Page 169 - Materials Chemistry, Second Edition

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in the study. As an example of the latter, consider an attempt to compare lung cancer
rates in a city with high ambient air pollution levels with rates in a city with less
pollution. Suppose the rates are higher in the more polluted city, even after account-
ing for smoking history, age distribution, and working background. To conclude that
ambient air pollution is causing those differences may be totally invalid. Instead,
different levels of radon may be in homes, for example, or differences that are
causing the cancer variations may exist in other indoor air pollutants associated with
the type of fuel used for cooking and heating.

4.6.1.4 Quantitative Structure Activity Relationship

When data do not exist or are limited for a given endpoint, the structure activity
relationship (SAR) has been used lately by scientists. It should be noted that SAR
techniques and methods, particularly for quantitative structure activity relationship
(QSAR) techniques, have been created recently and are not well developed in relation
to mammalian toxicology. The SARs, which are used for risk-assessment purposes,
are usually more of the “expert judgment” type.

4.6.2 ECOSYSTEMS (ENVIRONMENT)

The procedure for the environmental risk assessment of substances consists in
comparing the concentration in the environmental compartments (PEC) with the
concentration below which unacceptable effects on organisms will most likely not
occur (PNEC).
The PNEC values are usually determined on the basis of results from monospecies
laboratory tests or of established concentrations from model ecosystem tests, taking
into account adequate safety factors. A PNEC is regarded as a concentration below
which an unacceptable effect will most likely not occur. In principle, the PNEC is
calculated by dividing the lowest short-term L(E)C50 or long-term NOEC value by
an appropriate assessment factor. The assessment factors reflect the degree of uncer-
tainty in extrapolation from laboratory toxicity test data for a limited number of species
to the “real” environment. Assessment factors applied for long-term tests are smaller
because the uncertainty of the extrapolation from laboratory data to the natural envi-
ronment is reduced. For this reason long-term data are preferred over short-term data.
Because aquatic organisms are exposed for a short period to compounds with
an intermittent release patterns, short-term L(E)C50 values are used to derive a
PNEC water for these compounds. For most compounds, data will probably not be
present for sediment-dwelling organisms. Appropriate test systems are under devel-
opment but standardized guidelines are not yet available. A method to compensate
for this lack of toxicity data, known as the equilibrium partitioning method, is used
to derive a PNEC . Toxicity data are also scarce for the soil compartment. When
sed
such data are present, they will normally include only short-term studies. In cases
in which data are missing, the equilibrium partitioning method can be used to derive
a PNEC . For the atmosphere, biotic and abiotic effects like acidification are
soil
addressed. Due to the lack of suitable data and unavailability of adequate methods
to assess both types of effects, a provisional strategy is used.

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