CONSTRUCTION of the International Space Station (ISS) – a multinational effort – began in 1999. In its present configuration, the ISS is expected to carry a crew of three to six astronauts for up to 180 days. Because the space station will be a closed and complex environment, some contamination of its internal atmosphere and water system is unavoidable. Several hundred chemical contaminants are likely to be found in the closed-loop atmosphere and recycled water of the space station.
In 1992, the NRC provided NASA with guidelines for developing exposure guidance levels for airborne contaminants in Guidelines forDeveloping Spacecraft Maximum Allowable Concentrations (SMACs) for Space Station Contaminants (NRC 1992). That report provides guidance on the sources and types of data that should be used for establishing SMACs, approaches for performing risk assessments for carcinogenic and noncarcinogenic effects, and how to account for the effects of physiological changes induced by microgravity. SMACs have been established for 50 airborne contaminants using the NRC guidelines (NRC 1994; 1996a,b; 2000).
To protect space crews from contaminants in potable and hygiene water, NASA requested that the NRC develop guidelines, similar to those established by the NRC for airborne contaminants, for setting exposure guidance levels for spacecraft water contaminants. The NRC
was asked to consider only chemical contaminants, and not microbial agents. The NRC assigned this task to the Committee on Toxicology, and the Subcommittee on Spacecraft Water Exposure Guidelines, a multidisciplinary group of experts, was convened to develop guidelines for calculating exposure levels that will prevent adverse health effects and degradation in crew performance. These guidance levels are called spacecraft water exposure guidelines (SWEGs).
SWEGs are to be established for exposures of 1, 10, 100, and 1000 days. 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 1000 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.
SWEGs and SMACs differ from each other in two fundamental ways. The first is that SMACs are used for inhalation exposures, whereas SWEGs will be used for oral exposures. Second, the time scales used to set the guidance levels are different. SMACs were developed for 1-hr, 24-hr, 7-day, 30-day, and 180-day exposures, whereas SWEGs will be established for 1, 10, 100, and 1000 days. The reason for the difference is that exposure to water is more intermittent than is exposure to air and because it would be possible to refrain from drinking or using contaminated water for short periods in an emergency. Furthermore, there is a possibility that NASA could conduct missions that would last for up to 1000 days, so a long-term SWEG is needed.
Water used in NASA's space missions must be carried from Earth or generated by fuel cells. The water is used for drinking, food reconstitution, oral hygiene, hygienic uses (handwashing, showers, urine
flushing), and oxygen generation. Because of plans for longer space flights and habitation of the ISS, water reclamation, treatment, and recycling is required. Water for long space flights can be reclaimed from several onboard sources, including humidity condensate from the cabin, hygiene water (shower and wash water), and urine. Each of those sources will have a variety of contaminants. Humidity condensate will have contaminants released into the cabin from crew activities (e.g., by-products of crew metabolism, food preparation, and hygiene activities); from routine operation of the air revitalization system; from off-gassing of materials and hardware; from payload experiments; and from routine in-flight use of the crew health care system. Wash water will include detergents and other personal hygiene products. Urine contains electrolytes, small molecular weight proteins, and metabolites of nutrients and drugs. The urine is chemically treated and distilled before recycling, which causes a variety of by-products to be formed. Other sources of chemical contaminants include mechanical leaks, microbial metabolites, payload chemicals, biocidal agents added to the water to retard bacterial growth (e.g., silver, iodine), fouling of the filtration system, and incomplete processing of the water.
The possibility also exists that contaminants in the atmosphere can end up as toxic substances in the water system. The air and water systems of the ISS constitute a single life support system, and the use of condensate from inside the cabin as a source of drinking water could introduce some unwanted substances into the water system.
NASA's current water exposure guidelines are based on standards from the U.S. Public Health Service and the U.S. Environmental Protection Agency (EPA) for public drinking water. Those standards were established to protect the general public, including the elderly, persons with disabilities or compromised immune systems, and infants. Protecting sensitive individuals is necessary and appropriate for the safety of the public health, given the likelihood of lifetime exposures. However, exposure limits for the general public are not necessarily appropriate for spacecraft flight crews. Many of the limits are likely to be overly conservative – much stricter than would be necessary to protect healthy adult astronauts even for several years away from Earth. Other limits will be inadequate – microgravity, increased radiation, or other unique attributes of spaceflight could make astronauts more sensitive than are humans on Earth to a given contaminant. Moreover, water exposure guidance levels are not available for many contami-
nants that might be found in spacecraft water supplies. Data collected from space shuttle and Mir missions indicate that organic compounds found in processed water samples are vastly different from the list of target compounds developed by EPA for protection of public drinking-water supplies.
APPROACH TO THE STUDY
NASA briefed the subcommittee on the water reclamation systems for the ISS and its programmatic predecessors, and provided water-contaminant data collected from ground-based and in-flight tests. That information is presented in Chapter 2, and also includes a description of the treatment methods and monitoring strategies for spacecraft water. Using those data and the SMACs guidelines, the subcommittee considered the sources and types of data that should be used in establishing SWEGs. That assessment is provided in Chapter 3, with particular emphasis given to advancements made since the establishment of the SMACs guidelines in the areas of neurobehavioral toxicity, reproductive toxicity, mutagenicity, epidemiology, and toxicologic mechanisms. The subcommittee also considered available approaches for establishing exposure guidelines for waterborne contaminants. A review of those approaches and the subcommittee's recommendations for deriving SWEGs in provided in Chapter 4. That chapter also discusses ways to account for uncertainties associated with spaceflight, such as microgravity. Because it is not possible to conduct risk assessments on all the potential water contaminants, the subcommittee also considered prioritizing the contaminants for risk assessment. Approaches to ranking spacecraft water contaminants are provided in Chapter 5.
NRC (National Research Council). 1992. Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants. Washington, DC: National Academy Press.
NRC (National Research Council). 1994. Spacecraft Maximum Allowable
Concentrations for Selected Airborne Contaminants, Volume 1. Washington, DC: National Academy Press.
NRC (National Research Council). 1996a. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 2. Washington, DC: National Academy Press.
NRC (National Research Council). 1996b. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 3. Washington, DC: National Academy Press.
NRC (National Research Council). 2000. Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants, Volume 4. Washington, DC: National Academy Press.