Study of the relationships between bone lead levels and its variation with time and the cumulative blood lead index, in a repeated bone lead survey Jose  A. A. Brito,* ab Fiona E. McNeill, a David R. Chettle, a Colin E. Webber c and Claude Vaillancourt d a Department of Physics and Astronomy, Nuclear Research Building, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada L85 4K1. E-mail: britoj@mcmaster.ca; Fax: z1 905 528 4339; Tel: z1 905 525 9140 ext. 27420 b Atomic Physics Center of the University of Lisbon, Lisbon, Portugal c Department of Nuclear Medicine, Chedoke-McMaster Hospitals, Hamilton, Ontario, Canada d NovaPb, Montre Âal, Que Âbec, Canada Received 20th March 2000, Accepted 10th April 2000 Published on the Web 17th May 2000 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. 109 Cd excited K X-ray ¯uorescence 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 ®rst survey. Measurements of whole blood lead were available for the large majority of subjects. Tibia lead concentrations fell signi®cantly (pv0.001) in the 51 subjects with repeated bone lead measurements, from a mean of 39 mg Pb (g bone mineral) 21 to 33 mg Pb (g bone mineral) 21 . The change correlated negatively with the initial tibia lead concentration, producing an estimate for an overall half-life of 15 years, with a 95% con®dence 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 mg Pb (g bone mineral) 21 initially, 65 mg Pb (g bone mineral) 21 5 years later). Subjects not measured previously had average lead concentrations of 15 mg Pb (g bone mineral) 21 in the tibia and 13 mg Pb (g bone mineral) 21 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 signi®cant fall in the calcaneus lead was surprising. Attempts should be made to repeat this observation. If con®rmed, 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. Introduction Lead exposure{ in both the occupational and general environment has been decreasing. 1±3 This laudable and welcome decrease has resulted in a lessened emphasis on adverse effects of acute lead exposure as these have been reduced or even largely eliminated in a number of countries. Instead, attention has increasingly been directed to more subtle sub-clinical effects of lead and to the long term consequences of lead exposure. It has long been known that bone is the major storage site of lead in the body 4,5 and that the retention of lead in bone can be described in terms of years or decades. In this context, the development of methods of measuring lead in bone in vivo has provided a tool that has proved to be apt to the task of increasing information about the long term storage of lead. Detailed models of human lead metabolism have been developed and have been shown to be well suited to simulating the behaviour of lead over periods of weeks to months. 6,7 This re¯ects the time span of data covered by blood lead measurements. However, these models were developed before in vivo bone lead data were at all widely available and they do not account well for the pattern of bone lead accumulation and its relation to individual blood lead histories. The advent of a number of substantial sets of bone lead data from cross-sectional studies, for which extensive blood lead monitoring records were also available, has demonstrated the imperfections in the existing models. The high likelihood is that the model structures are essentially sound, but that some of the transfer coef®cients and rate constants, directly or indirectly impacting the accumulation and release of lead in bone, need adjustment. There is a substantial data set incorporating longitudinal measurements of lead in ®nger bone in active and retired lead workers, 8,9 but there are few longitudinal data on the tibia or other bones, using the more precise 109 Cd excited K X-ray ¯uorescence technique. 10 The opportunity arose in 1998 to perform repeat bone lead measurements in the workforce of a secondary lead smelter. The workforce had originally had such measurements in 1993. {Abbreviations: T93, tibia lead 1993; T98, tibia lead 1998; DTIB, changes in tibia lead between surveys; C93, calcaneus lead in 1993; C98, calcaneus lead in 1998; DCAL, changes in calcaneus lead between surveys; CBLI93, cumulative blood lead in 1993; CBLI98, cumulative blood lead in 1998; DCBLI, changes in cumulative blood lead between surveys; CUR98, current blood lead in 1998. DOI: 10.1039/b002855j J. Environ. Monit., 2000, 2, 271±276 271 This journal is # The Royal Society of Chemistry 2000