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The respirable fraction of lunar dusts may be toxic to humans. NASA has therefore determined that an exposure standard is necessary to limit the amount of respirable airborne lunar dusts to which astronauts will be exposed. The nominal toxicity that is expected from ordinary mineral dust may be increased for lunar dusts due to the large and chemically reactive surfaces of the dust grains. Human exposures to mineral dusts during industrial operations and from volcanic eruptions give researchers some sense of the relative toxicity of lunar dust, although the Earth-based analogs have serious limitations. Animal and cellular studies provide further evidence that mineral dusts can be somewhat toxic. Earth-based research of mineral dust has shown that freshly fractured surfaces are chemically reactive and can elicit an increased toxic response. Since lunar dust is formed in space vacuum from highly energetic processes, the grain surfaces can be expected to be indefinitely reactive on the lunar surface. NASA predicts that this chemical reactivity will change once the dust is brought into a habitable environment. Dust from lunar soil that was carried into spacecraft during the Apollo missions proved to be a nuisance. The lack of gravity, or the existence of gravity at a small fraction of the gravitational force of the Earth, increases the time during which dust remains airborne, thereby increasing the probability that these dust particles will be inhaled. Lunar dust particles that are generated by impaction in a deep vacuum have complex shapes and highly reactive surfaces that are coated with a thin layer of vapor-deposited mineral phase. Airborne mineral dust in a variety of forms has been shown to present a serious health hazard to ground-based workers. The health hazards that are associated with volcanic ash, which is a commonly used analog of lunar dust, have not been reported to be especially serious; however, this type of ash quickly loses its reactive surfaces and is often aggregated into particles that are not readily respirable into the deep lung. Crew members who will be at a lunar outpost can be directly exposed to lunar dust in several ways. After crew members perform spacewalks or EVAs, they will introduce into the habitat a large quantity of dust that will have collected on spacesuits and boots. Cleaning of the suits between EVAs and changing of the Environmental Control Life Support System (ECLSS) filters are other operations that could result in direct exposure to lunar dusts. In addition, if the final spacesuit design is based on the current spacesuit design, EVAs may cause dermal injuries, and the introduction of lunar dusts into the suits' interior, which may enhance skin abrasions. When the crew leaves the lunar surface and returns to microgravity, the dust that is introduced into the crew return vehicle will "float," thus increasing the opportunity for ocular and respiratory injury. ==Introduction== In 2004, President George W. Bush unveiled a plan directing NASA to return humans to the moon by the year 2015, and to use the lunar outpost as a stepping-stone for future human trips to Mars and beyond.〔(【引用サイトリンク】url=http://www.chinadaily.com.cn/en/doc/2004-01/15/content_299037.htm )〕 To meet this objective, NASA will build an outpost on the lunar surface near the south pole for long-duration human habitation and research. Because of the various activities that will require the astronauts to go in and out of this habitat on numerous spacewalks (EVAs), the living quarters at the lunar outpost are expected to be contaminated by lunar dust. The president's ''Vision for Space Exploration'' and charge to return to the moon have resulted in questions about health hazards from exposure to lunar dust. Lunar dust resides in near-vacuum conditions, so the grain surfaces are covered in "unsatisfied" chemical bonds, thus making them very reactive. When the reactive dust is inhaled, it can be expected to react with lung surfactant and pulmonary cells. The fine, respirable lunar dust could thus be toxic if the astronauts are exposed to it during mission operations at a lunar base. Although a few early attempts were made to understand the toxicity of the lunar dust that was obtained by the Apollo astronauts or the Luna probes, no scientifically defensible toxicological studies have been performed on authentic lunar dust. Awareness of the toxicity of terrestrial dusts has increased greatly since the original Apollo flights, which occurred circa 1970, in which the crew members were exposed to lunar dust for a relatively brief time. The first National Ambient Air Quality Standard (NAAQS) was issued by the Environmental Protection Agency (EPA) in 1971 and was indexed to total suspended particles (TSP) on a mass per unit volume basis. In 1987, this NAAQS was refined to include only particles that were of less than 10 μm in aerodynamic diameter (PM10) because this was the size that was most likely to reach the bronchial tree and deeper into the lung. Finally, in 1997, the EPA Administrator issued standards for particles that were less than 2.5 μm in aerodynamic diameter (PM2.5) based primarily on epidemiological associations of increased mortality, exacerbation of asthma, and increased hospital admissions for cardiopulmonary symptoms. None of these standards specified the composition of the particles. In fact, the last standard was a bit contentious because mechanisms of toxic action were not understood. In a review article, Schlesinger et al. list the properties of particulate matter that might elicit adverse effects. The properties that seem pertinent to lunar dust include: size distribution, mass concentration, particle surface area, number concentration, acidity, particle surface chemistry, particle reactivity, metal content, water solubility, and geometric form. In attempting to consider each of these properties, one property emerges as the most difficult to study; particle surface chemistry may be difficult to understand because the environment on the lunar surface is unlike any on Earth, and is likely to alter the surface of dust grains in a way that will render them highly reactive. Recreating the processes that could affect grain surface reactivity on the moon is difficult in an Earth-bound laboratory. Freshly fractured quartz is distinctly more toxic to the rat respiratory system than aged quartz. Quartz and lunar dust may have similar toxic properties, but breaking of surface bonds on mineral substrates has been shown to increase the toxicity of the well-studied mineral quartz. The site at which various sizes of particles are deposited is critical to an understanding of any aspect of their toxic action. The fractional regional deposition of particles shows that between 10 and 1 μm, the portion of particles that is deposited in the upper airways falls off from 80% to 20%, whereas the pulmonary deposition increases from near zero to about 20%. Pulmonary deposition, after falling off near 1 μm, peaks again near 40% for particles of 0.03 μm, whereas upper airway deposition remains low until a new peak deposition is found at less than 0.01 μm. The portion and pattern of deposition can be modified under conditions of reduced gravity; however, human data during flights of the gravity research aircraft show that particles in the 0.5 to 1 μm range are deposited less in the respiratory system at lunar gravity than at Earth gravity. This finding is consistent with the reduced sedimentation of the particles when the gravity is less. However, a larger portion of the particles is deposited peripherally in reduced gravity. The first encounter in which a particle deposits in the distal airways occurs with the broncho-alveolar lining fluid (BALF). The thickness of this fluid in the lung varies as the alveolar sacs expand and contract, but lies in the range of 0.1 to 0.9 μm. In the case of biological particles such as bacteria, this fluid opsonizes the particles to facilitate ingestion by macrophages. A similar process has been demonstrated for nonbiological carbonaceous particles. This process removes some components of the BALF that participate in opsonization, and it is postulated that this might enhance the toxicity of particles with a surface chemistry that is capable of selectively removing opsonizing components. The agglomeration of the grains is also affected by the interactions between the BALF and the grains. Preliminary data on authentic lunar dust has shown that in aqueous suspension, lunar particles agglomerate rapidly. Artificial surfactant has been found to greatly reduce this particulate agglomeration. Particles that are deposited in the pulmonary region are eliminated according to their surface area and chemical composition. If a particle is relatively soluble, its dissolution products end up in the bloodstream. Relatively insoluble particles are ingested by macrophages and removed by mucociliary clearance or the lymphatic system, or they persist in the interstitial areas of the lung. Ultrafine particles (<0.1 μm) that deposit in the upper airways have been shown, under some conditions, to translocate to the brain, whereas similar particles reaching the pulmonary regions can translocate to adjacent organs such as the liver. The effects of particles on the respiratory system include de novo causation of clinical disease as well as exacerbation of existing disease. If particulate inhalation is to directly cause disease, the exposure levels typically must be at levels that are encountered in industrial settings. For example, silicosis is a well-known disease of persons working for years in conditions in which dust containing quartz is inhaled. Epidemiological studies show that ambient dust levels such as those that are encountered in some cities can exacerbate respiratory conditions such as asthma and chronic obstructive pulmonary disease. At certain times, sand dust that originates in Asia or Arizona, for example, has been associated with exacerbation of allergenic respiratory inflammation. Of particular concern in addition to the respiratory system is the ability of small particles to affect the cardiovascular system. Epidemiological studies suggest that exposure to ambient particulate matter increases the incidence of angina, arrhythmia, and myocardial infarctions. The increased acute mortality that is associated with particle "events" is attributed to cardiovascular disease. Clinical studies involving concentrated ambient air particulate have shown increased blood fibrinogen and reduced heart-rate variability; exposure to ultrafine particles causes "blunted" repolarization response following exercise. The role of C-reactive protein in mediating the effect of ambient particle exposures on the causation of CAD has been reviewed. Batalha has drawn attention to the ability of particles to elicit vasoconstriction of small pulmonary arteries. Although the mechanistic details have not been fully elucidated, the evidence favors a strong link between exposure to particulates and to both acute and chronic heart disease.〔 There is some evidence from the Apollo missions that, in susceptible individuals, lunar dust exposure may lead to cardiovascular effects that are similar to those produced through exposure to air pollution. The fact that no accepted health standards or policies exist concerning exposure limits to lunar dust is a critical challenge to the design of vehicle systems in the CxP. The multi-center Lunar Airborne Dust Toxicology Assessment Group (LADTAG) was formed and responded to a request from the Office of the Chief Health and Medical Officer to "… develop recommendations for defining risk criteria for human lunar dust exposure and a plan for the subsequent development of a lunar dust permissible exposure limit." The LADTAG is composed of technical experts in lunar geology, inhalation toxicology, biomedicine, cellular chemistry, and biology from within the agency as well as of leading U.S. experts in these fields. Based on the opinions that were expressed by the LADTAG experts, NASA scientists will develop and execute a plan to build a database on which a defensible exposure standard can be set. LADTAG experts recommend that the toxicity of lunar dust on the lungs (pulmonary toxicity), eyes (ocular toxicity), and skin (dermal toxicity) be investigated, and that this investigation is to be conducted by the Lunar Dust Toxicity Research Project (LDTRP) using various assays including in vivo and in vitro methods. In an initial LADTAG workshop that was held in 2005, experts noted that they were unable to reconcile individual expert opinions to set an inhalation standard. The array of opinions from these experts spanned a 300-fold range (i.e., 0.01 to 3 mg/m3). The members of the LADTAG concluded that research is necessary to narrow this wide uncertainty range, the lower end of which cannot be met by known methods of environmental control, and that there is an urgency to determine the standard so that environmental systems for the lunar vehicle can be appropriately designed. Therefore, in keeping with the LADTAG experts' recommendations, members of the LDTRP have reviewed first-hand accounts of Apollo astronauts who were exposed to lunar dust during their missions as well as of terrestrial-based human exposures to dust generated in the mining industry and to volcanic ash. In accordance with the LADTAG recommendations to increase our evidence base, the LDTRP is conducting studies of Apollo spacesuits, filters, vacuum bags, and rock-collection boxes. These studies will enable us to focus our understanding of the grain-size distribution that is present in the lunar surface samples and in the habitat, but the dust surfaces are expected to be fully passivated. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Adverse health effects from lunar dust exposure」の詳細全文を読む スポンサード リンク
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