The factors affecting the accuracy and minimum detectable concentration of in vivo tibia lead measurement are discussed, and it is demonstrated that the use of a 109Cd source in a backscatter geometry and using the 88 keV coherently scattered photon for normalisation optimizes both criteria. The measurement is shown to be independent of variations in source-sample distance, thickness of overlying tissue and tibia size and shape. Applying the same technique in vitro to samples of human tibia and metatarsals, it is shown that the results are not significantly different (p approximately equal to 0.9) from atomic absorption spectrometry results from another laboratory. The results of Monte Carlo dose distribution calculations are presented and compared with measurements using thermoluminescent dosemeters: the mean absorbed dose to a 20 cm leg section is less than 0.1 mGy (10 mrad) and the maximum absorbed skin dose is 0.45 mGy (45 mrad). For this dose the minimum detectable lead concentration is 10 micrograms g-1. Finally, the technique has been applied to groups of normals and occupationally exposed workers, and the means have been shown to be significantly different, namely 10 and 31 micrograms g-1 respectively. In the normal subjects tibia lead correlated strongly with age (r = 0.63, p less than 0.001).

In-vivo measurements of lead concentrations in calcaneus (mainly trabecular bone) and tibia (mainly cortical bone) were performed by x-ray fluorescence (XRF) in 70 active and 30 retired lead smelter workers who had long-term exposure to lead. Comparison was made with 31 active and 10 retired truck assembly workers who had no known occupational exposure to lead. After physical examination, all participants provided blood and urine samples and answered a computerized questionnaire. Since 1950, blood lead has been determined repeatedly in lead workers at the smelter, which made it possible to calculate a time-integrated blood lead index for each worker. Lead concentrations in blood, urine, calcaneus, and tibia in active and retired lead workers were significantly higher than in the corresponding control groups (p < .001). The highest bone lead concentrations were found among retired lead workers (p < .001), which was the result of considerably higher lead exposure during 1940 to 1960. Lead concentrations in calcaneus in active lead workers were significantly higher than in tibia when expressed in ug of lead per gram of bone mineral, which suggests a quicker absorption over time in this mainly trabecular bone. The estimated biological half-times were 16 y in calcaneus (95% confidence interval [95% CI] = 11-29 y) and 27 y in tibia (95% CI = 16-98 y). A strong positive correlation was found between lead concentrations in calcaneus and tibia for all lead workers (r = 0.54; p < .001). A strong positive correlation was also found between the bone lead concentrations and the cumulative blood lead index. Blood lead, at the time of study, correlated well with bone lead concentrations in retired--but not in active--workers, reflecting the importance of the endogenous (skeletal) lead exposure. The findings in this study indicate that bone lead measurements by XRF can give a good index of long-term lead exposure. Tibia measurements offer a higher precision than calcaneus measurements. The method is of particular interest in epidemiologic studies of adverse health effects caused by long-term lead exposure.

In vivo tibia lead measurements of 20 non-occupationally exposed and 190 occupationally exposed people drawn from three factories were made using a non-invasive x ray fluorescence technique in which characteristic x rays from lead are excited by gamma rays from a cadmium-109 source. The maximum skin dose to a small region of the shin was 0-45 mSv. The relation between tibia lead and blood lead was weak in workers from one factory (r = 0 11, p > 0.6) and among the non-occupationally exposed subjects (r = 0 07, p > 0 7); however, a stronger relation was observed in the other two factories (r = 0 45, p < 0 0001 and r = 0 53, p < 0-0001). Correlation coefficients between tibia lead and duration of employment were consistently higher at all three factories respectively (r = 0-86, p < 0-0001; r = 0-61, p < 0-0001; r = 0 80, p < 0 0001). A strong relation was observed between tibia lead and a simple, time integrated, blood lead index among workers from the two factories from which blood lead histories were available. The regression equation from two groups of workers (n = 88, 79) did not significantly differ despite different exposure conditions. The correlation coefficient for the combined data set (n = 167) was 0-84 (p < 0-0001). This shows clearly that tibia lead, measured in vivo by x ray fluorescence, provides a good indicator of long term exposure to lead as assessed by a cumulative blood lead index.As a consequence of the well established toxicity of lead, workers occupationally exposed to it in the United Kingdom and other industrialised countries are subjected to regular monitoring of blood lead concentrations. In In vivo tibia lead measurements as an index ofcumulative exposure in occupationally exposed subjects is relatively stable, as with the tibia, it is feasible to normalise per mass of wet bone. The relation between wet bone mass and bone mineral in trabecular bone, however, is less well defined and changes with, among other things, age, particularly in women. Because our technique normalises to the gamma rays coherently scattered from both calcium and phosphorus, the most logical normalisation is therefore to the bone mineral mass. This is equivalent to quoting the lead content per mass of bone ash, a unit that is widely used for in vitro analysis. A possible alternative, particularly for those making biopsy measurements using atomic absorption spectrometry, is to normalise to the calcium content: however, the relation between the two procedures is readily established assuming bone mineral to consist of calcium hydroxyapatite (Ca10(P04)6(OH)2). As our measurement programme is being extended to include trabecular bone we have therefore chosen to normalise to bone mineral mass throughout.x Ray fluorescence, which involves stimulation of characteristic x ray emission from the element of interest using a beam of photons, has been used by several groups to measure bone lead. The first to do so were Ahlgren and co-workers,45 who measured the lead K. x ray emission (at 75 0 and 72-8 keV for K., and...

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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.