Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 465

further investigation of the fraction of vegetables that is home-grown has been
recommended for the future (Swartjes et al. 2007 ).
11.2.4.3 Correction for Relative Bioavailability in the Human Body
The relative oral bioavailability of contaminants in the human body (see Chapter 7
by Cave et al., this topic, for a detailed explanation) may be an important factor for
this pathway, though it is not as important as it is for exposure through the direct
ingestion of soil. Intawongse and Dean ( 2006 ) studied the oral bioavailability of cad-
mium, copper, manganese and zinc ingested in the leaves of lettuce and spinach and
the roots of radish and carrot and found that the bioaccessibility of metals in plants
varies between metals and between different plant types. However, there is only lim-
ited information on the differences between intake and uptake of contaminants via
a vegetable matrix. Therefore, the correction for bioavailability in the human body,
f bioavailability , is currently 1.0 in practically all existing exposure models.
11.2.5 Site-Specific Risk Assessment of Exposure Though
Vegetables Consumption
As with other pathways, a tiered approach can be used to assess the site-specific
human health risks through vegetable consumption from contaminated sites in a
scientifically-based and efficient way. Successively, in each tier the degree of con-
servatism decreases, while site-specificism increases. As a consequence, complexity
and hence effort and finances needed also increase in each tier. In a specific tier, if
unacceptable human health risks cannot be excluded, the Risk Assessment should
proceed to the following tier. The underlying principle is: simple when possible and
complex when necessary.
An example of this kind of tiered approach is given in Swartjes et al. ( 2007 ).
According to this approach, site-specific Risk Assessment is usually carried out in
higher tiers. Site-specific Risk Assessment can take the form of site-specific calcu-
lations, or include measurements of metal concentrations in vegetables grown on
the site. Tier 0 concerns a preliminary qualitative evaluation of the possibilities for
experiencing adverse human health effects due to vegetable consumption. In Tier
1 the actual total soil concentrations (average values or a specified percentile of
the measured values) are compared with Critical soil concentrations . These Critical
soil concentrations have been derived on the basis of a conservative exposure sce-
nario. Tier 2 offers the possibility for a detailed assessment of the site-specific
risks on the basis of calculation . The site-specific calculation of the contaminant
concentration in vegetables differs for metals, other inorganic contaminants and
organic contaminants. For metals, Freundlich-type plant-soil relations (dependent
of the total soil concentration and the major soil properties) (see Chapter 8 by
McLaughlin et al., this topic) and geometric means of the BioConcentrationFactors
(corrected for organic matter and clay contents) are combined. The accumulation
of other inorganic contaminants is based on passive uptake. The calculation of