Derivation of a Target Concentration of Pb in Soil Based on Elevation of Adult Blood Pressure

The increase in systolic blood pressure in males appears to be the most sensitive adult endpoint appropriate for deriving a health risk‐based target level of lead (Pb) in soil. Because the response of blood pressure to blood Pb concentration (PbB) has no apparent threshold, traditional approaches based on the application of a Reference Dose (RfD) are not applicable. An alternative approach is presented based on a model which predicts the population shift in systolic blood pressure from ingestion of Pb contaminated soil as a simultaneous function of exposure to Pb in soil, the baseline distribution of blood Pb concentration in the population and the baseline distribution of systolic pressure in the population. This model is analyzed using Monte Carlo analysis to predict the population distribution of systolic pressure resulting from Pb exposure. Based on this analysis, it is predicted that for adult males 18‐65 years old, exposure to 1000 ppm Pb in soil will result in an increase of approximately 1 mm Hg systolic pressure, an increase in the incidence of systolic hypertension (i.e., systolic pressure >140 mm Hg) of approximately 1% and an increase in PbB of 1–3 μg/dl. Based on the proposition that these adverse effects can be considered de minimis, 1000 ppm Pb in soil is proposed as a target soil concentration for adult exposure. Available data do not appear to be adequate to predict the newborn PbB level which would result from exposure to this soil level during pregnancy. SUMMARY AND CONCLUSIONS The increase in systolic blood pressure in adult males appears to be a reasonable health endpoint for the derivation of a target soil level of Pb for sites (such as those on industrial land) where exposure of children is precluded. Pb soil levels protective against a non‐de minimis increase in systolic pressure should also be protective against other known adverse adult endpoints. Based on the available data it is predicted that a concentration of 1000 μg Pb/g soil will result in a de minimis population‐based increase in systolic blood pressure where ingestion is the only significant route of soil exposure. Significant uncertainty exists as to the appropriate input distributions for some of the parameters in this model. As additional data become available, they can be used to refine this analysis.