Protocol for the Derivation of Environmental and Human ... - CCME

Part C, Section 4
Section 4
Exposure Scenarios
4.1 Assumptions about Exposure
In developing soil remediation guidelines to protect human health, one must ensure that exposure to
contaminants at the guideline concentration will not result in adverse human health effects. For the
purposes of guidelines development, the CCME assumes a chronic exposure scenario (i.e., lifetime
exposure to a remediated site). This is a conservative assumption which helps ensure that no limitations
will exist within the defined land use. The first step in setting soil guidelines is one of working backward
from the Tolerable Daily Intake (TDI) or critical Risk Specific Dose (RSD) for a contaminant, through
appropriate direct soil exposure pathways to a land use generic soil concentration. A second step
considers indirect exposure pathways and protects against cross-media contamination.
A schematic diagram of potential human exposure pathways within a multi-media context is provided in
Figure 18. Soil exposure pathways can result from direct or indirect exposure to soil, such as the direct
ingestion of contaminated soil or cross-media contaminant transfer from soil to another medium (e.g.,
water, air, or food). Direct exposure pathways include ingestion of soil/dust, dermal uptake of
contaminants in contact with the skin, and inhalation of soil particles into the lungs. Indirect exposure
pathways include ingestion of food grown on contaminated soil, inhalation of vapours resulting from the
volatilization of contaminants from soil into air, and ingestion of groundwater contaminated by the
leaching of contaminants from soil into groundwater. Development of guidelines is based on direct
exposure pathways, with indirect exposures used to provide a check on the guidelines to ensure that
generic guidelines are protective for a large majority of scenarios within a given land use.
The physical and chemical properties of a contaminant will determine its environmental fate. These
properties will also focus the possible important exposure pathways to humans. For example, the
dermal exposure pathway will be of prime importance for contaminants which are lipophilic and can
readily cross the epidermal layer of the skin. Similarly, contaminants with a high vapour pressure likely
to volatilize from soil to air are extremely important in the respiratory pathway.
Cross-media transfer of a chemical from contaminated soil to another medium, such as water, air, or
food, can result in indirect exposure to contaminants from soil. Each of these cross-media transfers can
be modelled using a number of assumptions about the exposure scenario. For example, the uptake of
soil contaminants by plants and the subsequent ingestion of contaminated homegrown produce by
people can be estimated for a given soil contaminant concentration.
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