Environmental Engineering Reference
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(exposure 1 /RfD 1 ) + (exposure 2 /RfD 2 )+
...
+ (exposure n /RfD n )
=
1
(5.8)
where
exposure 1 , exposure 2 ,
...
, exposure n is exposure to contaminant 1, 2,
...
, and n
[mg.kg bw 1
d 1 ];
·
RfD 1 ,RfD 2 ,
...
,RfD n is Critical Exposure of contaminant 1, 2,
...
, and n
kg bw 1
d 1 ].
[mg
·
·
From a quantitative Human Health Risk Assessment perspective, this procedure
is not correct, since it falsely assumes that the risk is linearly related to the Risk
Index (e.g., that a Risk Index of 0.6 is two times worse than a Risk Index of 0.3). In
spite of the lack of scientific foundation, Eq. ( 5.8 ) offers a practical way to account
for an increased risk when more contaminants are present.
In case of effects due to exposure of contaminants with a different endpoint, two
or more different threats to the human health risk must be accounted for. However,
since there is no standard procedure to quantify the overall effect of two differ-
ent human health threats, this is rarely done in Human Health Risk Assessments.
Generally, attention is focused on the contaminant that is expected to cause the most
serious health effect.
Comparable considerations can be made for non-threshold carcinogens. If they
act through the same mode of action on the same endpoint, exposure could be com-
bined. However, even in regard to different targets and modes of action, there may
be a potential for combined effects (Environment Agency 2008 ).
Typically, in practice effects from carcinogenic contaminants are often assumed
to be additive, regardless of target organ.
5.6 A Closer Look at Human Health Risk Assessment
5.6.1 Significance of Exceeding Toxicological Reference Values
If exposure due to soil contamination exceeds the Critical Exposure value, the ques-
tion arises as to what the significance of this exceeding means in terms of health
effects. The question is whether adverse health effects will indeed occur for the
human beings involved.
Exceeding a Toxicological Reference Value should be interpreted on a case-by-
case basis. The conclusion will depend on the toxic mechanism for the critical effect,
the toxic endpoint, the margin between effects at chronic and sub-acute/sub-chronic
exposure and the potential for different toxic effects at sub-acute/sub-chronic
exposure.
Generally, Toxicological Reference Values are set at safe levels that protect the
general population, including sensitive individuals. The protective nature of an ADI,
for example, predominantly depends on the accuracy of the toxicological studies,
as well as on the range and number of doses tested (Speijers 1999 ). In general,
the safety factors used are higher as less information is available. If exposure
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