Page 172 - Materials Chemistry, Second Edition

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The health risk to exposed populations from exposure to environmental pollut-
ants is characterized by the calculation of noncarcinogenic hazard quotient and
indices and/or carcinogenic risks. These parameters can then be compared with
benchmark criteria or standards in order to arrive at risk decisions about an envi-
ronmental pollution problem.

4.7.1.1 Carcinogenic Risk to Human Health
Cancer risk expresses the likelihood of suffering cancer due to a definite daily intake
of a pollutant. In this way, carcinogen risk is defined by the incremental probability
of an individual developing cancer over a lifetime as a result of exposure to carci-
nogenic substances. It can be described by a dose–response estimate. Cancer risk is
nonthreshold; this means that even the lowest doses have a small, or finite, probability
of generating a carcinogenic response. Although risk decreases with the dose, it does
not become zero until the dose becomes zero. For the characterization of cancer
risk, the specific exposure is compared with a corresponding health benchmark for
the relevant contaminant. The results correspond to the probability that cancer occurs,
–6
e.g., a value of 10 means that the probability exists that one person in a million
may get cancer due to the study exposition.
In this way, cancer risk is estimated as excess risk (ER). ER does not express
total cancer. It is an incremental risk due to exposure to the considered pollutant.
In general, risks associated with the inhalation and noninhalation pathways (oral
pathways) may be considered separately, and therefore different human health bench-
marks are required. They can be estimated in accordance with the following generic
relationships:

3
3
Inhalation risk = air ground-level concentration (GLC) (µg/m ) × unit risk (m /µg)
–1
Noninhalation risk = dose (mg/kg-day) × potency slope ((mg/kg-day) ) (4.4)
Therefore, different human health benchmarks are required. Inhalation risk of car-
cinogenic substances is assessed using the inhalation unit risk factor, while the
noninhalation risk is the oral cancer slope factor. The latter (cancer slope factors)
do not represent a safe exposure level, but relate the exposure with the probability
of causing carcinogenic effects (Gold et al., 1995).
A total pathway risk can be calculated by summing the cancer risk estimates to
each pathway. However since cancer risk describes the probability of developing cancer
over a lifetime, the entire duration of exposure must be considered for risk assessment.

4.7.1.2 Noncarcinogenic Risk to Human Health
Depending on the exposure level, adverse health effects other than cancer can be
associated with all chemical substances. Therefore, a noncancer risk characterization
is always a dose–response analysis that determines whether the actual human expo-
sure exceeds a defined exposure level. This critical exposure level represents the
threshold below which adverse effects are assumed to be unlikely and it is determined
in a toxicity assessment.

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