Chapter 3 presented the hazards from materials on Mars interacting dynamically with humans or critical systems involved in human missions. In Chapter 4, the committee presents hazards associated with the chemical reactivity of materials on Mars. As with radiation exposure standards, NASA standards for astronaut exposure to chemical hazards have only been docu-mented for operation in low Earth orbit (NASA, 1995).
As discussed in Chapter 2, the committee assumed that astronauts will conduct EVAs on the surface of Mars. The following analysis is based on the assumption that some dust and soil will be brought into the habitat through the airlock by returning astronauts, as was the case during the Apollo missions to the Moon. Even though some space suit designs have been proposed that would mitigate the dust and soil intrusion problem, astronauts will inhale some fraction of the dust and fine particle soil entering the habitat.
The committee was faced with the question of whether or not NASA must fully characterize the mineralogy of the dust and soil&—that is, the chemical composition and the physical form&—prior to the first human mission to Mars. The answer to this question is complicated by the fact that very little is known about Martian airborne dust and soil. The only factual data the committee was able to use during its deliberations was based on the analysis of meteorites from Mars that have landed on Earth (also known as SNC meteorites) and chemical and spectroscopic measurements of Mars soil taken by the Viking and Mars Pathfinder missions.
The committee's goal was to identify, if possible, the level below which particulate concentrations in a habitat must be kept to protect astronauts from dust and soil whose composition is not well known.
Given the unknown nature of the Martian dust and soil, the committee believed it was prudent to err conservatively by assuming a worst-case scenario. In choosing the “worst” toxic chemical hazards to humans, the committee considered inorganic substances separately from organic substances. With respect to inorganic substances, it identified certain toxic metals as the worst threat to humans at the lowest concentrations. The committee addressed organic substances in a different manner, choosing to look first for the presence of organic carbon, which will be discussed below.
The chemical health threat from Martian airborne dust and soil can be considered in terms of acute (short-term) and chronic (long-term) effects. Noncancer acute or chronic effects could result in lung injury in the form of silicosis or in other specific organ damage. Lung tissue could be damaged by the intrusion of acidic dust. Strong oxidants in the soil and dust are also potentially hazardous to astronauts. The effects of such damage would be observable in a short time and could interfere with the completion of a human mission.
Cancer is a potential chronic effect caused by the inhalation of Martian particulate matter containing certain toxic metals, asbestos-like fibers, or organic compounds. The committee concluded that the threat of asbestos-like materials being present in Martian dust and soil is not significant based on inferences about
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface 4 Chemical Environmental Hazards Chapter 3 presented the hazards from materials on Mars interacting dynamically with humans or critical systems involved in human missions. In Chapter 4, the committee presents hazards associated with the chemical reactivity of materials on Mars. As with radiation exposure standards, NASA standards for astronaut exposure to chemical hazards have only been docu-mented for operation in low Earth orbit (NASA, 1995). CHEMICAL INTERACTION OF MARTIAN SOIL AND AIRBORNE DUST WITH ASTRONAUTS AND CRITICAL EQUIPMENT As discussed in Chapter 2, the committee assumed that astronauts will conduct EVAs on the surface of Mars. The following analysis is based on the assumption that some dust and soil will be brought into the habitat through the airlock by returning astronauts, as was the case during the Apollo missions to the Moon. Even though some space suit designs have been proposed that would mitigate the dust and soil intrusion problem, astronauts will inhale some fraction of the dust and fine particle soil entering the habitat. The committee was faced with the question of whether or not NASA must fully characterize the mineralogy of the dust and soil&—that is, the chemical composition and the physical form&—prior to the first human mission to Mars. The answer to this question is complicated by the fact that very little is known about Martian airborne dust and soil. The only factual data the committee was able to use during its deliberations was based on the analysis of meteorites from Mars that have landed on Earth (also known as SNC meteorites) and chemical and spectroscopic measurements of Mars soil taken by the Viking and Mars Pathfinder missions. The committee's goal was to identify, if possible, the level below which particulate concentrations in a habitat must be kept to protect astronauts from dust and soil whose composition is not well known. Given the unknown nature of the Martian dust and soil, the committee believed it was prudent to err conservatively by assuming a worst-case scenario. In choosing the “worst” toxic chemical hazards to humans, the committee considered inorganic substances separately from organic substances. With respect to inorganic substances, it identified certain toxic metals as the worst threat to humans at the lowest concentrations. The committee addressed organic substances in a different manner, choosing to look first for the presence of organic carbon, which will be discussed below. The chemical health threat from Martian airborne dust and soil can be considered in terms of acute (short-term) and chronic (long-term) effects. Noncancer acute or chronic effects could result in lung injury in the form of silicosis or in other specific organ damage. Lung tissue could be damaged by the intrusion of acidic dust. Strong oxidants in the soil and dust are also potentially hazardous to astronauts. The effects of such damage would be observable in a short time and could interfere with the completion of a human mission. Cancer is a potential chronic effect caused by the inhalation of Martian particulate matter containing certain toxic metals, asbestos-like fibers, or organic compounds. The committee concluded that the threat of asbestos-like materials being present in Martian dust and soil is not significant based on inferences about
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface dust mineralogy from spectroscopy, measured soil composition, and plausible alteration mechanisms (Morris et al., 1995; Bell et al., 2000). The committee also concluded that Martian airborne dust could present the same chemical hazards as Martian soil, so soil and dust should be characterized in the same way. In addition, certain soils could contain harmful organic compounds. However, the oxidizing environment at the surface would most likely have destroyed any organic compounds contained in the surface layers of the soil. Notwithstanding these conclusions, in the absence of further data, astronauts should avoid direct skin contact with soil. Cancer versus Noncancer Risks Generally, EPA considers that there is no safe threshold (no safe level) for human exposure to certain genotoxic, cancer-inducing compounds. “Genotoxic” refers to the process by which cancer-causing compounds directly interact with DNA. In contrast, there is a defined threshold level above which noncancer effects are induced. An estimate of the safe inhalation concentration is referred to as the reference concentration (RfC). If hazardous compounds are below the RfC threshold, noncancer effects are not expected to occur. If the RfC of a toxic compound is exceeded, there could be harmful health effects. It should be noted that many compounds do not have RfCs established. An examination of the Integrated Risk Information System (IRIS) provided by the EPA reveals that the allowed safe concentrations for cancer are much more stringent than the safe concentrations that give comparable risk estimates for noncancer effects (Table 4.1 is a representative sample). Therefore, if NASA protects astronauts against the risk of developing cancer in the long term as a result of having been exposed to particulate matter on Mars, NASA will also be protecting astronauts from acute and short-term noncancer effects that could potentially interfere with mission success. Toxic Metals and Other Inorganic Elements Airborne dust and soil could contain trace amounts of hazardous chemicals, including compounds of toxic metals that are known to cause cancer over the long term if inhaled in sufficient quantities. Soil analyses conducted by the Viking missions established maximum possible concentration limits for a few toxic elements based on the detection capabilities of the instruments on the landers (Table 4.2), and Mars Pathfinder measurements established that chromium is present in Mars soil. Although analogous measurements have not been made on airborne dust, soil and dust are commonly assumed to have similar chemical compositions (McSween and Keil, 2000). TABLE 4.1 Representative Listing of Reference Concentrations for Cancer-Causing Compounds and for the Noncancerous Effects of Those Compounds from EPA's IRIS Database (milligrams per cubic meter) Compound Reference Concentration (RfC) Safe Dose for Noncancer Effects Concentration That Gives a Cancer Risk of 1 in 1,000,000a Beryllium 2 × 10-5 4 × 10-7 Acrylonitrile 2 × 10-3 1 × 10-5 Acrolein 2 × 10-5 No information given Acrylic acid 1 × 10-3 No information given Aniline 1 × 10-3 No information given Antimony trioxide 2 × 10-4 No information given Carbon disulfide 7 × 10-1 No information given Dichlorobenzene 8 × 10-1 No information given Styrene 1 × 100 No information given Chromium VI No information given 8 × 10-8 Arsenic No information given 2 × 10-7 Cadmium No information given 6 × 10-7 a Higher dose poses a greater risk. Based on a survey of Environmental Protection Agency (EPA) exposure risk estimates, the elements that are toxic at the lowest concentrations are hexavalent chromium (Cr VI), arsenic (As), cadmium (Cd), and beryllium (Be) (see Box 2.2). Hexavalent Chromium Chromium contained in naturally occurring geologic materials is primarily in the trivalent state (a +3 ion), which is a stable form of chromium and minimally toxic to humans. Hexavalent chromium (Cr VI, a +6 ion), the highly toxic form of chromium, is rarely encountered in natural geologic materials. In fact, hexavalent chromium is only found naturally on Earth in the rare mineral crocoite (PbCrO4) and is more often produced by humans for industrial purposes (ATSDR, 2000).
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface TABLE 4.2 Element Detection Capability on the Viking Landers Atomic Number Detection Lower Limit Potentially Hazardous Elements Note 1 to 11 No detection capability Be and F Fluorine amounts may be unusually high on Mars since halogen amounts are generally high. 12 to 28 Variable Ni, Co, Cr, Si Consists of major and minor elements. 29 to 42 150 ppma As, Se, Rb, Sr, Y, Zr, Mo Bromine (Br) was detected in the 100-150 ppm range. 43 to 75 No detection capability Cd No ability to place useful lower limits due to interference from major and minor elements. 76 to 92 300 ppm Hg, Ti, Pb a Bromine is an exception. See note in right column. The committee believes that hexavalent chromium is not present in abundance on Mars but cannot state this with absolute certainty. There are three reasons for being cautious about the presence of hexavalent chromium on Mars. Mars Pathfinder APXS (alpha-proton-x-ray spectrometer) data indicate that chromium, in an unknown valance state, is present on Mars in the soil at an average of 0.2±0.1 weight percent (Wanke et al., 2001). If even a modest fraction of this amount is hexavalent chromium, it would pose a serious health threat to astronauts operating on the surface. Hexavalent chromium is derived from trivalent chromium by an oxidation process. The surface of Mars is highly oxidizing, which makes it a suitable environment for generating hexavalent chromium. Hexavalent chromium reverts fairly easily to the trivalent state in the presence of organic compounds. The surface of Mars appears to be devoid of organic carbon (Biemann et al., 1977). The lack of organic carbon indicates that few or no organic compounds are present, so any hexavalent chromium present may not easily revert to the nontoxic trivalent state. To offset these concerns to some degree, it should be noted that the only cation known to combine miner-alogically with hexavalent chromium on Earth is lead (Pb2+), producing the oxide, crocoite (PbCrO4). Based on SNC meteorite analysis, lead is present on Mars at only about 72 parts per billion (ppb) (Lodders, 1998). Therefore, if the general mineralogy of Mars is similar to that of Earth, as is indicated by all experiments to date, the amount of hexavalent chromium on Mars is likely to be very small. The physical and chemical weathering processes that would cause rocks on Mars, which the meteorites represent, to be converted into airborne dust and soil could cause an increase in the concentration of certain toxic metals. Even given a thousandfold increase in the concentration of lead (from 72 ppb to 72 ppm) by such weathering processes, the hexavalent chromium that would combine with this lead would not exceed a concentration of about 18 ppm. For the purposes of this study, the committee made the conservative assumption that hexavalent chromium is present in Martian soil and airborne dust at no more than 150 ppm. This concentration is equivalent to nearly 10 percent of the chromium detected by Pathfinder and well above any expectations for the amount of crocoite present, as discussed above. It must be clear to the reader that all conclusions in this chapter will be based on this assumption and others as discussed shortly. Should the actual concentration of hexavalent chromium become known, the habitat filtration requirements discussed in this chapter will change accordingly, keeping in mind that other toxic elements, such as arsenic, might then be dominant. Given the severe toxicity and uncertainty of the amount of hexavalent chromium, the committee recommends that NASA conduct an in situ experiment prior to the first human mission to Mars to determine if hexavalent chromium is present in Martian soil and airborne dust at potentially hazardous concentrations. As is shown in Table 4.3, a 2-year exposure to Cr VI at a concentration of 150 ppm in 1 milligram per cubic meter (mg/m3) airborne particulate matter produces a cancer risk of 5 in 100,000, which is in the middle of
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface TABLE 4.3 Toxic Metal Inhalation Risk Metal Concentration Equivalent to Lifetime Exposure Representing a Risk of 1 in 1,000,000 (mg/m3) Maximum Cancer Risk to Astronauts for 2 Years of Exposure at 1 mg/m 3 Particulate Matter Containing 150 ppm of the Metal, Roundeda Chromium VI 8 × 10-8 5 in 100,000b Arsenic 2 × 10-7 2 in 100,000 Beryllium 4 × 10-7 1 in 100,000 Cadmium 6 × 10-7 1 in 100,000 NOTE/CALCULATIONS: The concentration of the metal that represents a cancer risk of 1 in 1,000,000 (10-6) is given in the EPA's Integrated Risk Information System (IRIS). For instance, for chromium VI, a risk of 1 in 1,000,000 results from a lifetime exposure (70 years) to a concentration of 8 × 10-8 g/m3. Risks are linearly related to exposure concentration. The astronauts will not be exposed to the metal for a lifetime but, in the worst case, for 2 years. Since the risks are proportional to the exposure, one can tolerate a higher concentration for a shorter period of time. The concentration equivalent to 2 years exposure to give a 10-6 risk is 70/2 times the lifetime risk. The following is a sample calculation for arsenic: 150 ppm of the metal in the particulate matter in 1 mg/m3 is 150 × 10-6, or 1.5 × 10-4 mg/m3 of metal. This is the (conservative) concentration to which the astronauts could be exposed for 2 years. The lifetime cancer risk of 1 in 1,000,000 (10-6) for arsenic is 2 × 10-7 mg/m3 (from IRIS). The concentration equivalent to 10-6 for a 2-year exposure is (70/2) × (2 × 10-7), or 7 × 10-6 mg/m3. The ratio of the exposure concentration to the concentration that gives a risk of 10-6 represents the risk times 10-6. For instance, 1.5 × 10-4 (exposure concentration) divided by 7 × 10-6 (exposure that gives a risk of 10-6) = 0.2 × 102 × 10-6, or 2 × 10-5, a risk of 2 in 100,000. a Assumptions/methods: (1) the metal is uniformly present in the respirable particulate matter at 150 ppm or less and (2) astronauts could be exposed constantly for 2 years at 1 mg/m3 particulates in air (maximum). Rounded to nearest whole number. b Risk of contracting cancer is 5 in 100,000. the acceptable risk range (ARR) established for the exploration of Mars by the committee. Thus, the committee recommends that the measurement quantify hexavalent chromium down to a concentration level that is 150 ppm or less. This measurement can take place at any location on the planet since the airborne dust on Mars is highly mixed and therefore considered to be uniform in composition. If this measurement cannot be made in situ, a sample of airborne dust and fine particles of Martian soil must be returned to Earth to determine if hexavalent chromium will pose a threat to astronauts operating on the surface of Mars. Recommendation: In order to evaluate if hexavalent chromium on Mars poses a threat to astronaut health, NASA should conduct a precursor in situ measurement to determine if hexavalent chromium is present in Martian soil and airborne dust at more than 150 parts per million (ppm). This measurement may take place anywhere on Mars where well-mixed, uniform airborne dust is present. If such a measurement is not possible, a sample of airborne dust and fine particles of Martian soil must be returned to Earth for evaluation. Other Toxic Inorganic Elements The principal constraint on human missions from other toxic elements in Martian soil and airborne dust is an upper limit on the abundance of arsenic. This limit is based on Viking mission instrument measurement limitations (Table 4.2). Based on ratios of the measured abundances of arsenic, cadmium, and beryllium in Martian meteorites to other elements created by similar geochemical processes, abnormally high concentrations of these elements are not expected in Martian soil and airborne dust (arsenic and cadmium data compiled by Lodders, 1998; beryllium analyzed by Lentz et al., 2001). These elements are generally present at a few tens of parts per billion in Martian meteorites. Martian soils are similar in major element chemistry to the meteorites, except for enrichment in salts (sulfates and chlorides) (Clark et al., 1982). The geochemical process that introduced salts probably also concentrated arsenic and cadmium, but not beryllium, in soils. However, even at concentrations a thousand times greater than the concentrations in meteorites, arsenic and cadmium levels would still be less than 150 ppm. The Need for Measurements Based on the predicted concentrations of hexavalent chromium, arsenic, beryllium, and cadmium, as a worst-case scenario, the committee has addressed the risk of astronauts inhaling particulate matter containing 150 ppm of these metals. At this concentration, a 2-year exposure period, and a maximum average particulate
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface level of 1 mg/m3, as discussed below, the committee was able to develop the risk estimates listed in Table 4.3. All of the risks fall within its ARRs. The significance of this result is that no additional measurements of the concentrations of toxic elements in Martian soil and airborne dust, except Cr VI, are necessary as precursors to human exploration. However, this conclusion is dependent on NASA's ability to maintain particulate concentration in the habitat at or below the maximum allowable level discussed in this section of the study. Instead of determining the concentrations of every toxic element in Martian soil and airborne dust, the committee determined that NASA can design around the potential hazard. Simply stated, if the habitat and/ or astronauts are equipped with appropriate air filtration systems, then their health will be protected from the risk of toxic metal exposure as long as the assumptions listed below hold true. Filtration systems that maintain a maximum average concentration of 1 mg of particulate matter per cubic meter of air will place astronauts in an ARR of between 1 in 10,000 and 1 in 100,000 of getting cancer during their lifetime from exposure to toxic elements in Martian soil and airborne dust (see Table 4.3). As noted in Chapter 2, in the section “Establishing Risk Standards,” the committee established the ARR as an appropriate risk for astronauts. The issue is also discussed in Box 2.2. It is natural that the operation of filtration systems will result in oscillations in airborne particulate matter concentration depending on the loading rate. For instance, when an astronaut returns from an EVA and introduces soil and dust into the habitat, the concentration of particulate matter in the air will go up for some period of time until returning to less than 1 mg/m3. It is possible to average a short-term, high-concentration exposure with a low-concentration exposure over a long period of time and obtain an average concentration meeting toxic concentration limits. However, this is not the intent of the specification offered in this report. EPA does not provide short-term or ceiling limits for exposure to chemicals, even though the intention is not to experience high-concentration fluctuations. The National Institute of Occupational Health and Safety (NIOSH) and the Occupational Safety and Health Administration (OSHA), on the other hand, do have short-term exposure limits for many chemicals. For most chemicals, this short-term maximum over a period of 15 to 30 minutes is between 1.5 and 2 times the specified average daily value. The committee believes it is reasonable to adopt this precedent in recommending a maximum concentration of respirable particles to no more than 1.5 times the average concentration for a duration not to exceed 30 minutes per day to protect against toxic elements (NIOSH, 2000). Assumptions The committee made the following assumptions in formulating the recommendation that NASA develop a system that filters particulate matter in astronaut habitat air to concentrations of 1 mg/m3 or less for the first human mission to Mars: Astronaut forays outside the habitat will contami-nate the environmental living space with material from the surface of Mars. Astronauts will primarily be exposed to toxic elements by breathing Martian dust, with only minuscule amounts of soil or dust contaminating food or contacting bare skin. Hexavalent chromium, arsenic, cadmium, beryllium, and other toxic metal concentrations are 150 ppm or less in soil and airborne dust. There are few or no negative additive or synergistic human health effects of the mixture of toxic metals and other chemicals, including other parameters such as acidity, in Mars soil or airborne dust. Considering metals individually is common practice since the quantification of combined effects with complex chemical mixtures is difficult. Radiation exposure in combination with exposure to toxic metals has little negative synergistic effect on human health. Toxic metals are not present in higher concentrations in the breathable portion of the airborne dust (small particles less than 1 micron in diameter) than in the Martian soil. The dust on Mars is homogeneous with respect to its trace element composition. Residence time on Mars for the astronauts will be at least 1.5 years. Acute exposures over a few months are outside the scope of this analysis. If a shorter mission takes place, exposure to toxic metals could be higher and still achieve the same ARR. All of the toxic metal is bioavailable&—that is, any metals entering an astronaut's body would be
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface completely absorbed. If the metals in Martian dust or soil are encapsulated in a coating of a nontoxic material or are otherwise rendered nonbioavail-able (e.g., by binding to other chemicals), the risk to astronauts will be reduced. What If Assumed Filtration Levels Cannot Be Attained? The committee believes, based on current filtration standards and the capability demonstrated on the International Space Station, that the filtration levels required to protect astronauts are readily achievable. However, if a filtration system cannot be designed to limit the average particulate inhalation exposure of an astronaut to 1 mg of particulate matter per cubic meter of air in the habitat, then a sample of airborne dust taken from the Martian atmosphere and soil must be analyzed to establish concentration levels of all toxic metals. The level of analytical precision required will be dictated by the filtration capability of the astronauts' habitat. Although in situ measurements of Martian dust and soil with a resolution of parts per million are possible in principle, the committee judged that limitations on the ability of robotic instruments to measure trace elements would probably necessitate the return of soil and airborne dust samples to Earth if the specified filtration level cannot be achieved. The following analyses and/or protocols must be established for the soil and airborne dust samples to ensure astronaut safety on the surface of Mars if appropriate filtration levels cannot be attained: Trace element abundance must be measured with parts per million resolution to determine the levels of toxic metals in the soil and airborne dust. The chemical and physical form of the toxic metals may need to be determined, depending on their concentrations. If the ARR of 1 in 10,000 to 1 in 100,000 due to inhalation of toxic metals cannot be maintained by using filtration systems to reduce airborne particulate concentration, then animal testing should be conducted to determine the integrated effects of soil and dust exposure on living systems. Under these circumstances, the chemical and physical form of the toxic metals should also be determined, since bioavailability could be an issue. Airborne Respirable Particulate Matter It should be very clear to the reader that, in the view of the committee, the 1 mg/m3 specification is the maximum acceptable respirable particle average concentration to which astronauts should be exposed. This concentration level will protect astronauts from exposure to toxic metals, which&—of all inorganic chemicals&— the committee considers to pose the greatest health risk to astronauts. Filtering at or below the recommended 1 mg/m3 average with a 1.5 mg/m3 peak concentration should be readily achievable for NASA. The 1 mg/m3 particulate level is equivalent to very dirty industrial city air and is about 20 times greater than the 0.05 mg/m3 average standard set for the International Space Station (Green and Lane, 1964; NASA, 2000). Indeed, to minimize risks from exposure, the committee strongly believes that filtering should be implemented below 1 mg/m3, to as low a concentration as is reasonably achievable in the Martian habitat. The committee notes that there are no risk estimates similar to those presented in the EPA IRIS database for general respirable particulates, so an analysis similar to the one above for toxic metals is not possible. However, maintaining a particulate concentration below 1 mg/m3 would also be consistent with EPA National Ambient Air Quality Standards (NAAQS). Current and proposed NAAQS for particulate concentrations are shown in Table 4.4. TABLE 4.4 EPA National Ambient Air Quality Standards for Particulate Concentrations Pollutant Measure Value (mg/m3) Type PM 10&—diameter <10 micrometers Annual arithmetic mean 24-hour average 0.05 0.15 Primary and secondarya Primary and secondary PM 2.5 (proposed)&—diameter <2.5 micrometers Annual arithmetic mean 24-hour average 0.015 0.065 Primary and secondary Primary and secondary a Primary standards are intended to protect public health in general, including sensitive populations such as children, the elderly, and asthmatics. Secondary standards are intended to protect the public welfare, including protection of animals, vegetation, buildings, etc.
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface The mean annual particulate matter (PM) 10 standard (for coarse respirable particles less than 10 microns in diameter) is the same as the International Space Station standard for total particulate concentrations. The lower PM 2.5 standard (for fine respirable particles less than 2.5 microns in diameter) is not currently being enforced by EPA. Although the distribution of Martian dust particle sizes has not been fully characterized, the mean particle diameter of 3.4 microns suggests that a significant fraction of the dust will fall in the coarse respirable PM 10 size category. It is noteworthy that the NAAQS for particulate concentrations are designated to serve as both primary and secondary health standards. Primary standards are intended to protect public health in general, including sensitive populations such as children, the elderly, and asthmatics. Secondary standards are intended to protect the public welfare, including protection of animals, vegetation, buildings, etc. The NAAQS also consider long-term exposure to the hazards. Thus, because the specifications are set to protect sensitive populations over long periods of exposure, NAAQS are conservative for healthy astronauts. It would not be unreasonable for NASA to adopt higher concentration standards for human missions to Mars with durations of 2 years or less. NASA's Advanced Environmental Monitoring and Control Program has recognized the need for general particulate monitoring in space habitats, recommending that respirable particles less than 10 microns in diameter be quantified in the range 0.01 to 10 mg/m3 (NASA, 1996). This measurement range is well suited for monitoring particulates at the concentrations the committee has determined are necessary to protect astronauts from exposure to toxic elements. If NASA chooses to limit respirable particulate concentrations to below 1 mg/m3, the cancer risk from toxic elements will also be reduced, since the relationship between the risk of getting cancer and the allowable concentration of airborne particulate matter is taken to be linear. This means that if NASA allows 10 mg/m3 of particulate matter in the habitat, the risk range of getting cancer will increase tenfold. On the other hand, if the concentration of particulate matter is one order of magnitude lower, that is, 0.1 mg/m3, the protection to the astronauts increases by one order of magnitude. Biological Degradation and Equipment Corrosion There are high concentrations of sulfur and chlorine in Martian soil (Clark et al., 1982; Wanke et al., 2001). This implies that both the soil and airborne dust might be acidic, which could pose a hazard if they were introduced into an astronaut habitat. When inhaled by astronauts, acidic soil and dust could degrade their lung tissue and, if humidified and allowed to penetrate control units inside the habitat, could corrode sensitive critical equipment, such as control circuits. Even with the filtration systems discussed above, the filtration level may not be stringent enough to protect astronaut health and critical mechanical equipment from dust and soil that are extremely acidic. On the other hand, strong oxidants detected in Martian soil by the Viking biology experiment would be inactivated by humidification inside the astronaut habitat. It is therefore essential that NASA implement proper humidification in conjunction with the filtration system as part of habitat atmosphere conditioning. The committee concluded that even if strong oxidants are present, if the dust level is maintained at 1 mg/m3 or less and appropriate humidification systems are in place, there will be negligible risk associated with oxidation on the Martian surface.1 The Need for Measurements The committee recommends that NASA measure the pH and buffer capacity of Martian soil and airborne dust so that mission planners may better understand the potential corrosive effects of the soil and airborne dust on astronauts and critical systems inside the habitat. This measurement could be made on the surface of Mars or from a sample of soil and airborne dust collected from the Martian atmosphere and returned to Earth. Recommendation: In order to evaluate the potential corrosive effects of Martian soil and airborne dust on humans and critical systems in a humidified environment, NASA should measure the pH and buffer capacity of soil and airborne dust either via an in situ experiment or on Earth with returned samples of soil and airborne dust collected from the Martian atmosphere. 1 It should be noted that the Viking results were unexpected at the time, and the full nature of the oxidants has not been determined by follow-up experiments.
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface As stated above, the committee determined that oxidizing agents in soil and airborne dust do not pose a health hazard to astronauts if proper filtration and humidification levels are maintained. However, if NASA decides not to implement the necessary engineering controls or for other science-related reasons chooses to measure the oxidation properties of Martian airborne dust and soil, then the measurement should be performed on the surface of Mars rather than via a sample return. The committee is concerned that the oxidants might dissipate during a sample return transfer unless the sample is maintained in near-Martian conditions during transit. If NASA chooses to measure the oxidizing characteristics of the Martian environment, the committee recommends exposing a variety of materials, such as space suit material, to the Martian atmosphere and observing the effects of oxidants on the materials by optical or other measurement techniques. Hazardous Organic Compounds Organic carbon “includes all compounds of carbon, including straight chain, closed ring and combinations, except such binary compounds as the carbon oxides, carbides, carbon disulfide, etc.” (Lewis, 1997). Organic compounds are the proverbial building blocks of life. However, their presence does not necessarily indicate that life is or ever was present. Certain organic compounds can be highly toxic to humans, even if those compounds are not associated with a life-form. This threat should be evaluated in planning the first human mission to Mars. Organic compounds on Mars, if present, could have come from several sources, including meteorite impact, photochemical synthesis, and Martian biologic activity. At the two Viking landing sites, it was determined that within the limits of the gas chromatography-mass spectrometer experiment, the Martian soil contained no organic compounds to a detection limit of 1 ppb (Biemann et al., 1977). Experimental conditions restricted detection to organic compounds that could be volatilized and/or pyrolyzed (i.e., released from the soil) at up to 500 degrees Celsius. While this constraint precluded the direct detection of living organisms and very high molecular weight materials such as some polymers, the temperature was high enough to volatilize all known organic compounds within the mass detection range of the mass spectrometer (12 to 215 atomic mass units). The temperature was also sufficient to pyrolyze many larger organic compounds into smaller products detectable by the mass spectrometer. Overall, the gas chromatography-mass spectrometer results strongly indicate the absence of appreciable quantities of organic carbon on the Martian surface. This lack of detectable quantities of organic carbon is most likely a result of the abundance of a strong oxidizing agent on the surface that is produced by ultraviolet radiation from the Sun. Any hazard would most likely come from handling subsurface samples that might contain organic compounds. SNC meteorite samples indicate that there may be very small amounts (in the parts per billion range) of organic compounds in the subsurface, which would not represent a hazard. The committee also believes that there will not be any threat from organic compounds in the airborne dust, because oxidants in the atmosphere would have broken down those compounds. The Need for Measurements The committee concludes that if organic carbon is not detected in Martian soil, there is no hazard from organic compounds. If organic carbon is present, it may present a hazard to the astronauts through one of two mechanisms: toxicity or infection from a life-form. The former is discussed in this chapter, while the latter is deferred to the discussion of biohazards in Chapter 5. From a review of the EPA IRIS database, the committee found that organic chemicals pose a risk similar to that posed by toxic metals at the same concentration. Therefore, the committee concludes that if organic carbon is present at a concentration of more than 150 ppm in soil to which astronauts might be exposed, a possible threat exists. Filtration systems that reduce astronaut exposure to organic carbon to concentrations less than 150 ppm would mitigate this threat. If experiments determine that organic carbon is present in concentrations greater than 150 ppm, the subsurface soil should be considered a toxic hazard until proven otherwise. NASA must then determine which compounds constitute the organic carbon by returning a sample from that specific location to Earth. The reader will learn in Chapter 5 that the need to assess the potential threat posed by a hazardous life-form consisting of organic carbon requires a more stringent measurement of organic carbon concentration. For this reason, the committee's recommendation on the measurement of organic carbon on Mars is deferred to Chapter 5.
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Safe on Mars: Precursor Measurements Necessary to Support Human Operations on the Martian Surface TOXICITY OF MARTIAN ATMOSPHERIC GASES Based on previous in situ measurements, the Martian atmosphere has been determined to be composed predominantly of carbon dioxide (95 percent), with nitrogen, argon, and oxygen (all nontoxic) present in abundances greater than 0.1 percent (Owen, 1992). There is a small amount of toxic carbon monoxide (0.07 percent), as well as traces of ozone up to 0.2 ppm. Scientists have used ultraviolet and infrared remote sensing techniques to search for a variety of candidate trace gases in the Martian atmosphere. No organic molecules or toxic gases, such as those containing N, S, P, or Cl, have been detected down to limits of 0.01 ppm (Owen, 1992). Even if a habitat were vented completely to the Martian atmospheric pressure of 0.6 kPa (6 mbar) and then refilled with the habitat's breathable mixture at 100 kPa (1 bar), the dilution factor would be over 160. In this scenario, the astronauts would be exposed to less than 0.6 percent carbon dioxide, 4 ppm of carbon monoxide, and 1 ppb ozone by volume. All of these amounts are well below the current NASA standard for these toxic gases. In addition, the atmospheric revitalization systems on spacecraft include systems for removing carbon dioxide and contaminants. The committee expects that the same capabilities would be provided in a human habitat on Mars. In addition, any highly reactive species, such as hydroxide radicals or other highly oxidizing species, created by photochemical processes in the Martian atmosphere by ultraviolet radiation would quickly evolve to less-hazardous chemical forms upon coming into contact with habitat airlock surfaces. Thus, sufficient knowledge is already available to ascertain that the Martian atmosphere does not pose a toxic risk for astronauts, and no further characterization is required. Long-term oxidizing effects on materials continuously exposed externally is a separate problem, as discussed earlier in this chapter. If NASA chooses to measure the oxidation properties of the Martian atmosphere, the committee recom-mends&—as it did with respect to measuring the oxidation properties of soil and airborne dust&—that this measurement be done on the surface of Mars rather than via a sample return. The committee has the same concerns&—that is, that the oxidants might dissipate during a sample return transfer unless the sample is maintained in near-Martian conditions during transit. If NASA chooses to measure the oxidation characteristics on Mars, the committee recommends exposing a variety of materials, such as space suit material, to the Martian atmosphere and to assess the effects of superoxidants or other radicals on the materials. REFERENCES Agency for Toxic Substances and Disease Registry (ATSDR). 2000. Toxicological Profile for Chromium, 259, September. 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