Biologic effects of airborne particulate lead on continuously exposed rats and rhesus monkeys

The biological effects of airborne particulate lead were studied in rats and rhesus monkeys (Macaca mulatta) exposed nearly continuously (22 hours per day) at a concentration of 21.5 mug Pb/cubic meter of air. The year-long exposure elicited an increase in the concentration of lead in the blood to 28 mug/100 ml in the rats and 17 mug/100 ml in the monkeys. Levels of lead in control animals were about 5 mug/100 and 4 mug/100 ml, respectively, in the rats and monkeys. The elevated blood-lead levels reached a maximum during the first few months of exposure and did not increase significantly after that time. Elevated concentrations of lead were also detected in lung, liver, kidney and bone of both species. While concentrations of lead were nearly always less than 1 mug/g in control animals, they rose to 2-3 mug/g in soft tissues of exposed animals and up to 6 mug/g in bone. Differences between lead levels in animals exposed six months or twelve months were very small. The concentration of lead decreased in the soft tissues of animals removed from the chamber after one year's exposure but remained elevated in bone. The activity of the enzyme delta-amino-levulinic acid dehydrase (ALAD) in circulating erythrocytes was inhibited about 70% in rats exposed to the airborne lead. No reduction of the enzyme activity occurred in brain or liver tissue of either the rats or monkeys. Normal levels of ALAD in erythrocytes of the monkeys were too low to permit meaningful measurements. In spite of the reduced ALAD activity in rat erythrocytes, there was no concomitant alteration in the excretion of heme precursors. In all other respects, there appeared to be no deleterious effects on the animals from exposure to the airborne particulate lead. No changes were detected in serum chemistry or hematology nor were there any gross, microscopic or ultrastructural pathologic changes observed.