The fall of zinc protoporphyrin levels in workers treated for chronic lead intoxication.

A temporal fall of zinc protoporphyrin (ZPP) levels in whole blood was observed in 51 patients with occupational chronic lead intoxication who were removed from exposure, treated with intravenous calcium disodium edetate (EDTA), and followed for periods up to 2,273 days. ZPP levels fell, with a mean half-life of 68 days, to a mean baseline level of 36 micrograms/dl of whole blood. The baseline ZPP level was positively associated with the length of exposure (p less than .01) and the blood lead half-life (p less than .001). The amount of EDTA received had no apparent effect on ZPP levels. These data suggest that the fall of ZPP levels is largely a function of red blood cell turnover. The baseline ZPP level appears to be a useful biologic index of the biologically active pool of lead for at least two years after removal from exposure.