As noted previously, lead has a residence time in blood of only 30–45 days (Rabinowitz et al., 1976, 1977). Hence, measurements of lead in blood reflect recent exposure. A blood lead concentration might be more useful when lead exposure can be reliably assumed to have been at a given concentration, as in occupationally exposed adults, than when intermittent exposure is occurring or has occurred, as in children exposed to leaded paint. With the recent development of complementary K-line and L-line XRF techniques to measure bone lead stores noninvasively (Somervaille et al., 1985, 1986, 1988; Rosen et al., 1989, 1991; Wielopolski et al., 1989; Rosen and Markowitz, 1993), it now appears possible to assess directly a time-averaged compartment of lead in bone, where its residence time is months to years; however, wide clinical application of XRF still needs to be developed (Rabinowitz et al., 1976, 1977; Chamberlain et al., 1978). XRF has the potential to relate bone lead stores (and blood lead concentration), as predictive outcome measures, to the development of early expressions of lead toxicity, such as biochemical, electrophysiologic, and neurobehavioral indexes (Rosen et al., 1989, 1991; Rosen and Markowitz, 1993). In a pioneering study, Needleman et al., (1979) found that short-term and long-term neurobehavioral deficits caused by lead were closely correlated to lead concentrations in compact bone, as reflected in tooth lead concentrations (Needleman et al., 1990). Moreover, because blood lead concentrations decrease once excessive lead exposure ends and a course of chelation treatment has been successfully completed, blood lead concentration is likely to underestimate high bone concentrations (Christoffersson et al., 1986; Rosen et al., 1991; Rosen and Markowitz, 1993).

KXRF methods developed in Sweden (Ahlgren et al., 1980) and England (Somervaille et al., 1985, 1986, 1988; Chettle et al., 1989; Armstrong et al., 1992) have demonstrated the clinical utility of bone lead measurements in industrially exposed adults. The minimal detection limit for lead, with 3 mm of overlying tissue, was about 10 µg/g of wet bone (Somervaille et al., 1988). Specifically, studies by the English group showed that KXRF measurements are a good indicator of long-term lead exposure, as assessed with the cumulative blood lead concentration index in 20 control subjects and 190 workers in lead industries (Somervaille et al., 1988). Moreover, Christoffersson et al. (1986), using the KXRF technique, showed progressive decreases in tibial lead content with a mean half-time (short-term and long-term) of