Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Lead concentrations in various tissues of mammals also have been correlated with toxicity
(Ma, 1996). The tissues commonly analysed for Pb are blood, liver, and kidney. Typical
baseline levels of blood Pb are approximately 4 to 8 µg/dL for small mammals, and 2 to 6 µg/dL
for mature cattle. Typical baseline levels of Pb in liver are 1 to 2 mg/kg dw for small mammals.
Typical baseline levels of Pb in kidney are 0.2 to 1.5 mg/kg dw for mice and voles, but shrews
typically have higher baseline levels of 3 to 19 mg/kg dw. Ma (1996) concluded that Pb levels
less than 5 mg/kg dw in liver and 10 mg/kg dw in kidney were not associated with toxicity, but
that levels greater than 5 mg/kg dw in liver and greater than 15 mg/kg dw in kidney could be
taken as a chemical biomarker of toxic exposure to Pb in mammals. Humphreys (1991) noted
that the concentrations of Pb in liver and kidney can be elevated in animals with normal blood Pb
concentrations (and without exhibiting clinical signs of Pb toxicity), because Pb persists in these
organs longer than in blood.
Uptake of Pb from soil into small mammals was also modeled as part of Eco-SSL
development (U.S. Environmental Protection Agency, 2005a). The relationship derived between
Pb in the soil and Pb in the whole-body of a small mammal was taken from Sample et al. (1998)
and is as follows:
Ln(Cmammal) = 0.4422 * Ln(Csoil) + 0.0761
Where Cmammal is the concentration of Pb in small mammals (dry weight) and Csoil is the
concentration of Pb in the soil. Similar to the uptake equations for plants (Eq. 8-1) and
earthworms (Eq 8-2), the equation for mammalian uptake recognizes that the ratio of Pb
concentration in small mammals to Pb concentration in soil is not constant.
AX7.1.2.4 Resistance Mechanisms
AX7-44
(AX7-3)
Many mechanisms related to heavy metal tolerance in plants and invertebrates have been
described, including avoidance (i.e., root redistribution, food rejection), exclusion (i.e., selective
uptake and translocation), immobilization at the plant cell wall, and excretion (i.e., foliar
leakage, moulting) (Tyler et al., 1989; Patra et al., 2004). The following section reviews the
recent literature on the resistance mechanisms of plants and invertebrates through mitigation of
Pb (1) toxicity or (2) exposure.