for periods up to 24 hours (h). The 1-d SWEG is intended to prevent irreversible harm and degradation in crew performance. Temporary discomfort is permissible provided 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 d. In contrast with the 1-d SWEG, longer-term SWEGs are intended to provide guidance for exposure under the expected normal operating conditions in spacecraft.
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 spaceflights and habitation of the ISS, water reclamation, treatment, and recycling is required. Water for long spaceflights can be reclaimed from several on-board 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 (for example, 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. It 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 (such as silver and iodine), fouling of the filtration system, and incomplete processing of the water.
It is also possible that contaminants in the spacecraft atmosphere will 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.