Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 239

becomes. It is often claimed that the Toxicological Reference Values are too con-
servative. Dourson et al. ( 2001 ), however, showed that a substantial number of
Toxicological Reference Values derived from human data are even lower than the
values derived from animal studies.
Toxicological Reference Values that are used worldwide vary strongly, as a result
of differences in the interpretation of the toxicological studies and in the derivation
of assessment factors. Part of this variation is also due to different policy decisions,
for instance, in regard to the acceptable choice of excess lifetime cancer risk for
genotoxic carcinogens.
Nevertheless, Toxicological Reference Values such as the TDI or Reference dose
are useful as long as their derivation is transparent and the exact meaning of the
values is realised. A confidence rating on the Critical Exposure value can improve
knowledge about the uncertainty for a certain contaminant, as listed in the USA IRIS
database. Since the Toxicological Reference Values generally are conservative, it is
unlikely that adverse human health effects will occur when Estimated Exposure does
not exceed the Critical Exposure. One mistake that could be made is to conclude
that health effects will arise if the Estimated Exposure at a site exceeds the Critical
Exposure value by any margin. The correct conclusion is that there is an increased
risk of health effects and that this increased risk may be considered unacceptable
within the regulatory context.
5.5 Risk Characterisation
Risk Characterisation combines the result of the two preceding steps in the Risk
Assessment framework, that is, the Hazard and Exposure Assessments. By compar-
ing Estimated (predicted or observed) Exposure to Critical Exposure, a conclusion
with regard to human health risk at a site can be drawn.
5.5.1 Site-Specific Risk Assessment
Site-specific Risk Assessment is appropriate if a specific (potentially) contaminated
site must be appraised. For site-specific Risk Assessment purposes, the Estimated
Exposure relates to the exposure that takes place at that specific site (site-specific
exposure). The combination of this site-specific exposure and Critical Exposure, the
Risk Characterisation, results in a risk qualification. This risk qualification is either
expressed in terms of 'good' or 'bad' (when the Critical Exposure is not exceeded, or
is exceeded, respectively). From a political perspective the risk qualification usually
is expressed in terms like acceptable or unacceptable human health risk. These
terms are commonly derived from a scientific range of acceptable levels, combined
with political arguments such as ethics, social impact and costs.
In a more sophisticated way, the result of a site-specific Human Health Risk
Assessment is expressed as a Risk Index , that is, the quotient between the site-
specific exposure and the Critical Exposure. Except for an absolute qualification