Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 240

('good' or 'bad'; acceptable or unacceptable, when the Risk Index is lower or higher
than 1.0, respectively) the Risk Index allows for classification of risk qualifications,
although this is a subjective process, into classes such as, for example, a 'very high
human health risk', when the Risk Index exceeds a value of 10. Moreover, a Risk
Index offers possibilities for the scaling of human health risks, which is useful in
terms of priority setting. It should be noted, however, that in regard to classification
and ranking of risks, the Risk Index assumes a linear relationship with seriousness
of human health effects, whereas dose-response curves generally are not linear. Thus
a Risk Index of 10 is generally not 10 times worse than a Risk Index of 1.

5.5.2 Soil Quality Standards

Generally, Soil Quality Standards are derived for generic Risk Assessment purposes,
that is, for Risk Assessments not related to a specific site. For the derivation of
Soil Quality Standards, the Estimated Exposure relates to the potential exposure.
Potential exposure is defined as the exposure that would occur under specific, stan-
dardized conditions in terms of geographical conditions (e.g., relating to soil type,
soil properties, depth of groundwater table) and human behaviour (also exposure
characteristics: e.g., relating to residence time, amount of home-grown vegetable
consumption). To be able to calculate the potential exposure, a generic expo-
sure scenario must be constructed, in which the above-mentioned conditions and
characteristics are described and quantified.
In regard to the derivation of Soil Quality Standards, the Risk Characterisation
is best explained graphically (see Fig. 5.5 ). In this graph the relevant potential total

RfD

Total soil concentration (mg.kg -1 dw )

C soil solubility human health-based SQS

Fig. 5.5 Average life-long human exposure ( potential exposure ) as a function of total soil concen-
tration; the critical exposure (reference dose; RfD); the resulting human health-based soil quality
standard (human health-based SQS); C soil solubility = concentration in soil at which the water

solubility is reached