1) A physiologically based pharmacokinetic model was used to predict blood lead levels in astronauts exposed to lead from microgravity-accelerated bone loss. The model found that for most astronauts, long duration spaceflights should cause minimal health concerns from lead toxicity. 2) The biggest influence on blood lead levels in microgravity is the amount of lead stored in bones at launch. For typical astronauts, drinking water lead levels under 9 μg/L were not predicted to increase blood lead levels compared to pre-flight. 3) Spacecraft water exposure guidelines for lead of 9 μg/L were set for missions of 100 and 1000 days, as this level should not increase blood lead over average pre-launch values for