available with < 5 µg/L Pb (Esernio-Jenssen et al. 1999), but it is nevertheless desirable for the analyzing laboratory to characterize the background Pb contamination in each new lot of tubes to ensure that reported concentrations are not compromised by contamination. The choice of anticoagulant is important because EDTA, as a strong metal-chelating agent, may be difficult to obtain without some back- ground contamination and may give mislead- ingly high plasma-Pb results because of selective extraction of Pb bound to erythro- cytes. The use of heparin is problematic because heparinized blood is more prone to form fibrin clots after several hours. These issues were evaluated by Smith et al. (1998) in some detail; they compared commercial Vacutainer-type tubes with ultracleaned collec- tion tubes containing either EDTA or heparin. As there are no commercial blood collection tubes available that are certiﬁed for ultra-low Pb measurements, the analyzing laboratory should prepare precleaned polyethylene tubes containing ultra low-Pb anticoagulants. There are many reports of plasma-Pb measurements where validation data are either weak or absent. For example, some simply cite successful participation of the analyzing labora- tory in quality assurance (QA) programs for whole blood Pb operated by the CDC and the College of American Pathologists (Hernandez- Avila et al. 1998), whereas others neglect to cite any kind of QA program (Dombovari et al. 2001). Participation in QA schemes designed specifically for whole BPb, while commendable, does not address the much more challenging analysis for plasma-Pb. This problem is compounded by the lack of certi- fied reference materials for either serum or plasma-Pb (Cake et al. 1996). For these rea- sons, production of plasma or serum reference materials that have Pb concentrations certiﬁed close to current human values is urgently needed to support method validation. Saliva Lead Saliva has been proposed as a diagnostic specimen for various purposes, as it is easily collected (Silbergeld 1993). However, in the absence of consistent and dependable saliva Pb measurements, it is not generally accepted as a reliable biomarker of Pb exposure. Saliva shows large variations in its ion content throughout the day, coupled with changes in salivary ﬂow rates before, during, and after meals. Variations also arise depending on the manner in which saliva collection is stimulated (or not) and on the nutritional and hormonal status of the individual. Some data suggest an association between Pb levels in saliva and those in either plasma or blood (Omokhodion and Crockford 1991; Pan 1981). Moreover, it has been argued that Pb in saliva is the direct excretion of the Pb fraction in diffusible plasma namely, the frac- tion not bound to proteins) (Omokhodion and Crockford 1991). Despite the associations reported in the literature, the older saliva Pb concentrations are quite high, and the values vary among studies. Recent data suggest much lower saliva Pb levels, in both exposed and unexposed subjects (Koh et al. 2003; Wilhelm et al. 2002). According to Wilhelm et al. (2002), Pb content in the saliva of unexposed children is usually < 0.15 µg/dL. Uncontrolled variation in salivary flow rates, lack of standard or certified reference materials, and absence of reliable reference val- ues for human populations are major factors that limit the utility of saliva Pb measurements. In addition the very low levels of Pb present in saliva limit the range of suitable analytical tech- niques, thereby further diminishing the utility and reliability of this biomarker for evaluating Pb exposure. Hair Lead Hair is a biological specimen that is easily and noninvasively collected, with minimal cost, and it is easily stored and transported to the laboratory for analysis. These attributes make hair an attractive biomonitoring substrate, at least superficially. Because Pb is excreted in hair, many have suggested it for assessing Pb exposure, particularly in developing countries where specialized laboratory services may be unavailable and resources are limited (Schumacher et al. 1991). However, an exten- sive debate is ongoing about the limitations of hair as a biomarker of metal exposure gener- ally. Here we limit our discussion to Pb expo- sure, although many of the issues for Pb, such as preanalytical concerns for contamination control, sampling, and reference ranges, also apply to other metals. The ability to distinguish between Pb that is endogenous, namely, absorbed into the blood and incorporated into the hair matrix, and Pb that is exogenous, namely, derived from exter- nal contamination, is a major problem. During the washing step it is assumed that exogenous Pb is completely removed, whereas endogenous Pb is not. However, no consensus exists about how removal of exogenous Pb is best accom- plished. Some publications that describe the use of hair for assessing Pb exposure reference a hair washing method proposed by the International Atomic Energy Agency (IAEA) in 1978. The approach entailed washing hair specimens with acetone/water/acetone (Ryabukin 1978). However, a recent study (Morton et al. 2002) demonstrated that the IAEA method failed to remove exogenous Pb from hair. Another issue is the signiﬁcant variation in the Pb concentration profile among various subpopulations according to age, sex, hair color, and smoking status (Wolfsperger et al. 1994). Moreover, geographic, racial/ethnic, Environmental Health Perspectives • VOLUME 113 | NUMBER 12 | December 2005 Biomarkers for monitoring lead exposure and ecologic factors can also affect Pb distribu- tion in hair within a given population. Thus, it is difﬁcult to establish reference ranges because confounding factors impose restrictions on the interpretation of individual results. No consen- sus exists on the length of the hair specimen to be collected, or the amount, or the position on scalp. Variations in Pb content between single hairs from the same individual can be as high as ± 100%, particularly in the distal region (Renshaw et al. 1976). Recently, the ATSDR established an expert advisory panel to review current knowledge about the use of hair analysis for trace metals in biomonitoring (ATSDR 2001). The general consensus was that many scientiﬁc issues need to be resolved before hair analysis can become a useful tool in understanding environmental exposures. Although hair analysis may be able to answer some speciﬁc questions about envi- ronmental exposure to a few substances, it often raises more questions than it answers. The scientiﬁc community currently does not know the range of Pb contamination levels typically found in human hair. Without reli- able data on baseline or background hair con- tamination levels in the general population, health agencies cannot determine whether results from a given site are unusually high or low (ATSDR 2001). In addition to the preanalytical issues and the absence of reliable reference ranges, the quality of analytical techniques used for deter- mining Pb, as well as other trace metals, in hair has been questioned. In a recent interlaboratory study of commercial laboratories that speciﬁ- cally market the test for trace metals in hair, interlaboratory agreement was judged very poor, with wide discrepancies observed for Pb as well as for other elements (Seidel et al. 2001). Urinary and Fecal Lead The determination of Pb in urine (urine-Pb) is considered to reﬂect Pb that has diffused from plasma and is excreted through the kidneys. Collection of urine for Pb measurements is noninvasive and is favored for long-term bio- monitoring, especially for occupational expo- sures. However, a spot urine specimen is particularly unreliable because it is subject to large biological variations that necessitate a cre- atinine excretion correction. Urine-Pb origi- nates from plasma-Pb that is filtered at the glomerular level; thus, according to some authors (Tsaih et al. 1999), urine-Pb levels that are adjusted for glomerular ﬁltration rate may serve as a proxy for plasma-Pb. Hirata et al. (1995) found a better correlation between the concentration of plasma-Pb and urine-Pb than between BPb and urine-Pb for lead workers with low levels of Pb exposure. Manton et al. (2000), using high-precision Pb isotope ratio measurements, found the concentration of urine-Pb to be about 10% of that in whole 1671