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

other metals is inconsistent (Påhlsson, 1989). Therefore, no generalizations can be made about
the relative toxicity of metal mixtures. For example, An et al. (2004) conducted acute, 5-day
bioassays on cucumber exposed to Pb, Pb + copper, Pb + cadmium, or Pb + copper + cadmium
in a sandy loam soil of pH 4.3. Shoot and root growth were measured. Depending on the tissue
and metal combination, additivity, synergism, or antagonism was observed in the responses to
these metals. In fact, the response in roots was not consistent with the response in shoots for the
binary mixtures. However, the combined effects were greater in the roots than the shoots, which
may be explained by the tendency for Pb and other heavy metals to be retained in the roots of
plants. In addition, the pattern of metal bioaccumulation into plant tissue did not always
correlate with the toxic response. However, antagonism was observed in the response of roots
and shoots exposed to all three metals, and this was reflected in the decreased accumulation of
metals into plant tissues. The authors hypothesized that this may be due to the formation of less
bioavailable metal complexes (An et al., 2004).
He et al. (2004) found that selenium and zinc both inhibited the uptake of Pb into Chinese
cabbage (Brassica rapa) and lettuce (Lactuca sativa). Zinc applied at 100 mg/kg or selenium
applied at 1 mg/kg decreased the uptake of Pb (present in soil at 10 mg/kg as Pb-nitrate) into
lettuce by 15% and 20%, respectively, and into Chinese cabbage by 23 and 20%, respectively.
Selenium compounds were evaluated to determine whether they could change the
inhibition of ALAD in liver, kidney, or brain of mice exposed to Pb-acetate (Perottoni et al.,
2005). Selenium did not affect the inhibition of ALAD in the kidney or liver, but it did reverse
the ALAD inhibition in mouse brain.
Co-occurrence of cadmium with Pb resulted in reduced blood Pb concentrations in rats
(Garcia and Corredor, 2004). The authors hypothesized that cadmium may block or antagonize
the intestinal absorption of Pb, or the metallothionein induced by cadmium may sequester Pb.
However, this was not observed in pigs, where blood Pb concentrations were greater when
cadmium was also administered (Phillips et al., 2003). The effect on growth rate also was
additive when both metals were given to young pigs (Phillips et al., 2003).
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