Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 246

One alternative possibility is to indirectly include background exposure by using
an appropriate Critical Exposure, for example, by using additional safety factors
(often of 10 or 100). Such a limit value is at the protection level of Negligible Risk,
for instance. This risk level is supposed to cover, among other things, the effect of
background exposure.
The role of background exposure is different for non-threshold carcinogens .For
these contaminants, an acceptable risk level is set, which corresponds to the excess
risk that is accepted for soil contamination. Therefore, background exposure does
not need to be incorporated in the Risk Characterisation.
5.5.5 Combined Exposure
In the great majority of contaminated sites show contamination with more than one
contaminant in soil and/or groundwater. Sometimes, the same combinations of con-
taminants are found in soils and groundwater, at other sites an incoherent cocktail of
contaminants is present. As a consequence, humans generally are exposed to more
than one contaminant at the same time.
Non-carcinogenic contaminants can act independently from each other, or can
influence the overall effect due to combined exposure. Although the exposure rate
differs for each contaminant, the impact of this combined exposure must be assessed
in a Human Health Risk Assessment. Exposure to more than one contaminant can
increase the human health risk. For the magnitude of this increment, three differ-
ent possibilities exist, depending on the composition of the contaminant cocktail.
According to the first possibility, contaminants do not influence the potency of
the other contaminants. In this case, two types of addition exist, dose addition and
response addition . For contaminants with the same toxicological endpoint (e.g., tar-
get organ) that act through a common mode of action, dose addition is appropriate.
For these contaminants the doses can be summed up, if necessary after accounting
for differences in potency, and then compared to the Critical Exposure. If contami-
nants have the same endpoint, but act through a different mode of action, response
addition applies. In that case, the responses should be added. In practice, this means
that the combined effect of contaminants showing dose addition can lead to a neg-
ative health risk appraisal, even if the separate exposures do not exceed the Critical
Exposure. In contrast, the combined effect of contaminants showing response addi-
tion will not exert a negative health risk appraisal when the separate exposures do not
exceed the Critical Exposure (Wilkinson et al. 2000 ). The second possibility is that
the exposure to several contaminants enhances the overall effect more than linearly,
which are called synergistic effects. Third, a less than linear increase in effects is
possible, known as antagonistic effects. Quantification of synergy and antagonism
is difficult and, therefore, seldom done. In practice, linear addition of exposure is
often performed for contaminants that have the same mode of action.
As a simplified approach to dose addition, the different Risk indices (ratio of
exposure to Critical Exposure value) are added up, while the criterion for 'possible
unacceptable human health risk' is: