464 HAZARDOUS WASTES
the topography of the land and its uses. The behavior patterns
of the individual—that is, the time and places occupied, the
food and drink ingested and its sources, and the possibility of
direct ingestion of contaminated soil through work or play—
must be factored in to obtain a realistic dose estimate.
Estimation of the carcinogenic hazard of a substance
has become a very complex procedure; however, the general
idea can be presented in a model that calculates the indi-
vidual or aggregate risk based on linear extrapolation from
experimental data to zero dose of the curve relating dose to
the probability of cancer.
This assumes that there is no threshold for the incident of
cancer, i.e., the only exposure for which there is zero prob-
ability of cancer is zero exposure. The experimental data
most convincing are those derived from studies of the human
population supported by documented exposures and inci-
dences of cancer. In most cases such data are not available;
therefore animal-study data are used, with conservative fac-
tors introduced to compensate for lack of information about
interspecies relationships. The safety factors that have been
used have been as large as 10,000—that is, in some cases the
effects on humans are assumed to take place at exposures
that can be as much as 10,000 times less than those causing
similar effects on the surrogate species.
The doses are commonly measured in terms of mil-
ligrams of chemicals absorbed in the body each day per
kilogram of body weight over a lifetime. For the part that is
inhaled, it is customary to convert dosage units into micro-
grams per cubic meter in the inhaled air. For the purposes of
this conversion, a body weight of 70 kg is assumed and the
inhaled volume is 20 cubic meters per day.1mg
kg-day70kg
person1
20person-day
m
g
mg3500g
m33
1000Therefore, a bodily intake of 1 mg per kilogram of body
weight per day is equivalent to the inhalation of air contain-
ing 3500 μ g of the chemical per cubic meter.
The unit risk value (URV) is the cancer probability
corresponding to the inhalation over a lifetime of 70 years of
air containing 1 μ g per cubic meter of the substance—that is,the risk is the ratio of the probability of cancer to the micro-
gram per cubic meter inhaled:URVProbability of cancer
gm^3
The URV multiplied by the average concentration inhaled
over a lifetime is the individual lifetime risk of cancer:URVg
m 3 individual life time risk
The individual risk multiplied by the number in the popula-
tion is the aggregate lifetime risk.
A commonly used value to describe a hazardous substance
is the atmospheric concentration that over a lifetime will result
in one cancer per million inhabitants.URVg
m 3 106 1
g
m1(^36) URV 10
HAZARDOUS-WASTE INFORMATION
Information about Superfund locations and hazardous-
waste activities in your zip code, city, county, or state can
be obtained over the Internet from the U.S. Environmental
Protection Agency’s (EPA) Envirofacts Data Warehouse.
REFERENCES
Book of Chemical Lists (CD-ROM), Business and Legal Reports Inc.
CERCLA/Superfund Orientation Manual, 1992 EPA Office of Solid Wastes
and Emergency Response
National Center for Environmental Assessment, EPA
RCRA Orientation Manual, Jan. 2003, EPA 530-R-02–016
Toxicology Excellence for Risk Assessment, [email protected]
EDWARD F. FERRAND
Edward F. Ferrand Associates
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