McKay, Donald. "Front matter" Multimedia Environmental Models ...

Exposure routes vary greatly in magnitude from chemical to chemical, depending
on the substance’s physical chemical properties such as K AW and K OW, and it is not
usually obvious which routes are most important.
Data from the environmental fate models can provide a sound basis for estimating
risk when used to assess quotients and to determine dominant exposure routes. If
such information can be presented to the public, the individuals will be, at least in
principle, able to choose or modify their lifestyles to minimize exposure and presumably
risk. Individuals then have the freedom and information to judge and
respond to acceptability of risk from exposure to this chemical compared to the
other voluntary and involuntary risks to which they are subject.
©2001 CRC Press LLC
8.15 THE PBT–LRT ATTRIBUTES
A regulatory issue in which evaluative mass balance models are playing an
increasingly important role is in assessing the persistence, bioaccumulation, toxicity
and long-range transport (PBT-LRT) attributes of chemicals. If chemicals that display
these undesirable attributes can be identified, they can be considered for regulation,
as has been done by UNEP for the “dirty dozen” high-priority substances
discussed earlier. Monitoring data are usually too variable to enable them to be used
directly in this priority setting task, and monitoring is impossible for chemicals not
yet in use. Since there are many thousands of chemicals of commerce that require
assessment, and (it is hoped) most are innocuous, there is an incentive to develop a
tiered system in which there are minimal data demands initially and perhaps 90%
of chemicals evaluated are rejected as of no concern. The remaining 10% of potential
concern can be more fully evaluated in a second tier with a similar rejection ratio.
A third tier may be needed to select the (perhaps) 100 top priority chemicals from
a universe of 100,000 chemicals in a three-tier system. The challenge is to devise
models or evaluation systems that will accomplish this task efficiently.
Webster et al. (1998) have suggested using a Level III model similar to EQC,
but with advection shut off, to evaluate persistence. Gouin et al. (2000) have
described an even simpler Level II approach. This has the advantage that no “modeof-entry”
information is required. Regardless of which model is used, it seems
inevitable that models will play a key role in assessing persistence or residence time,
since these quantities cannot be measured directly in the environment.
Bioaccumulation can be evaluated most simply by calculating the equilibrium
bioconcentration factors as the product of lipid content and K OW. For more detailed
evaluation involving considerations of bioavailability, metabolism, and possible biomagnification
from food uptake, the Fish model or a variant of it can be used. In
some cases, a food web model may be required to determine if significant biomagnification
occurs. Foodweb can be used for this purpose. Mackay and Fraser (2000)
have suggested such a three-tiered approach for assessing the bioaccumulation
potential of chemicals.
Toxicity is not within our scope here.
Long-range transport (LRT) presents an interesting challenge because, like persistence,
LRT cannot be measured in the environment. It can only be estimated using