Serum lead, whole blood lead, and lead in both tibia and calcaneus were measured in each of 49 active lead workers. Serum lead correlated more strongly with both in vivo bone lead measurements than did whole blood lead. The ratio of serum lead to whole blood lead varied from 0.8% to 2.5% and showed a positive correlation with tibia, and an even stronger correlation with calcaneus lead. This implies that lead released from bone (endogenous exposure) results in a higher proportion of whole blood lead being in serum than is the case for exogenous exposure. This observation needs to be confirmed, and the relationships amongst the parameters must be studied further, particularly in former or retired lead workers. If confirmed, since at least a portion of lead in serum is readily diffusible and thus toxicologically more immediately significant than lead bound to red cells, the health implications of endogenous exposure may have to be reassessed. © 1996 Wiley‐Liss, Inc.

The study aims were to: (i) investigate long term human lead metabolism by measuring the change of lead concentration in the tibia and calcaneus; and (ii) assess whether improved industrial hygiene was resulting in a slow accumulation of lead in an exposed workforce. 109Cd excited K X-ray fluorescence was used to measure tibia lead and calcaneus lead concentrations in 101 workers in a secondary lead smelter. 51 subjects had had similar bone lead measurements 5 years previously. Most of the other subjects had been hired since the first survey. Measurements of whole blood lead were available for the large majority of subjects. Tibia lead concentrations fell significantly (p<0.001) in the 51 subjects with repeated bone lead measurements, from a mean of 39 microg Pb (g bone mineral)(-1) to 33 microg Pb (g bone mineral)(-1). The change correlated negatively with the initial tibia lead concentration, producing an estimate for an overall half-life of 15 years, with a 95% confidence interval of 9 to 55 years. Adding continuing lead exposure and recirculation of bone lead stores to the regression models produced half-life estimates of 12 and 9 years, respectively, for release of lead from the tibia. The repeat subjects showed no net change in calcaneus lead (64 microg Pb (g bone mineral)(-1) initially, 65 microg Pb (g bone mineral)(-1) 5 years later). Subjects not measured previously had average lead concentrations of 15 microg Pb (g bone mincral)(-1) in the tibia and 13 microg Pb (g bone mineral)(-1) in the calcaneus. The rate of clearance of lead from the tibia (9 to 15 years) is towards the more rapid end of previous estimates. The lack of a significant fall in the calcaneus lead was surprising. Attempts should be made to repeat this observation. If confirmed, it would have implications for models of lead metabolism. The relatively low lead concentrations in the non-repeat subjects are reassuring. However, observation after a longer period of employment would be desirable.

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Reflections from the front lines of the free trade struggles.