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

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Concept<br />

For a given metal or metalloid (hereafter referred to as metal), the term speciation refers<br />

to its chemical <strong>for</strong>m or species, including its physicochemical characteristics that are relevant to<br />

bioavailability. As a result <strong>of</strong> the direct impact these factors <strong>of</strong>ten have on a metal’s<br />

bioavailability, the term “bioaccessibility” has been adopted to define those factors.<br />

Speciation Role<br />

The accumulation <strong>of</strong> metals in the lithosphere is <strong>of</strong> great concern. Unlike organic<br />

compounds, metals do not degrade and, thus, have a greater tendency to bioaccumulate. It is<br />

now well accepted that knowledge <strong>of</strong> the bulk, toxic characteristic leaching procedure (TCLP),<br />

or synthetic leaching procedure (SLP) concentrations <strong>for</strong> any metal is not a controlling factor in<br />

understanding a metal’s environmental behavior or in developing remedies <strong>for</strong> its safe<br />

management. Although these tests are essential to site characterization and management, they<br />

<strong>of</strong>fer no insight into risk assessment. Rather, it is the metal’s bioavailability (the proportion <strong>of</strong> a<br />

toxin that passes a physiological membrane [the plasma membrane in plants or the gut wall in<br />

animals] and reaches a target receptor [cytosol or blood]), which plays a significant role in the<br />

risk assessment <strong>of</strong> contaminated media.<br />

The NRC review (NRC, 2002) on bioavailability defined bioavailability processes in<br />

terms <strong>of</strong> three key processes:<br />

• contaminant interactions between phases (association-dissociation/bound-released),<br />

• transport <strong>of</strong> contaminants to organism, and<br />

• passage across a physiological membrane.<br />

As mentioned previously, the first process is more commonly referred to as speciation.<br />

The speciation <strong>of</strong> a toxic metal in the environment is a critical component <strong>of</strong> any ecosystem<br />

health risk assessment. Four important toxicologic and toxicokinetic determinants relating<br />

speciation to bioavailability are the (1) chemical <strong>for</strong>m or species, (2) particle size <strong>of</strong> the metal<br />

<strong>for</strong>m, (3) lability <strong>of</strong> the chemical <strong>for</strong>m, and (4) source.<br />

AX7-4

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