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 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...

Objectives-To investigate whether there is an excess of leukaemias in 0-15 year old children among those living in close proximity (within 100 m) of a main road or petrol station. Methods-Data for 0-15 year old children diagnosed between 1990 and 1994 in the United Kingdom West Midlands were used. Postcode addresses were used to locate the point of residence which was compared with proximity to main roads and petrol stations separately, and to both together. Odds ratios (ORs) were calculated with solid tumours as a control, and incidence ratios (IRs) with population density as a control. Results-The method based on solid tumours as a control showed ORs of 1.61 (95% confidence interval (95% CI) 0.90 to 2.87) and 1.99 (95% CI 0.73 to 5.43), for those living within 100 m of a main road or petrol station respectively. When population was used as a control, the estimated IRs for leukaemia were 1.16 (95% CI 0.74 to 1.72) and 1.48 (95% CI 0.65 to 2.93) for residence within 100 m of a main road or petrol station respectively, but neither reached significance at the 95% level. Results for residence in close proximity to both a main road and petrol station were inconsistent, but there were few. The influence of socioeconomic factors as represented by the Townsend deprivation index on leukaemia incidence was not significant and the results were not explicable on the basis of impact of social class. Conclusions-The results are suggestive of a small increase in risk of childhood leukaemia, but not solid tumours, for those living in close proximity to a main road or petrol station. This increase in risk is not, however, significant and a larger study is warranted to establish the true risk and causes of any increase in risk. (Occup Environ Med 1999;56:774-780)

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.

Lead in bone can be measured in vivo using gamma-rays from a 109Cd source to excite lead K X-rays. Normalization of lead X-ray amplitudes to that of the elastically backscattered 88 keV gamma-rays produces a determination of the concentration of lead in bone mineral that is accurate and insensitive to variations in measurement or bone geometry. For in vivo tibia measurements, a typical precision (1 SD) of +/- 5 micrograms lead (g bone mineral)-1 is achieved for an effective dose equivalent of 2.1 microSv. Measurement can be made of any superficial bone site, but precision will vary approximately as the inverse of the square root of the mass of bone mineral sampled. The apparatus required for this technique is readily transportable, and mobile laboratory facilities are easily established.ImagesFIGURE 7.

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In vivo bone lead measurements have been made on a group of about 120 people, most of whom were lead exposed workers. Two different x-ray fluorescence (XRF) techniques were used to make measurements at three bone sites. Finger lead was measured using 57Co sources, and lead measurements were made in both tibia and calcaneus with a technique based on 109Cd sources. The results of the bone lead measurements correlated strongly with each other and with the index of cumulative exposure, thus confirming the value and reliability of these in vivo measurements as a tool in the study of chronic lead exposure. Measurement precision, +/- 1 standard deviation, was highest for tibia +/- 7.4 micrograms (g bone mineral)-1, +/- 16.6 micrograms (g bone mineral)-1 for the calcaneus and lowest for phalangeal lead +/- 25.0 micrograms (g bone mineral)-1. Maximum absorbed doses to the skin were comparable for all three measurements (1-3 mGy). The mean whole body dose equivalents were all low, but that for the finger measurement, 0.1 microSv, was significantly less than for the calcaneus and tibia measurements 3-5 microSv.

Atomic absorption spectrometry and x-ray fluorescence have been used to determine the lead content of metatarsal and tibia bone samples. For a range of bone lead levels from 6.5 to 83 micrograms g-1 of ashed bone there is no evidence of a systematic difference between the two techniques of more than 1 microgram g-1. There is, however, some evidence that random differences between the two in vitro analyses applied to the same bone sample are larger than can be accounted for by known measurement uncertainties. Variations in bone composition could account for these differences. Because the x-ray fluorescence technique is applied in an identical way to in vivo analysis, it is concluded that the uncertainties in in vivo measurements are small.