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4.5.3 ECOLOGICAL EXPOSURE ASSESSMENT
Ecosystems may be exposed to chemical substances during all stages of their life-
cycle — from production to disposal or recovery. For each environmental compart-
ment potentially exposed, the exposure concentrations should be evaluated. The
objective of the ecological exposure assessment is to estimate the concentration to
which an environmental compartment is or may be exposed.
A chemical may be released into the environment and is then subject to physical
dispersal into the air, water, soil, or sediment. The chemical may then be transported
spatially and into the biota and perhaps be chemically or otherwise modified or
transformed and degraded by abiotic processes (such as photolysis, hydrolysis, etc.)
and/or by microorganisms present in the environment. The resulting transformation
may have different environmental behavior patterns and toxicological properties
from those of the chemical. Nonetheless, it is the nature of exposure scenarios to
determine the potential for any adverse impacts. The amount of a target species’
exposure to environmental contamination is based on the maximum plausible expo-
sure concentrations of the chemicals in the affected environmental matrices. The
total daily exposure (in mg/kg-day) of target species can be calculated by summing
the amounts of constituents ingested and absorbed from all sources (e.g., soil,
vegetation, surface water, fish tissue, and other target species) as well as those
absorbed through inhalation and dermal contacts.
The process for the environmental risk assessment of a substance is based on
the comparison of the concentration in the environmental compartment (predicted
environment concentration, PEC) with a concentration below which unacceptable
effects on organisms will most likely not occur (predicted no effect concentration,
PNEC) as shown in Section 4.6.2. Therefore the aim of exposure assessment for the
environment is the evaluation of PEC. It can be derived from available monitoring
data and/or model calculations.
Analytical processes used to estimate receptor exposure to chemicals in various
contaminated media (such as a wildlife or a game species’ daily chemical exposure
and the resulting body burden) are similar to those discussed under human health
risk assessment.

4.6 DOSE–RESPONSE AND EXPOSURE–RESPONSE
FUNCTIONS

Some experts argue that it is unnecessary to determine first if a chemical is hazardous.
Their philosophy stems from the first definition of toxicology, given by Paracelsus
(1493–1541) over 450 years ago: “All substances are poisons; there is none which is
not a poison. The right dose differences a poison and a remedy.” In other words, all
chemicals have the potential to be hazardous, depending on the dose; therefore, exposure
assessment could potentially take the place of hazard identification.
Dose–response and exposure–response evaluation, the third component of the
risk assessment process, involves the characterization of the relationship between
the dose administered or received and the incidence or severity of an adverse effect
in the exposed population or ecosystem. Characterizing the dose–response

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