Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p6
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 6/8)
Character Range: 1045942–1048828

from ages 2 to less than 16 years of life and no adjustment for exposures for ages 16 years and older.
This process should be considered for individual contaminants where there is clear evidence of a mutagenic mode of action.

    5.4.3          Metal speciation
A chemical 'species' is the specific form of an element defined by its oxidation (valency) state and/or complex or molecular structure. Some of these structural levels are more important for risk assessment than others. In particular, valency state and inorganic and covalent organometallic speciation are of great importance in determining the toxicity of metals and metalloids (WHO 2006). Elements occur in soil in either the solid phase or in the soil solution. In the solid phase, ions can be bound to soil components by means of ion exchange or surface complexes or they can occur as minerals or be co-precipitated as minerals in soil. In the soil solution they can occur as free ions or complexes.

Standard chemical analysis provides a measure of 'total' metal in soil or water, expressed as a concentration of the elemental form. This is not particularly informative as a means of assessing how toxic the soil or water could be.

Further difficulty is introduced because toxicological research rarely focuses on the metal species most likely to be present in soil. Typically, the focus is on the most toxic forms and on those that are of particular health concern as a result of their presence in food, consumer products or in the workplace. This means that the available TRVs for metals and metal compounds may significantly overestimate the toxicity of the metals in soil and water.

Some examples of variations in toxicity with chemical species include the following:
    * Cr(III) is considered to have low toxicity to humans while Cr(VI) is carcinogenic
    * inorganic As(III) compounds are carcinogenic while arsenobetaine is essentially non-toxic
    * inorganic tin (Sn) compounds are considered essential for plants and some animals but tributyltin (TBT) is an endocrine disruptor.
The chemical species of a metal can affect its toxicokinetics by influencing its absorption, distribution, biotransformation and elimination. It is therefore important that risk assessments should consider the species rather than the elemental constituent in order to create meaningful data.

Some typical elemental species in soil are summarised in Table 7.

The speciation of metals in soil and water can be determined to some extent by chemical analysis; however, this is expensive and the data obtainable is limited. Assumptions regarding speciation will normally have to be made using available understanding of the site conditions and site history. It is recommended that where analytical information is not available, risk assessments considering metals should account for metal species using a reasonable worst-case