in charge of decision-making must be answerable for their actions to all those who may be affected by these actions. Transparency is also part of implementing the value of procedural justice. It concerns the fairness of the process by which information is shared intentionally between individuals and/or organizations. Finally, inclusiveness is usually referred to as ‘stakeholder participation,’ which is how the value is applied in practice.

Relationship between the key principles of the system and the core ethical values

The principle of justification, which implies that a decision that alters radiation exposure should do more good than harm, is supported by the ethical value of beneficence/non-maleficence. The optimization principle is informed by prudence. For example, this becomes evident, when considering that optimization is often applied at rather low doses of radiation where uncertainties of the scientific evidence on radiation-related health effects are large or where such evidence does not exist. Instead, one must rely on the extrapolation of knowledge obtained at higher doses by applying the LNT concept. With restrictions on individual exposures to limit inequity between individuals and the need to account for the views and concerns of stakeholders, justice and dignity come into play. Finally, the principle of application of dose limits, which implies that the radiation dose of any individual should not exceed the level of exposure considered tolerable for the exposure situation under consideration, relates to beneficence/non-maleficence, justice, and dignity. While each principle of the System is related to more than one core value, it is noted that the principle of application of dose limits is mainly related to justice.

Issues that make radiological protection in space special

In ICRP Publication 123, which deals with the assessment of radiation exposure of astronauts in space, the Guest Editors raised the question, “are astronauts occupationally exposed in the ICRP sense, and are they subject to the traditional principles of the ICRP system of radiological protection (i.e., justification, optimisation, and limitation)?” [15]. Space inhabitants live 24-hours a day in space whether they are “on duty” or not. Other questions may be raised, for example, whether the dose quantities typically used in radiological protection on Earth are appropriate, given that it is “difficult to imagine a class of radiation exposure that is more complex in terms of its radiation fields, and more challenging in terms of dose assessment, than that presented to astronauts by the radiation environment of space.” [15]

In fact, one of the conclusions of ICRP Publication 123 was that “not all concepts of quantities defined for radiological protection applications on Earth are appropriate for applications in space missions, especially when risk assessment is an important task. A radiation weighting factor (w_R) of 20 for all types and energies of heavy ions in the definition of equivalent dose is not justified.” Furthermore, operational dose quantities of area monitoring in space were not recommended. Still, measurement of radiation fluence rates, energy distributions of relevant particles, and the corresponding linear energy transfer distributions were recommended [15].

For occupational exposure on Earth, ICRP recommends an annual limit of 20 mSv effective dose, averaged in a defined 5-year period, with no annual dose in a single year exceeding 50 mSv [8]. This approach would require significant modifications, given that long-term exploratory space missions, such as to Mars, will be associated with radiation doses exceeding such a dose limit [2]. Further, astronauts usually have careers of much less than 50 years and begin much later than 18 years of age. Such conditions require consideration of higher mission limits that may approach concern for early or late tissue effects in more sensitive individuals. Thus, in this context, a critical question that may arise is about the application of the limitation principle when such a situation arises. One may even question whether a dose-based or a risk-based limit should be applied and whether a threshold system is appropriate. These considerations demonstrate that alternative approaches may be needed to deal with the increased radiation-related risks to which astronauts are exposed.

For example, over the previous decade, NASA has opted for a risk-based, rather than a dose-based, approach to limiting the exposure of its astronauts. Specifically, career doses for NASA astronauts should not exceed a risk of exposure-induced death (REID) of 3% at a 95% confidence level [16]. Exposure up to that career limit means accepting the calculated risk that three out of 100 astronauts would die at some point in their lifetime from cancer due to the radiation they had received during their space missions. Because radiation-related risks are somewhat different for males and females, this approach implied different career dose limits and, thus, different permissible mission durations for males and females, which called into question the desired equity of job opportunities.

Consequently, NASA has recently developed an alternative approach and proposed a new “standard”. This standard is based on a REID of 3% calculated for cancer mortality in the most vulnerable group of astronauts––35-year-old females––(it is a general understanding that females are more sensitive to radiation than males, younger females are more sensitive than older females, and astronauts younger than 35-year-old are unlikely). In terms of effective dose, this translates into a career limit of 600 mSv, which applies to both males and females. However, long-duration space missions imply the possibility of mission doses being higher than the proposed standard of 600 mSv. This means that, strictly speaking, the 600 mSv standard is not a “dose limit” in the sense practiced on Earth and that some sort of exception (waiver) must be implemented to deal with possible overexposures exceeding such standards. This new NASA approach was reviewed and agreed upon by a committee of the US National Academies of Sciences, Engineering, and Medicine (NASEM). The committee recommended that the required exception (waiver) should be rare, “judicious, transparent, and informed by ethics” and that NASA should “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.” [17] The NASEM committee found that “If a human spaceflight mission cannot meet current health standards, or if inadequate information exists to revise a health standard,…” that it would not be ethically acceptable to “… (1) liberalize the NASA health standards, (2) establish more permissive “long duration and exploration health standards,”…”.

It is worth mentioning that on Earth, the System developed by ICRP does not include any systematic differentiation between recommendations on limits for males and females. For example, a single occupational dose limit for the annual effective dose of 20 mSv, averaged over 5 years, is recommended by ICRP (although it is well known that there are individual differences in radiation sensitivity between males and females) due to legal and ethical considerations. However, some stratification elements are included in the System; for example, special recommendations are made for pregnant women [8].

For exposures of humans to cosmic radiation in space, one might argue that such exposures should be considered as an existing exposure situation, as cosmic radiation already exists when a decision on control must be taken. This is consistent with the fact that ICRP has recommended that aircrew exposure be considered as an existing exposure situation [18]. In practice, however, others have decided to treat aircrew exposure as a “planned exposure situation” and apply dose limits as in other occupational settings [19]. In space, emergency exposure situations might happen; for example, should a Solar Particle Event occur on the Sun, high energy particles, including protons and helium ions, could strike a space vehicle and result in high doses and dose rates to astronauts within the vehicle. Clearly, this situation would require immediate action, such as the configuration of provisional shielding for the vehicle occupants.

Another example where there might be differences between radiological protection on Earth and in space relates to the health consequences of radiation exposure considered relevant. Currently, the detriment is one of the key concepts or tools in the radiological protection toolbox used by ICRP (see above). Although the detriment includes a number of parameters not related to exposure to ionizing radiation (cancer lethality, years of life lost, quality of life), in terms of radiation-related health effects, the major contributors to detriment are solid tumors and, to a lesser extent, leukemia and heritable effects, all of which are classified by ICRP as “stochastic” effects. The radiation-related risk considered tolerable for occupational exposures is informed by occupational risks typically encountered in other professions. With the approximated overall fatal risk coefficient of 5% per Sv, ICRP recommended a dose limit of 20 mSv per year, averaged over 5 years of exposure [8], [20]. However, more similar to “less safe” terrestrial occupations (which are becoming safer over time), spaceflight activities impose additional non-radiation-related risks to the participants, and those risks should be considered in the overall risk assessment.

In space, in addition to solid cancers, leukemia, and heritable effects, other radiation-related health effects are significant and need to be considered to guarantee mission success.

The abovementioned aspects provide only a glimpse of the numerous issues relevant to radiological protection in space and their potential relevance to radiation protection on Earth. However, they may demonstrate that various aspects must be considered, including purely scientific issues, ethical considerations, and practical aspects. Clearly, addressing all these challenges in detail would be beyond the scope of TG115. Fortunately, many currently active ICRP task groups deal with topics of high relevance for radiological protection in space that could inform TG115 in its efforts to develop a comprehensive framework for risk and dose assessment for the radiological protection of astronauts. This is also relevant for space tourism. At the same time, some of these task groups will also benefit from the experience gained through the work of TG115. Some of these task groups are mentioned below based on the Terms of Reference defined by the ICRP Main Commission when these task groups were established.

Relevant active ICRP task groups

Protection of astronauts from cosmic radiation during space missions requires consideration of many aspects. For example, characterizing the involved radiation fields and quantifying radiation doses to astronauts is challenging since cosmic radiation includes many radiation qualities at a vast range of energies. ICRP Publication 123 Assessment of Radiation Exposure of Astronauts in Space has already addressed some aspects of radiological protection of astronauts in space [15]. This publication focused on the challenges in dosimetry related to the radiation fields in space, which differ from those on Earth. In addition, it covered the radiation environment in space, quantities used in radiological protection, measurement methods, radiation fields inside spacecraft and on planetary surfaces, radiation fields and doses in the human body, and a short section on operational radiological protection.

Because radiation doses to astronauts span a wide range of values, mainly depending on the type and duration of the involved space missions, relevant radiation-related health effects include tissue reactions and stochastic effects. Thus, the work of TG115 must be informed by many disciplines in natural sciences, including physics, material sciences, biology, medicine, epidemiology, etc. Furthermore, the possibility of relatively high mission doses, the specific working conditions within a relatively small working environment (i.e., the space vehicle), and the presence of other stressors and sources of risk whose interactions with ionizing radiation are largely unknown pose many additional questions that cannot be answered by natural sciences alone but would require further inputs from disciplines among the social sciences and humanities.

Fig. 1 provides an overview of those Task Groups working on topics relevant to TG115. The Terms of Reference defined by the ICRP Main Commission for each Task Group are provided on the ICRP website (www.ircp.org).

Figure 1.

TG 115 and its relation to other existing ICRP Task Groups (see www.icrp.org for more details).

Summary

In summary, the ICRP system of radiological protection has guided the development and implementation of national and international standards and regulations on radiological protection of mainly workers, the public and patients, and the environment from exposure to radiation on Earth, in the past for more than 90 years. For radiological protection of astronauts, who are exposed to cosmic radiation in space as well as to the associated secondary radiations generated in spacecraft and in the human body, ICRP published its Publication 123 in 2012 and established its TG115 in 2019 to further address topics related to dose and risk assessment. Collaborations between TG115 and many other ICRP task groups are essential; these task groups may provide input in developing future radiological protection for human space activities, and in turn, the work of TG115 may also help inform radiological protection on Earth.

In this special issue on space radiation research, the reader will find various publications ranging from measurements and dosimetry of space radiation exposure to health effects and risks associated with such exposure. Among these publications, three describe work done within ICRP TG115 addressing space agency standards for dose and risk assessment of ionizing radiation exposures for the International Space Station (ISS) crew [2], a comparison of dose and risk estimates between ISS partner agencies for a 30-day lunar mission [1], and the impact of interpopulation variability of survival curves on the values and uncertainty of the estimates of the time-integrated radiation risk of cancer [3].

Further papers on space radiation measurements and dosimetry include those on ground-based passive generation of Solar Particle Event spectra [21], a description of the ESA Active Dosimeter (EAD) system on board the ISS [22], a Monte Carlo model for ion mobility and diffusion for characteristic electric fields in nanodosimetry [23], and a Geant4-DNA Monte Carlo study of radium deposition in human brain tissue [24].

Space radiation effects and risk assessment represent additional very important aspects when it comes to space travel, in particular for long-term exploration missions. Consequently, this special issue also includes studies on light flashes and other sensory illusions perceived in space travel and on the ground [25], application of multi-method-multi-model inference to radiation-related solid cancer excess risk models [26], radiation-related cancer risk assessment using dosimetric calculations of organ dose equivalents [27], and evaluation of Parkinson's disease mortality among U.S. radiation workers and veterans [28].

All these publications in this special issue will be useful to the work of ICRP TG115 to develop a consistent framework of radiological protection in space and on Earth.