Measurements of bone lead concentrations in the tibia, wrist, sternum, and calcaneus were performed in vivo by x ray fluorescence on active and retired lead workers from two acid battery factories, office personnel in the two factories under study, and control subjects. Altogether 171 persons were included. Lead concentrations in the tibia and ulna (representative of cortical bone) appeared to behave similarly with respect to time but the ulnar measurement was much less precise. In an analogous fashion, lead in the calcaneus and sternum (representative of trabecular bone) behaved in the same way, but sternal measurement was less precise. Groups occupationally exposed to lead were well separated from the office workers and the controls on the basis of calculated skeletal lead burdens, whereas the differences in blood lead concentrations were not as great, suggesting that the use of concentrations of lead in blood might seriously underestimate lead body burden. The exposures encountered in the study were modest, however. The mean blood lead value among active lead workers was 1-45 pmol l1 and the mean tibial lead concentration 21 1 pg (g bone mineral)-'. posure. Calcaneal lead concentration, by contrast, was strongly dependent on the intensity rather than duration of exposure. This indicated that the biological half life of lead in calcaneus was less than the seven to eight year periods into which the duration of exposure was split. Findings for retired workers clearly showed that endogenous exposure to lead arising from skeletal burdens accumulated over a working lifetime can easily produce the dominant contribution to systemic lead concentrations once occupational exposure has ceased.Lead is a widely used toxic metal that accumulates in the body. It is concentrated in bone, which contains over 90% of the body burden in adults.' Occupational exposure to lead is routinely monitored by determination of blood lead concentrations, which largely reflect recent average exposure as the half life of lead in blood is of the order of 35 days.2 Blood lead concentration has been shown to be associated with indicators of adverse effects on haem synthesis, such as free erythrocyte protoporphyrin,' and with neurophysiological4 and psychological effects.56The relation between blood lead concentration and exposure is, however, not necessarily linear7 and, in particular, it has been recognised that in a model of a skeletal subcompartment, the lead should be considered readily exchangeable and constitute an intrinsic source of lead input to the blood.

The objective of the study was to identify those factors that should affect treatment planning for patients who have lower third molars, using decision-analytic techniques. Utility values based on data from 104 patients indicated that the respondents considered that postoperative complications (except mild pain and temporary paresthesia) reduced health to a greater degree than did complications following non-intervention. A decision analysis indicated that the maximum expected utility of prophylactic third-molar surgery (60.25) was lower than that for non-intervention (76.96). The decision was sensitive to changes in the probabilities of occurrence of recurrent pericoronitis (threshold = 0.52), resorption of an adjacent tooth (threshold = 0.29), loss of an adjacent tooth (threshold = 0.32), and cystic change (threshold = 0.34). These thresholds are much higher than the incidence of problems affecting the lower third molar shown by a concurrent clinical audit and literature review. This study therefore suggests that lower third molars should not be removed prophylactically.

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