Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 220

• exposure through soil ingestion (oral exposure);

• exposure through vegetable consumption (oral exposure);

• exposure through inhalation of indoor air, excluding airborne dust particles

(inhalation exposure).

For these pathways the following intercompartimental relationships must be
derived:

•

the relationship between contaminant concentrations in soil and in vegetables;

•

the relationship between contaminant concentrations in groundwater or soil, and
in indoor air.

Note that no contact medium is concerned with regard to pathway exposure
through soil ingestion. In fact, soil material could be regarded as a contact medium
in and of itself for this exposure pathway.

Exposure Through Soil Ingestion

Exposure through soil ingestion is an important pathway, especially for immobile
contaminants, since exposure through soil ingestion is independent of that part of
the contaminant which is in the pore water. Exposure rates are generally higher for
children. There is some controversy in regard to the significance of exposure through
soil ingestion for adults. Davis and Mirick ( 2005 ) concluded that soil ingestion
levels in children and adults within the same families were not correlated. Oral expo-
sure through soil ingestion depends on the soil (or soil particle) ingestion rate, the
concentration in the soil (or soil particles) and the availability of contaminants in the
human body. The pathway exposure through soil ingestion, including the parameter
identification, is described in detail in Bierkens et al. ( Chapter 6 of this topic).
Exposure through soil ingestion by children strongly depends on the children's
activities (e.g., Freeman et al. ( 2005 ), who investigated the ingestion of pesticides by
hand loading on the basis of analyses of the children's activities from videotapes).
An important input parameter in the calculation of exposure through soil ingestion
is the relative bioavailability factor in the human body (e.g., Ruby et al. 1999 ). This
contaminant-specific factor (generally between 0.0 and 1.0) covers the difference
between intake (external exposure) and uptake (internal exposure) and reflects the
ratio of the bioavailability in the soil matrix as compared to the bioavailability in
a reference matrix (e.g., food, or the matrix on which the Toxicological Reference
Value is based). This relative bioavailability factor is used to correct for the fact
that a contaminant in soil can show a reduced uptake in the body compared to that
same contaminant present in food or water. The availability in the human body is
dependent on the fraction released from the soil matrix in the stomach during diges-
tion in the gastrointestinal tract ( the accessibility ), the fraction transported across
the intestinal epithelium and reaching the portal vein ( absorption ), and the possi-
ble metabolism of the contaminant in the intestinal epithelium and/or in the liver.
The theory behind bioavailability in the human body and the derivation of the rel-
ative bioavailability factor in the human body is described in detail in Cave et al.
( Chapter 7 of this topic).