Human Health Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 241

exposure is given as a function of total soil concentration, both for a metal and an
organic contaminant. Moreover, the Critical Exposure for the specific contaminant is
given in this graph as a reference Dose (RfD). The human health-based Soil Quality
Standard is defined as the total soil concentration that corresponds with a poten-
tial exposure equal to the RfD. Note that for metals the relationship between total
soil concentration and exposure is assumed to be linear in many existing exposure
models. For organic contaminants this relationship often shows a non-linear kink at
the soil concentration S soil solubility, at which the water solubility is reached. The
reason for this is that at increasing total soil concentrations, the concentration in the
pore water remains at its maximum level and exposure via contaminants in the pore
water remains at the same level with increasing total soil concentration.
In fact, the procedure described above is a simplification of a more advanced
procedure in which oral and inhalation exposures are treated separately, since they
generally impact different target organs. To derive the human health-based limit
value, or human health-based Soil Quality Standard, the oral and inhalation expo-
sures are related to the oral and inhalation Critical Exposure values. The human
health-based Soil Quality Standard is defined as the concentration of a contaminant
in the soil for which the sum of the oral and inhalation Risk indices equals 1:

( oral exposure/ RfD oral )+( inhalation exposure/ RfD inhalation ) = 1 (5.7)

where oral exposure is the sum of exposure from all oral exposure

pathways [mg kg bw − 1 d − 1 ], RfD oral

· · is the Reference dose for oral exposure

kg bw − 1 d − 1 ], [mg inhalation exposure is the sum of exposure from all inhalation

exposure pathways [mg

· ·

kg bw − 1 d − 1 ], and RfD inhalation is the Reference dose for · ·

d − 1 ].
The Reference dose for inhalation exposure is generally derived from the
Reference concentration (RfC), or Tolerable Concentration in Air (TCA).
Strictly speaking, the above equation is only applicable in the case of contami-
nants where the critical effect is systemic (i.e., affecting the whole body) and is the
same for all exposure pathways. If a local effect is considered critical for one of the
exposure routes (e.g., exposure through inhalation) or the critical effect differs per
route (e.g., carcinogenicity for one of the exposure pathways), then exposures from
the oral and inhalation routes should not be combined in the Risk Characterisation.
Dermal exposure requires special attention. Dermal exposure is directly calcu-
lated as an absorbed dose, while Critical Exposure values are often not available.
Therefore, dermal exposure is generally combined with oral exposure. However,
the metabolisation and differences in absorption between the oral and the dermal
routes actually should be accounted for to the fullest extent possible. A Critical
Exposure value for systemic effects through the dermal exposure routes could
be calculated from the oral Critical Exposure value, and the oral bioavailability
or absorption factor (US Environmental Protection Agency 2004 ). Alternatively,
the absorbed dermal exposure could be converted into a pseudo-oral exposure
by multiplying it with the same oral bioavailability factor and then combining it
with the oral exposure pathways. Corrections for bioavailability should only be

kg bw − 1 inhalation exposure [mg · ·