Page 292 - Materials Chemistry, Second Edition

276 4 Life Cycle Impact Assessment

The third expression provides the entry into the primary compartment n (air,
water, soil). This entry can be taken from the inventory (LCI).
The result of the characterisation (S) is the sum of all substances i considering
all compartments m and n:
S =Σ Σ Σ S (4.28)
i m n i
A simple HTP or TBS Equations 4.24 and 4.25 with a weighting by OEL or
ADI values is a special case of Equation 4.28 with the assumption n = m = air,
NEC i m = OEL or ADI with a neglect of the fate factor which signifies the degrada-
i
i
tion and transfer between the environmental media.
An attempt to assess various toxicities individually or at least by groups is further
complicated. A category human toxicity and a characterising model HTP or S
necessarily produces a range of subcategories which correspond to the selected
toxicity endpoints.

Attempts for a Specification of Exposure Factors Difficulties with respect to exposure
factors for human toxicity are somehow similar to those of the impact category
ecotoxicity (see Section 4.5.3.3). A first determination of exposure factors has been
made by Guin´ ee and Heijungs. 302) The HTP definition by the authors considers
the intake of pollutants by air (respiratory) and by nutrition (orally). The dermal
absorption was so far neglected. The exposure via an environmental medium is
estimated by a Mackay-III model that describes the flow equilibrium between the
media air, water, soil and sediment within a global Unit-world 303) -Box-Model.This
model considers degradation processes. These are however only adequately speci-
fied for a number of substances which is especially valid for biological degradation.
Quantification of abiotic degradation in the air however is near to appropriate. 304)
For an application of the Mackay model – a similar application is valid for other
distribution models – the so-called flow/pulse problem occurs: in this model the
mass input is considered as a continuous mass flow, for example x kg/d into
−1
compartment air. The inventory however supplies a load per fU (kg fU )withan
unspecified dispersion in space and time. This load can be approximated as pulse
of uncertain characteristic, the target medium being known from the inventory.
As a work-around, Guin´ ee and Heijung proposed to refer exposure and effect to
an arbitrarily selectable reference substance which would eliminate conversion
(load/flow) and result in a dimensionless toxicity potential HTP. The HTP of the
reference substance is set equal to one. Although this approach seems logical it
is not convincing due to its highly artificial character which does not reflect the
diverse toxic effects. Therefore, those rather complicated calculations will not be
discussed in detail here. Heijungs and co-workers were later able to demonstrate
that pulses in Mackay-like models, like, for example in the Dutch model USES,

302) Guin´ ee and Heijungs (1993) and Guin´ ee et al. (1996a).
303) Mackay (1991) and Kl¨ opffer (1996a, 2012b). Four media or compartments (air, water, soiland
biota) are designated as boxes or compartments with a possible reference to subcategories in
more complex models.
304) Kl¨ opffer and Wagner (2007a).