Lead (Pb) is Now a Non-Threshold Substance: How Does this Affect Soil Quality Guidelines?

Risk-based concentrations (RBCs) for lead (Pb) in soil were estimated using equations for development of Canadian soil quality guidelines. Based on the latest toxicological assessments by various health agencies, risk specific doses for Pb were defined for children (for impacts on intelligence quotient IQ]) and adults (for impacts on systolic blood pressure SBP] as well as protection of fetal effects in the case of pregnant women). The analysis suggests that a RBC in soil of 180 μg/g (dry weight) for residential and other areas where children routinely play is protective of a 1 IQ point decrement on a population basis and may actually be associated with decrements of less than 0.2 to 0.35 IQ points when the weight of evidence is considered. For soils that children do not contact on a frequent basis, RBCs as great as 8800 μg/g are considered to be protective of a 1 mmHg SBP increase in adults (as well as IQ effects to the fetus). It is stressed that non-soil sources of Pb may be even more important than soil. The approach may also be useful in jurisdictions outside of Canada as the importance of considering IQ decrements on a population basis rather than an individual basis and uncertainties in soil ingestion rates are considered.