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The human health benchmarks most widely used are the RfD for oral exposure
and the RfC for inhalation of contaminants. RfD and RfC are lower-bound estimates
of the NOAEL (no observed adverse effects level) of a pollutant, expressed for the
different types of human exposure. These human health benchmarks are established
for chronic exposure and do not account for acute toxicity of a pollutant.
Noncancer risks are expressed by the hazard quotient (HQ), which relates the
exposure to the RfD and RfC, respectively (U.S. Environmental Protection Agency,
1989). HQ refers only to the potential for some individuals to be affected and cannot
address the absolute level of risk. If HQ is higher than 1, this does not necessarily
indicate a potential health risk. In consequence, noncarcinogenic quantitative esti-
mates only identify the exposure level below which adverse effects are unlikely but
say nothing about incremental risk for higher exposure. Although cancer risks are
expressed as an increased probability of the occurrence of carcinogenic effects due
to additional exposure, noncancer risks are assessed for the total exposure to a
pollutant.

4.7.2 ECOLOGICAL RISK
Having conducted the exposure assessment and the dose (concentration)–response
(effect) for all environmental compartments, risk characterization is carried out by
comparing the PEC with the PNEC. This is done separately for each of the protection
goals identified before, for instance: aquatic ecosystem, terrestrial ecosystem, atmo-
sphere, top predator, microorganisms in sewage treatment plants, etc.
For the risk characterization of the aquatic and terrestrial ecosystems a direct
comparison of the PEC and PNEC values must be carried out. If the PEC/PNEC
ratio is greater than one, the substance is “of concern” and further action must be
taken.
For the air compartment, only a qualitative assessment of abiotic effects is carried
out. If there are indicators that one or more of these effects occur for a given
substance (for example, for ozone depletion substances), expert knowledge and
consulting such as that provided by the responsible body in the United Nations
Environment Program (UNEP) will be necessary.

4.8 IMPACT PATHWAY ANALYSIS (IPA)

Impact pathway analysis (IPA) has been lately introduced as a simplified way to
assess the environmental fate and exposure of emissions to air; it allows the expres-
sion of effects in physical impact parameters, such as cancer cases and restricted
activity days, that can be evaluated in monetary terms. IPA takes into account
damages on a regional level due to pollutants with a long residence time and last
used exposure–response functions based on epidemiological studies and also to
dose–response functions based on toxicological tests.
IPA is also known as a damage function or “bottom-up” approach that traces
the passage of pollutants from the place where they are emitted to the endpoint, i.e.,
the receptor that is affected by them. The approach provides a logical and transparent
way of quantifying environmental damages, i.e., externalities. This methodology

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