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.