(Todd and Chettle 1994). The radiation dose is not a limiting factor in using this technique with humans, as demonstrated by Todd and Chettle (1994). Calibration is usually performed with Pb- doped plaster-of-Paris phantoms (Todd et al. 2002a). Method accuracy has been evaluated through comparison of XRF data from cadaver specimens with electrothermal atomic absorp- tion spectrophotometry data (Todd et al. 2002b). However, XRF sensitivity and preci- sion for Pb still constitute an analytical chal- lenge. In addition to sample-to-sample reproducibility, XRF can also display a certain amount of imprecision associated with each calculated bone-Pb value (Ambrose et al. 2000). This uncertainty, estimated using a goodness-of-ﬁt statistic from the curve ﬁtting of the background, ranged from 3 to 30 µg/g Pb; clearly, this represents a problem for meas- urements of low-level Pb namely, young chil- dren and nonexposed populations. Another problem inherent to the XRF technique is photon scattering due to overlying tissue or subject movement during the measurement period (Ambrose et al. 2000). Normalization of the Pb signal to the calcium backscatter sig- nal appears to solve this problem. Precision depends on the amount of tissue overlying the bone: the greater the thickness of tissue, the poorer the precision. Todd and Chettle (1994), comparing the K-shell with L-shell precisions with 3 and 6 mm of overlying soft tissue, reported that K-XRF precision worsens by only 5%, whereas L-XRF precision worsens by 49% for greater thickness. The precision of the L-XRF method is much more severely affected by the strong attenuation of the Pb L-shell X rays. Todd et al. (2001) reported contiguous inhomogeneities in the distribution of Pb toward the proximal and distal ends of the tibia bones. They speculated that the region of lower Pb concentration has lower blood ﬂow in the Haversian canals and, consequently, less Pb available for uptake into bone matrix dur- ing bone remodeling (Todd et al. 2001). Trabecular bone has a larger surface area and a greater volume of blood delivered per unit of time compared to cortical bone. In addition there are more active osteons per gram in tra- becular bone to accomplish resorption and deposition. Hernandez-Avila et al. (1998) reported that, in individuals with no history to occupational Pb exposure, bone-Pb (in particu- lar trabecular Pb) exerts an additional indepen- dent influence on plasma-Pb after control for BPb. Thus, an appropriate selection of the pre- cise bone type to be analyzed for Pb content must be made before commencing. Moreover, further research on the relationship between various bone-Pb subcompartments and other Pb measures is warranted. Tooth Lead Like bone, teeth accumulate Pb over the long term. However, there is some evidence that teeth are superior to bone as an indicator of cumulative Pb exposure because the losses from teeth are much slower (Maneakrichten et al. 1991). Moreover, deciduous teeth are relatively easy to collect and analyze; exfoliation generally occurs after the age of 6 years. Teeth are also very stable for preservation purposes. Chronic Pb exposure from mouthing activ- ity in early childhood may be camouﬂaged by “dilution” effects during periods of rapid skele- tal growth in young children and adolescents and may not be detected by a single BPb meas- urement. However, most published data on tooth-Pb have been based on whole tooth analysis, with no attempt to distinguish among tooth types (different teeth are formed at dif- ferent ages) or to differentiate the Pb concen- tration in enamel from that in dentin (enamel contains much more Pb, by mass, than does dentin). The inﬂuence of age and/or sex have also not been considered (Brown et al. 2002). Furthermore, use of deciduous teeth is only possible for children over 6 years in age. Recently, Gomes et al. (2004) proposed a solution to the limitations mentioned above by using an in vivo enamel biopsy of children. In this approach superficial minerals are leached from teeth and Pb is determined by electrothermal atomic absorption spectro- photometry. One important drawback to this approach is that, because an accumulation gradient for Pb has not yet been established for enamel, only biopsies of a given depth can be compared to one another. Another issue related to tooth-Pb measurements is whether Pb that accumulates in the ﬁrst few microme- ters of the enamel surface was incorporated posteruptively (e.g., from the mouth, saliva, food) rather than during the period when the tooth was mineralized inside the bone. An interesting and potentially valuable aspect of tooth-Pb measurements is their capacity to elucidate the history of Pb expo- sure. Teeth are composed of several distinct tis- sues formed over a period of several years, and different parts of the tooth can bind Pb at dif- ferent stages of the individual’s life. Therefore, a section of tooth can yield historical informa- tion on the individual’s exposure to Pb. For example, the enamel of all primary teeth, and parts of the enamel from some permanent teeth, are formed in utero and thus may pro- vide information on prenatal exposure to Pb. This information could be valuable in improv- ing our understanding of dose−effect relation- ships for embryonic anomalies, particularly neurotoxic dysfunction. The dentine of the primary teeth provides evidence of exposure during the early childhood years, when hand-to-mouth activity is usually an important contributor to Pb body burden (Gulson Environmental Health Perspectives • VOLUME 113 | NUMBER 12 | December 2005 Biomarkers for monitoring lead exposure 1996b). However, enamel Pb levels may be useful for indirectly estimating the Pb compo- sition of the mother’s bone (Gulson 1996b). More recently there has been some inter- est in using laser ablation ICP-MS to examine Pb distribution in tooth profiles. This approach offers spatially resolved measure- ments of trace element distribution that can be compared to a temporal axis via reference to the neonatal line, enabling researchers to use not only the Pb concentration of the entire tooth but also the specific amount of Pb in each tooth layer, namely, a time line of Pb exposure. Nevertheless, some serious chal- lenges remain before this technique can be fully exploited (Uryu et al. 2003). Conclusions Thus far an impressive body of data has been established based on the use of alternative bio- markers for monitoring exposure to Pb. However, it is still unclear to what extent such data are superior to the information obtained from BPb measurements. Clearly, many of the limitations identiﬁed in the foregoing sections must be resolved before alternative biomarkers can be accepted as superior indicators of Pb exposure. At this time BPb measurements are still the most reliable indicator of recent Pb exposure, although serial BPb measurements may offer a better assessment of temporal ﬂuc- tuations in Pb absorption. If reliable and repro- ducible plasma-Pb measurements can be obtained, these may offer better correlation with toxic effects. However, we do not yet know what a single plasma-Pb value means, in terms of health effects; in the absence of a nor- mal reference range, the clinical utility for indi- vidual assessment is problematic. Further research on this issue is needed, especially for children and adults with low to moderate Pb exposure. Further efforts are also warranted in the further development and continued use of well-established analytical protocols, as well as in the estimation of random and systematic errors. Efforts are needed to create regional ref- erence ranges of nonexposed populations for each biomarker, to acquire data related to long- term and short-term exposures, and to evaluate the inﬂuence of nutritional status and ethnicity (genetic polymorphisms). A critical question that might be asked with respect to an individual’s bone-Pb meas- urement is what does it mean in terms of health risk or, perhaps, clinical management? To answer this question, we may need to dis- tinguish between bone-Pb measurements in children and pregnant women, namely, those with high bone turnover rate compared to (nonpregnant) adults. In children, bone-Pb may have little effect on BPb levels, but it may help us to estimate the extent to which BPb derives from endogenous sources and the possi- ble contribution to the labile plasma-Pb pool. 1673