Environmental Engineering FOURTH EDITION

RiskAnalysis 41

the tumor-suppressing factor is overwhelmed. The length of time between exposure to
a risk factor and expression of the adverse effect is called the latency period. Cancers
in adults have apparent latency periods of between 10 and 40 years. Relating a cancer
to a particular exposure is fraught with inherent inaccuracy; it is exceedingly difficult
to isolate the effect of a single carcinogen when examining thirty or forty years of a
person’s life. Many carcinogenic effects are not identifiable in the lifetime of a sin-
gle individual. There are a few instances in which a particular cancer is found only
on exposure to a particular agent (e.g., a certain type of hemangioma is found only
on exposure to vinyl chloride monomer) but for most cases, the connection between
exposure and effect is far from clear. Many carcinogens are identified through animal
studies, but one cannot always extrapolate from animal results to human results. The
U.S. Environmental Protection Agency (EPA) classifies known animal carcinogens
for which there is inadequate evidence for human carcinogenicity as probable human
carcinogens.
There is a growing tendency to regulate any substance for which there is any
evidence, even inconclusive evidence, of adverse health effects. This is considered a
conservative assumption, but may not be valid in all cases. Such a conservative posture
toward regulation and control is the result of the cumulative uncertainty surrounding
the epidemiology of pollutants.

EXPRESSION OF RISK

In order to use risks in determining pollution standards, as EPA does, it is necessary to
develop quantitative expressions for risk. The quantitative expressions reflect both the
proportionality of the risk factor to the adverse effect and the statistical significance of
the effect.
Risk is defined as the product of probability and consequence, and is expressed
as the probability or frequency of occurrence of an undesirable event. For example,
if 10% of the students in a course were randomly given an “F,” the “risk” of getting
“F’ is 0.1 “F’ per total number of grades assigned. The probability is 0.1 and the
consequence is “E” The units in which risk is expressed incorporate both the probability
and some measure of consequence. In discussing human health or environmental risk,
the consequences are adverse health effects or adverse effects on some species of plant
or animal. The frequency of Occurrence of adverse health effects in a population is
written as

X

N

F=--,

where

(3.1)

F = frequency,

X = number of adverse health effects, and

N = number of individuals in the population.