John T. James and Noreen Kahn-Mayberry of the NASA Johnson Space Center, report that 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 dust 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, we expect the grain surfaces to be reactive indefinitely on the lunar surface.
We predict 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 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.
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