Uncertainties in the relationship between tibia lead and cumulative blood lead index.

Uncertainties in the relationship between bone Pb and cumulative blood lead index (CBLI), including evidence of nonlinearity and differences between the sexes, should be appropriately recognized when setting workplace blood Pb limits to achieve target bone Pb concentrations.

Schwartz and Hu (2007) recommended a maximum occupational tibia Pb concentration of 15 μg/g. They stated that, based on the slope of the relationship between tibia Pb and CBLI calculated by Hu et al. (2007), a tibia Pb of 15 μg/g can be avoided by limiting the CBLI to < 200–400 μg-years/dL.

Hu et al. (2007) acknowledged the uncertainty in the slope of the relationship between tibia Pb and CBLI. However, over the range of cumulative Pb exposures that would produce a tibia Pb concentration of 15 μg/g, the slope of the relationship between tibia Pb and CBLI may be less than the slope of 0.05 [95% confidence interval (CI), 0.046–0.055] μg/g per μg-years/dL calculated by Hu et al. (2007).

Table 1 presents slopes and mean tibia Pb concentrations among subjects of eight published studies. Gerhardsson et al. (1993) reported a slope of 0.022 μg/g per μg-years/dL (no uncertainty reported) and Armstrong et al. (1992) reported a slope of 0.10 (± 0.02) μg/g per μg-years/dL. These represent a greater range of slopes than reported by Hu et al. (2007).

Table 1

Various slopes of the relationship between tibia Pb and CBLI, and related mean tibia Pb concentration among study subjects.

Study	No.	r	Slopea	Mean Pb (μg/g tibia bone mineral)
Gerhardsson et al. (1993)	100	0.60	0.022	16.9
Erkkilä et al. (1992)	91	0.66	0.028 ± 0.003	21.1
Somervaille et al. (1988)	79	0.86	0.050 ± 0.003	31.0
Somervaille et al. (1988)	88	0.82	0.060 ± 0.005	32.3
Cake (1994)	53	0.70	0.059 ± 0.009	39
Fleming et al. (1997)	367	0.83	0.056 ± 0.002	40.6
Hu et al. (1991)	12	0.92	0.061 ± 0.008	46
Armstrong et al. (1992)	15	0.87	0.10 ± 0.02	54.8

Units are μg/g bone mineral per μg-year/dL.

These data also suggest that the tibia Pb versus CBLI slope may not be constant, with lower slopes evident for lower tibia Pb and CBLI levels. This trend has been noted previously (Chettle 2005; Fleming et al. 1997). For tibia Pb concentrations of approximately 15 μg/g, a slope of approximately 0.025 μg/g per μg-years/dL seems equally plausible as the slope calculated by Hu et al. (2007).

A slope of 0.025 μg/g per μg-years/dL yields an allowable CBLI of 600 μg-years/dL, or an average annual blood Pb concentration of 15 μg/dL for 40 working years. This compares to 5–10 μg/dL for 40 working years associated with Schwartz and Hu’s (2007) recommended CBLI of 200–400 μg-years/dL.

These slopes are also based on studies of predominantly male subjects and may not account for differences in Pb toxicokinetics between the sexes (McNeill et al. 2000; Popovic et al. 2005).