Longitudinal changes in bone lead concentration: implications for modelling of human bone lead metabolism

In this study, 539 occupationally exposed subjects received in vivo bone le ad measurements using Cd-109 excited K X-ray fluorescence (Cd-109 K XRF). O f these subjects, 327 had previously been measured five years earlier. Meas urements were made from both tibia and calcaneus samples, taken to reflect cortical and trabecular bone, respectively. Changes in tibia lead concentra tion related negatively to initial tibia lead concentration and positively to both lead exposure between the measurement dates and initial calcaneus l ead concentration. This finding confirmed and strengthened the interpretati on of an earlier study involving fewer subjects. With the larger data set i t was possible to examine subgroups of subjects. This showed that people ag ed less than 40 years had a shorter half-life for the release of lead from the tibia (4.9, 95% CI 3.6-7.8 years) than did those older than 40 (13.8, 9 5% CI 9.7-23.8 years). Similarly, less intensely exposed subjects (lifetime average blood lead less than or equal to 25 mug dL(-1)) had a shorter tibi a lead half-life (6.2, 95% CI 4.7-9.0 years) than those with a lifetime ave rage blood lead > 25 mug dL(-1) (14.7, 95% CI 9.7-29.9 years). Age and meas ures of lead exposure were strongly correlated; nevertheless, age matched s ubgroups with high and low intensity exposures showed clearance rates that were significantly different at the 10% level, with the lower exposure inte nsity again being associated with the faster clearance. These findings impl y that current models of human lead metabolism should be examined with a vi ew to adjusting them to account for kinetic rates varying with age and prob ably also with exposure level.