Estimate of safe human exposure levels for lunar dust based on comparative benchmark dose modeling

Abstract

Brief exposures of Apollo astronauts to lunar dust occasionally elicited upper respiratory irritation; however, no limits were ever set for prolonged exposure to lunar dust. The United States and other space faring nations intend to return to the moon for extensive exploration within a few decades. In the meantime, habitats for that exploration, whether mobile or fixed, must be designed to limit human exposure to lunar dust to safe levels. Herein we estimate safe exposure limits for lunar dust collected during the Apollo 14 mission. We instilled three respirable-sized (∼2 μ mass median diameter) lunar dusts (two ground and one unground) and two standard dusts of widely different toxicities (quartz and TiO₂) into the respiratory system of rats. Rats in groups of six were given 0, 1, 2.5 or 7.5 mg of the test dust in a saline-Survanta® vehicle, and biochemical and cellular biomarkers of toxicity in lung lavage fluid were assayed 1 week and one month after instillation. By comparing the dose--response curves of sensitive biomarkers, we estimated safe exposure levels for astronauts and concluded that unground lunar dust and dust ground by two different methods were not toxicologically distinguishable. The safe exposure estimates were 1.3 ± 0.4 mg/m³ (jet-milled dust), 1.0 ± 0.5 mg/m³ (ball-milled dust) and 0.9 ± 0.3 mg/m³ (unground, natural dust). We estimate that 0.5–1 mg/m³ of lunar dust is safe for periodic human exposures during long stays in habitats on the lunar surface.

• Benchmark dose
• inhalation
• lunar dust
• mineral dust

Acknowledgements

We thank David McKay and Bonnie Cooper of Johnson Space Center (JSC) for preparation of the lunar dusts, Alan H. Feiveson of JSC for statistical consulting, Jon Rask of Ames Research Center for excellent technical assistance, Vincent Castranova for welcoming us to work in his laboratory at the NIOSH and Patti C. Erdely, Terrence G. Meighan, Mark Barger and Shih-Houng Young of the NIOSH for technical expertise. We thank Jane Krauhs for editorial assistance and Cynthia Bush for graphics assistance.