Acceptable Limits for n-Hexane in Spacecraft Atmospheres.

INTRODUCTION:The Spacecraft Maximum Allowable Concentrations (SMACs) for C2-C9 alkanes set by NASA in 2008 under the guidance and approval of the National Research Council specifically excluded SMACs for n-hexane. Unlike other C2-C9 alkanes, n-hexane can cause polyneuropathy after metabolism in humans or rodents and so requires more stringent SMACs than the other members of this group do. This document reviews the relevant published studies of n-hexane toxicity to develop exposure duration-specific SMACs for n-hexane of 200 ppm for 1 hour, 30 ppm for 24 hours, and 2.4 ppm for 7 days, 30 days, 180 days, and 1000 days.Garcia HD. Acceptable limits for n-hexane in spacecraft atmospheres. Aerosp Med Hum Perform. 2021; 92(12):956-961.

Establishment of exposure guidelines for lead in spacecraft drinking water.

BACKGROUND: Setting Spacecraft Water Exposure Guidelines (SWEGs) for lead (Pb) in spacecraft drinking water has special challenges related to estimating the increase in blood lead levels (PbB) due to the release of lead to systemic circulation via microgravity-induced bone loss. METHODS: The effects on the PbB of lead in drinking water (PbW) and lead released from bones, and changes in lead exposure before, during, and after spaceflight, were evaluated using a physiologically based pharmacokinetic model that incorporated environmental lead exposure on Earth and in flight and included temporarily increased rates of osteoporosis during spaceflight. RESULTS: The model predicts that in 2030 (the earliest potential launch date for a long-duration mission), the average American astronaut would have a PbB of 1.7 microg x dl(-1) at launch and that, while in microgravity, PbB levels would decrease at PbW values less than about 9 microg L(-1) because of reduced exposure within the spacecraft to environmental lead. Astronauts with high concentrations of lead stored in bones could experience increases in PbB due to microgravity-accelerated release of lead from bones. While the resultant in-flight PbB would depend on their preflight bone lead levels, their PbB will not be significantly further elevated (< 1 microg x dl(-1)) by consuming water with a PbW of < or = 9 microg x dl(-1). Selection of a SWEG that would not result in an increase in blood lead is prudent given uncertainties about health effects at low exposures. CONCLUSION: A SWEG of 9 microg x L(-1) would protect astronauts on long-duration spaceflights by ensuring that PbB values will not exceed prelaunch levels.

Safe human exposure limits for airborne linear siloxanes during spaceflight.

BACKGROUND: Low molecular weight siloxanes are used in industrial processes and consumer products, and their vapors have been detected in the atmospheres of the Space Shuttle and International Space Station. Therefore, the National Aeronautics and Space Administration (NASA) developed spacecraft maximum allowable concentrations (SMACs) for siloxane vapors to protect astronaut health. Since publication of these original SMACs, new studies and new risk assessment approaches have been published that warrant re-examination of the SMACs. OBJECTIVE: To reevaluate SMACs published for octamethyltrisiloxane (L3) for exposures ranging from 1 hour to 180 days, to develop a 1000-day SMAC, and to expand the applicability of those values to the family of linear siloxanes. METHODS: A literature review was conducted to identify studies conducted since the SMACs for L3 were set in 1994. The updated data were reviewed to determine the sensitive toxicity endpoints, and current risk assessment approaches and methods for dosimetric adjustments were evaluated. RESULTS: Recent data were used to update the original 1-hour, 24-hour, 30-day, and 180-day SMACs for L3, and a 1000-day SMAC was developed to protect crewmembers during future exploration beyond Earth orbit. Group SMACs for the linear siloxane family, including hexamethyldisiloxane (L2), L3, decamethyltetrasiloxane (L4), and dodecamethylpentasiloxane (L5), were set for exposures of 1-hour to 1000 days. CONCLUSION: New SMACs, based on acute pulmonary and neurotoxicity at high doses only achievable with L2 and potential liver effects following longer-term exposures to L2 and L3, were established to protect crewmembers from the adverse effects of exposure to linear siloxanes.

Modeling of blood lead levels in astronauts exposed to lead from microgravity-accelerated bone loss.

INTRODUCTION: Most astronauts experiencing prolonged microgravity undergo accelerated bone loss at a whole-body rate of 0.5-1% per month, with some load-bearing bones losing mass at normalized rates up to about 2.6% per month. The accompanying release of lead (Pb) stored in bones would increase the concentration of Pb in the blood (PbB), thereby complicating efforts to set acceptable Pb concentrations for spacecraft drinking water (PbW). METHODS: A physiologically based pharmacokinetic (PBPK) model was modified to permit modeling the effects on PbB of temporarily increased rates of bone loss and various PbW concentrations. RESULTS: The model predicts that, for the average American astronaut, the increase in PbB due to Pb released from bones would be more than offset by decreases in ingested or inhaled spacecraft environmental Pb, so that calculated PbB levels actually decrease in microgravity when PbW < about 9 microg Pb x L(-1). Measured PbB in astronauts before and immediately after 6-mo stays on the International Space Station (ISS) support these results. Currently, PbW on the ISS averages 0.6 microg Pb x L(-1) and PbW on Earth at the Johnson Space Center averages about 5 microg Pb x L(-1). CONCLUSIONS: Most astronauts on long spaceflights will not be adversely affected by the release of lead from bones into the blood. A small percentage of astronauts (assuming there could be any who would have high concentrations of lead in their bones) could be at risk of experiencing elevated levels of PbB due to microgravity-accelerated release of Pb from their bones, depending on their individual rate of bone loss.

Ocular toxicity of authentic lunar dust.

BACKGROUND: Dust exposure is a well-known occupational hazard for terrestrial workers and astronauts alike and will continue to be a concern as humankind pursues exploration and habitation of objects beyond Earth. Humankind's limited exploration experience with the Apollo Program indicates that exposure to dust will be unavoidable. Therefore, NASA must assess potential toxicity and recommend appropriate mitigation measures to ensure that explorers are adequately protected. Visual acuity is critical during exploration activities and operations aboard spacecraft. Therefore, the present research was performed to ascertain the ocular toxicity of authentic lunar dust. METHODS: Small (mean particle diameter = 2.9 ± 1.0 µm), reactive lunar dust particles were produced by grinding bulk dust under ultrapure nitrogen conditions. Chemical reactivity and cytotoxicity testing were performed using the commercially available EpiOcularTM assay. Subsequent in vivo Draize testing utilized a larger size fraction of unground lunar dust that is more relevant to ocular exposures (particles <120 µm; median particle diameter = 50.9 ± 19.8 µm). RESULTS: In vitro testing indicated minimal irritancy potential based on the time required to reduce cell viability by 50% (ET50). Follow-up testing using the Draize standard protocol confirmed that the lunar dust was minimally irritating. Minor irritation of the upper eyelids was noted at the 1-hour observation point, but these effects resolved within 24 hours. In addition, no corneal scratching was observed using fluorescein stain. CONCLUSIONS: Low-titanium mare lunar dust is minimally irritating to the eyes and is considered a nuisance dust for ocular exposure. No special precautions are recommended to protect against ocular exposures, but fully shielded goggles may be used if dust becomes a nuisance.