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Introduction

Human spaceflight is inherently risky, with numerous potential hazards posed at each phase of a mission, including launch, inflight during the mission, and landing. Potential health risks during spaceflights include short-term health consequences from being in microgravity (e.g., nausea, blurred vision), as well as long-term health consequences that arise, or continue, months or years after a flight (e.g., radiation-induced cancers, loss of bone mass). Additional health considerations are risks posed by exposure to environmental contaminants onboard spacecraft. Because the International Space Station and spacecraft are closed environments, some contamination of the air and water will occur. Even with onboard air and water purification systems, chemicals will accumulate in the air and water supplies as they recirculate or are recycled onboard. Therefore, it is necessary for the National Aeronautics and Space Administration (NASA) to identify hazardous contaminants and determine exposure levels that are not expected to pose a health risk to astronauts.

The National Academies of Sciences, Engineering, and Medicine have a long history of supporting NASA in the setting of chemical exposure guidelines. This effort began in 1968 with the National Research Council (NRC) Space Science Board’s Panel on Air Standards for Manned Space Flight, which provided guidance on provisional guidelines for 39 chemicals (NRC 1968). In 1972, NASA requested that another panel be formed to review those exposure guidelines and to set new guidelines where appropriate. The new guidelines were necessary to provide engineering benchmarks to guide the development of advanced life-support systems for long-duration space missions. The Panel on Air Quality in Manned Spacecraft established 1-hour, 90-day, and 6-month exposure guidelines for 52 compounds (NRC 1972).

In 1990, the NRC and NASA resumed the effort to set chemical exposure guidelines in anticipation of launching a manned space station. Because several hundred atmospheric chemicals would likely be found in the complex, closed environment of the space station, an understanding of how contaminants are generated and the concentrations that are likely to pose a health hazard to crew members was needed to design the trace contaminant control system. An NRC committee developed methods for determining spacecraft maximum allowable concentrations (SMACs) for airborne contaminants, which included guidance on

the types of toxicologic information to consider and methods for calculating appropriate exposure guidelines (NRC 1992). NASA subsequently used those methods to derive SMACs for numerous compounds (see Table 1-1), and its documents were reviewed by the committee to ensure that they had been developed in accordance with the methods and were scientifically justified. SMACs are defined as the maximum concentration of airborne substances (e.g., gas, vapor, or aerosol) that will not cause adverse health effects, significant discomfort, or degradation in crew performance. SMACs are classified into short-term (1 or 24 hours) and longer-term (7, 30, 180, and 1,000 days) durations. The 1- and 24-hour SMACs are to be used in emergency situations, such as accidental spills or fire. Temporary discomfort is permissible as long as there is no effect on the ability to respond to the emergency. The longer-term SMACs are intended to avoid adverse health effects (either immediate or delayed) and to avoid degradation in performance of crew after continuous exposure for as long as 1,000 days. The need for a 1,000-day SMAC was introduced by NASA in the early 2000s. Five volumes of SMAC documents and guidelines were published between 1994 and 2008 (NRC 1994, 1996a, 1996b, 2000a, 2008a).

In 2000, a similar procedure was used to begin establishing spacecraft water exposure guidelines (SWEGs). To provide a space crew with an adequate water supply, it is necessary to recycle spacecraft wastewater during long-duration flights. Water can be recovered onboard from sources such as humidity condensate, used hygiene water, and urine, and controls are needed to prevent contaminants from reaching concentrations that might pose a health risk to astronauts. Thus, at the request of NASA, an updated set of methods for establishing exposure guidelines was developed, focusing on special considerations for water contaminants (NRC 2000b). Like SMACs, SWEGs are set for short-term (1 day) and longer-term (10, 100, and 1,000 days) durations. The 1-day SWEG is a concentration of a substance in water that is judged to be acceptable for the performance of specific tasks during rare emergency conditions lasting for periods up to 24 hours. The 1-day SWEG is intended to prevent irreversible harm and degradation in crew performance. Temporary discomfort is permissible as long as there is no effect on judgment, performance, or ability to respond to an emergency. Longer-term SWEGs are intended to prevent adverse health effects (either immediate or delayed) and degradation in crew performance that could result from continuous exposure in closed spacecraft for as long as 1,000 days. In contrast with the 1-day SWEG, longer-term SWEGs are intended to provide guidance for exposure under what is expected to be normal operating conditions in spacecraft, and includes consideration of the taste and smell of the water. The exposure durations of the guidelines for water differ from those of air because exposure to water is more intermittent than is exposure to air and because it is possible to refrain from drinking or using contaminated water for short periods in an emergency. NASA developed SWEGs for numerous compounds (see Table 1-1), which were reviewed by the committee and published in three volumes between 2000 and 2008 (NRC 2004, 2007, 2008b).

TABLE 1-1 Chemicals with SMAC or SWEG Guidelines

SMACs and SWEGs are established for use by the US space program, so they are designed for astronauts who have been medically screened and have undergone rigorous testing. Thus, the guidelines are based on the understanding that the astronaut population consists of healthy adults with no preexisting medical conditions (e.g., asthma).

THE PROCESS OF DEVELOPING AND REVIEWING SMACS AND SWEGS

The following process has been used by NASA and previous NRC committees to establish SMACs and SWEGs. NASA identifies the air and water contaminants of concern to its spaceflight program and determines which chemicals should undergo a comprehensive assessment to establish SMACs or SWEGs. NASA staff and contractors conduct literature-based toxicologic assessments of the selected chemicals, which involve the performance of literature searches, summarization of the literature, and selection of relevant studies from which to derive exposure guidelines. Acceptable exposure concentrations are calculated for health end points of concern, and exposure guidelines are proposed. NASA’s assessments are presented at NRC committee meetings, and the committee’s review and recommendations are documented in interim reports. If substantive changes are required that could affect the proposed SMACs or SWEGs, the committee reviews these changes at subsequent meetings. After the committee has approved a SMAC or SWEG document, it is published. The same process will be used for updating SMACs and SWEGs and for establishing exposure values for new chemicals.

ADVANCEMENTS IN RISK ASSESSMENT

Improved approaches for performing literature-based toxicologic assessments to support risk assessment have been outlined in several NRC reports (e.g., NRC 2009, 2011, 2014). These improvements are directed at programs of the US Environmental Protection Agency for the purpose of ensuring public health protection, so they have been developed for purposes outside of NASA’s purview and many aspects are not appropriate for setting SMACs and SWEGs (e.g., protecting the health of children). However, certain themes are relevant to NASA, such as the need for transparency and the importance of incorporating biologically-based, mode-of-action, and quantitative approaches into assessments as much as possible. The use of quantitative approaches over qualitative assessments is encouraged when estimating exposure guidelines, as mathematical models and statistical analyses can now be used at various steps of the risk assessment process, such as analyzing dose-response data to estimate doses associated with a low level of response, to quantify species differences, or to pool data from multiple studies. Transparency is an overarching aspect of performing toxicologic assessments, because it is critical that an assessment is understandable, that enough information is presented so that it would be possible to reproduce the assessment, that modeling approaches and assumptions are supported, and that departures from default approaches are adequately justified.

STATEMENT OF TASK

In light of updated approaches to conducting toxicologic assessments, NASA requested that an ad hoc committee be convened to assist the agency with developing spacecraft exposure guidelines for individual air and water contaminants. The committee will build on and update the Academies’ previous work for NASA on developing SMACs for air contaminants and SWEGs for water contaminants. The committee was asked to perform the following tasks:

• Update the guidelines and methods for developing SMACs and SWEGs.
• Assist NASA with identifying chemicals that need updated SMACs or SWEGs and new chemicals for which SMACs or SWEGs should be developed.
• Review the scientific basis of NASA’s proposed SMACs and SWEGs and ensure they have been developed in accordance with the updated guidelines.

This report addresses the first two tasks of updating the guidelines and methods for developing exposure guidelines for use on spacecraft and outlining procedures that can be used for choosing chemicals to evaluate. NASA will use the updated methods to reevaluate some of the existing exposure values and to develop SMACs and SWEGs for new chemicals, and the committee will subsequently review the proposed exposure guidelines to ensure that they were derived in accordance with current risk assessment practices. The updated SMACs and SWEGs will be published in future reports.

APPROACH TO THE STUDY

The Academies convened the Committee on Spacecraft Exposure Guidelines in 2015. Members of the committee have expertise in general toxicology, inhalation toxicology, neurotoxicology, toxicokinetics, mechanisms, industrial hygiene, occupational health, and risk assessment. Two public meetings were held to familiarize the committee members with the original methods used by NASA to develop SMACs and SWEGs, and to gather other information relevant to updating the assessment methods. The committee focused on identifying refinements that could be made to NASA’s existing procedures that would bring them more in line with advances that have been made in risk assessment.

ORGANIZATION OF THE REPORT

This report is organized into the Introduction, two additional chapters, and two appendixes. Chapter 2 outlines the process of deriving SMACs and SWEGs and identifies refinements in risk assessment practices that NASA should begin to use in updating its existing guidelines and in deriving guidelines for additional

chemicals. Chapter 3 reviews approaches to selecting and ranking contaminants for assessment. Appendix A provides biographical information on the committee members, and Appendix B provides examples of report outlines that might be used for organizing future SMAC and SWEG documents.

REFERENCES

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Meyers, V.E., H.D. Garcia, T.S. McMullin, J.M. Tobin, and J.T. James. 2013. Safe human exposure limits for airborne linear siloxanes during spaceflight. Inhal. Toxicol. 25(13):735-746.

NRC (National Research Council). 1968. Atmospheric Contaminants in Spacecraft. Washington, DC: National Academy of Sciences.

NRC. 1972. Atmospheric Contaminants in Manned Spacecraft. Washington, DC: National Academy of Sciences.

NRC. 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, DC: National Academy Press.

NRC. 1994. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 1. Washington, DC: National Academy Press.

NRC. 1996a. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 2. Washington, DC: National Academy Press.

NRC. 1996b. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 3. Washington, DC: National Academy Press.

NRC. 2000a. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 4. Washington, DC: National Academy Press.

NRC. 2000b. Methods for Developing Spacecraft Water Exposure Guidelines. Washington, DC: National Academy Press.

NRC. 2004. Spacecraft Water Exposure Guidelines for Selected Contaminants, Volume 1. Washington, DC: The National Academies Press.

NRC. 2007. Spacecraft Water Exposure Guidelines for Selected Contaminants, Volume 2. Washington, DC: The National Academies Press.

NRC. 2008a. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 5. Washington, DC: The National Academies Press.

NRC. 2008b. Spacecraft Water Exposure Guidelines for Selected Contaminants, Volume 3. Washington, DC: The National Academies Press.

NRC. 2009. Science and Decisions: Advancing Risk Assessment. Washington, DC: The National Academies Press.

NRC. 2011. Review of the Environmental Protection Agency’s Draft IRIS Assessment of Formaldehyde. Washington, DC: The National Academies Press.

NRC. 2014. Review of EPA’s Integrated Risk Information System (IRIS) Process. Washington, DC: The National Academies Press.

Ramanathan, R., J.T. James, and T. McCoy. 2012. Acceptable levels for ingestion of dimethylsilanediol in water on the International Space Station. Aviat. Space Environ. Med. 83(6):598-603.