Guidelines for Developing Spacecraft Maximum Allowable Concentrations for Space Station Contaminants (1992)

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Sources of Space-Station Contaminants Many sources of atmospheric contaminants will be present in the space station. Most sources will release only small amounts of material into the air, but contaminants may build up during operation of a closed vessel. Recognition of the many sources of atmospheric con- tamination has helped in eliminating major sources of contaminants and in developing methods to control and decrease contamination from any source. Table 5 lists possible space-station contaminants (Leban and Wagner, 1989). Major sources of contaminants include off-gas- sing of cabin materials, components, and equipment and metabolic waste products of crew members. All nonmetallic materials used in the interior of the orbiter crew compartment are known to off-gas contaminant compounds. These include cabin construction materials, electrical insulation, paints, lubricants, adhesives, and degradation of nonmetallic console and equipment structures. The heat produced by equipment operation increases off-gassing. Minor sources of contaminants in the spacecraft include internal decomposition of hydraulic fluids, electrical equipment, plastics, oil, leakage from environmental or flight control systems, volatile food components, volatile components of personal hygiene articles, and reaction products from the environmental control and life-support system (ECLSS). Overheating of electrical components and fire can cause some struc- tural materials (such as plastics) to emit toxic gases, vapors, and par- ticulate matter. Pyrolysis of plastics generates a variety of contam- inants, depending upon the composition of the polymer, including hydrogen chloride, carbon monoxide, hydrocyanic acid, formal- dehyde, and vinyl chloride. All structural components and other materials to be carried on board are tested to identify and quantify (under test conditions) off- gassing compounds (NASA, 1991). Off-gassing rates for each com- pound presented in Table 5 are based on these test results and on the quantities of each material projected to be present in the space station. 23

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34 GUIDELINES FOR DEVELOPING SMACS Escape of liquid or gaseous chemicals from water reclamation, cool- ing, and propellant systems; combustion; thermal decomposition; and vaporization could lead to a major contamination event. For these reasons, only ammonia cooling loops and hydrazine propellants will be permitted to be located outside the pressurized modules. Complete and multiple containments of processing and experimental apparatus are required by NASA if hazardous chemicals are present. Nevertheless, the possibility of accidental release of chemical, radioac- tive, or biological agents cannot be completely ruled out. Such re- leases could produce serious contamination and high concentrations in space-station air. Some of the commercial-type facilities expected to be used could accidentally release a significant amount of chemicals. In the space station, cleaning and degreasing agents, adhesives, disinfectants, and lubricants will be used in cleaning and maintenance procedures. Another potential source of contamination is repair of metallic or plastic objects involving soldering, welding, or drilling. These repair processes often involve localized heating and may release toxic gases, vapors, or particles. Chemical and physical processes in cabin air involving sunlight (high-energy solar radiation) and some airborne contaminants may result in the formation of highly toxic chemicals such as ozone, nitro- gen dioxide, and other photochemical products (peroxyacetyl nitrate, hydrogen peroxide, or free radicals). The formation of such products is a complex, nonlinear function involving many factors including the intensity and spectral distribution of sunlight, the concentration of the many precursors (aldehydes or carbon monoxide), the ratio between organic compounds and the oxides of nitrogen, and the reactivity of organic precursors. One resulting product, ozone, is highly toxic at low concentrations, causing a number of pulmonary and extrapulmon- ary effects. Due to the highly toxic nature of this gas, the potential hazard associated with such exposure in the spacecraft environment is a cause for significant concern. The principal sources of contaminants from metabolic waste prod- ucts of humans are urine, feces, flatus, and expired air. Animal urine and dander are potential sources of contaminants; however, NASA believes they are not a problem. Urine will be a contaminant in the space station only if the design is found to be ineffective in handling urine, leading to leakage or spillage. Urinary contaminants would be only a nuisance; in the absence of an infection of the urinary tract, urine is sterile.

SOURCES OF SPACE-STATION CONTAMINANTS 35 With feces, again the problem is proper mechanical and personal handling. Experience in spaceflights has indicated that bowel regular- ity (one or two stools per day) facilitates management. A method of vacuum cleaning of air contaminants has been devised in addition to surface cleaning. Flatus is an appreciable source not only of periodic discomfort due to odor but also of toxic gases, which must be removed from the cabin air. As studied by Galloway (1971), flatus of men fed a low-residue formula diet was 104 mL/12 hr (mean) in contrast to 286 mL/12 hr from men fed a diet of processed foods similar to those given to astro- nauts in the Gemini flight series. When gas-producing foods are eat- en, flatus passage may increase to 60-120 mL/hr (Calloway, 1968). Small volumes of flatus tend to have a composition like (ingested) air, whereas large volumes contain more carbon dioxide and flammable gases produced by bacteria, principally hydrogen and methane. These gases, in addition to being expelled, are absorbed into the bloodstream and then appear in expired air. Calloway and Murphy (1969), by measuring both expired air and flatus, found potential total daily volumes of 730 mL of hydrogen and 380 mL of methane in men on a spaceflight-type diet. Although the investigators raised the possibility that these amounts of hydrogen and methane "could constitute a fire hazard in a closed chamber," it seems unlikely that, in the sizeable space planned for the space station and with functional environmental controls, concentrations of these compounds would be high enough for risk of explosion. The malodorous compounds in flatus are indole, skatole, mercaptans, ammonia, and hydrogen sulf ide. The amounts of these compounds and others in flatus depend on two principal factors, the numbers and kinds of enteric organisms present and the substrate for these bacteria provided by diet. The importance of minimizing the number and amount of gas-producing foods for space diets is evident. In any event, the atmosphere of the space station will be continually cleansed. The principal expired respiratory gas is carbon dioxide at 15-20 L/hr, though possibly as much as four or five times that in extrave- hicular exertion (Calloway, 1971). Small amounts of carbon monox- ide, hydrogen, methane, and ammonia are also expired. Finally, skin cells are in a constant state of turnover, and there is a constant daily flaking of desiccated dead-cell remnants. This dusty material, though relatively minor in amount, would be a contaminant if a regular washing and wiping routine were not followed by astro- nauts in the space station. Studies of normal subjects (Calloway, 1971)

36 GUIDELINES FOR DEVELOPING SMACS have shown that several nitrogen-containing and organic compounds and minerals are lost from the skin with insensible perspiration and still more with active sweating. Measurements of actual losses from the skin in space are limited to those taken on the Gemini VII space- flight (Lutwak et al., 1969). Mineral losses were found to be minor during this 14-day flight. Sweating was minimal, as would be expected because most astronaut activities, although they require good neuromuscular coordination, need little physical exertion except dur- ing extravehicular operations. Since mineral and other losses from the skin increase into the significant range only with active sweating, the possibility of contamination from metabolic losses from the skin will be reduced by minimizing the likelihood of sweating, presumably by suitable temperature control and by moderation in physical activity. There is a possibility of potential contamination from dust mites, which produce proteins that are potent allergens. However, according to NASA, dust mites have not proved to be a problem. The water reclamation system in the space station may be a poten- tial source of toxic chemicals in the cabin air. The system may con- tribute several types of contaminants to space-station air. 1. Chemicals originally collected in the cabin condensate and then revolatilized. 2. Components of hygiene wastewater. 3. Chemicals actually produced in water treatment. Reclaimed water after treatment is the major source of drinking water. Any chemicals present in the air will obviously be condensed with water in proportion to their partial pressure and water solubility. For most nonpolar volatile chemicals, cycling through the water sys- tem is likely to contribute little to the hazards first encountered in the air. On the other hand, chemicals that are both polar and volatile (e.g., alcohols) may concentrate and even accumulate in the water system. Under such circumstances, the major portion of the exposure to the chemical may come via water. Hygiene wastewater will contain soaps, detergents, and other chem- icals present in personal hygiene products that might be expected from showering and laundering clothing. Most of these chemicals are rela- tively nonvolatile and unlikely to contaminate the air. These chemi- cals are removed fairly efficiently by water treatment processes. The water reclamation system has the potential to introduce novel compounds into the air. The use of oxidants to disinfect and chemi-

SOURCES OF SPACE-STATION CONTAMINANTS 37 cally degrade water contaminants is the major means by which such chemicals are produced. For example, the use of persulfate and sul- fur ic acid to treat urine results in the generation of cyanogen chloride. If this reaction product were to elude subsequent treatment processes, it could introduce a sufficient amount into the confined space of the shower to produce acute respiratory irritation. Repeated exposure to such irritants over a 90- to 180-day period could have long-term ef- fects. Similar but less well-defined problems might result from the use of iodine (or other reactive chemical) as the residual disinfectant in the water system. Crews from several space-shuttle flights have reported eye and respiratory tract irritation associated with the presence of airborne particles and floating debris in the shuttle cabin. The debris included paint chips, metal shavings, food particles, and fibrous materials, including fibers from clothing, paper wipes, and fiberglass. A panel assembled by NASA on Airborne Particulate Matter in Spacecraft (NASA, 1988) recommended that particle concentration in the cabin should be "as low as reasonably achievable" (ALARA). The panel noted that simple technology could be used to discern the par- ticle concentration in the atmosphere because the particles are likely to have many of the characteristics of nuisance dust. The panel ques- tioned whether the reported symptoms of eye and respiratory tract irritation were due to those particles. The symptoms also could result from exposure to gases such as nitrogen dioxide, ozone, or formalde- hyde present in the air. The recommendations of ALARA particle levels should allow, how- ever, for design of appropriate particle-control technology. The panel recommended the following numerical limits: 1. For flights of 1 week or less: 1 mg/m3 limit for particles <10 /jm in AD (aerodynamic diameter) + 1 mg/m3 for particles 10-100 too. in AD. 2. For flights of >1 week and up to 6 months: 0.2 mg/m3 for particles 10-100 pro. in AD. No specific limit for particles >100 pun in AD was recommended because adequate particle cleanup to meet the above values should result in acceptable levels of larger particles. In recommending these limits, the panel considered acute and chronic irritation of the respiratory tract and eyes to be the primary concerns. In selecting the 1-mg/m3 limit for short flights, the panel

38 GUIDELINES FOR DEVELOPING SMACS considered that if the irritation reported was due to particles and not gases, this limit should protect from irritation of the respiratory tract. This exposure limit was based on data from exposure of healthy humans to submicrometer-sized aerosols of sulf uric acid at concentra- tions as high as 1 mg/m3 with no signs of respiratory tract or eye ir- ritation. The limit for flights of longer duration was lowered by a factor of five to allow for the uncertainties about the toxicity of the particles. The 0.2-mg/m3 limit is the same as that set for U.S. sub- marines where conditions are somewhat similar to spacecraft. Obviously, exposure to chemicals from water and air are not neatly separable problems. Therefore, it is important that assessments of risks from certain concentrations of chemicals in air include consider- ation of the potential risks associated with possible buildup of the chemicals in the potable water system. Attention should be paid to defining the toxic effects and exposure scenarios for chemicals that are confined in the water system only. Sources of contaminants have been categorized into three subgroups according to predictability of release. Category 1 sources are those that release contaminants continuously or frequently or are associated with a specific routine activity. A principal characteristic of sources in this category is that their contaminant-generation rates can be pre- dicted with a high degree of accuracy. Systems usually can be de- signed to keep exposures to compounds emitted from these sources at or below the SMAC values. The emission rates given in Table 5 are based on sources of this type. Category 2 sources involve events such as inadvertent, accidental, or emergency releases of contaminants. Events leading to such re- leases include leaks, spills, failure of storage vessels, and overheating of components. Contaminants that may be released from category 2 sources include all gases and liquids normally kept on the space station and the thermal and chemical breakdown products of solid materials (for example, electrical insulation). Unlike category 1 sources, poten- tial release rates of category 2 sources cover many orders of magni- tude, even for a single source. Thus, planning for control of such releases involves examining various failure scenarios. Reliability anal- ysis to establish the likelihood of failure events is an important step in the risk assessment of contaminants from category 2. Category 3 sources are those involving accidental release from ex- periments performed on the space station. Some possible contaminants may be identified by an examination of the manifests of the materials for the experiments. Other contaminants may result from chemical

SOURCES OF SPACE-STATION CONTAMINANTS 39 reactions within an experimental module; recognition of such contam- inants requires a detailed understanding of the module and the experi- mental work or production to be carried out. Because every effort will be made to eliminate releases from experimental modules through the use of triple-containment systems, almost all releases from category 3 sources, as from category 2, are likely to be accidental. Unlike category 2, it is not possible to foresee with any certainty the contaminants that may be released from these sources or the magni- tude of release before space-station design and launch because many experiments will be performed over the space station's lifetime. Novel compounds may be released in the space station (e.g., new alloys or crystals during water recycling). Evaluation of such compounds usually will be required after identification because toxicity data on new compounds frequently are not available.