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TABLE 4.3
Example of RfD Values and Potential Factors
RfD Oral cancer factor Inhalation cancer
(mg/day/kg) (kg/day/mg) factor (kg/day/mg)
As 3.0 × 10 –4 1.75 50
Cd 5.0 × 10 –4 — 6.3
Cr 1.0 — 42
Ni 2.0 × 10 –2 — 1.19
Pb 6.0 × 10 –3 — —
Hg 3.0 × 10 –4 — —
Sn 6.0 × 10 –1 — —
Zn 3.0 × 10 –1 — —
PCDD/Fs 4.0 × 10 –9 1.56 × 10 5 1.16 × 10 5
Source: IRIS (1996), EPA Integrated Risk Information System, available online at:
http://www.epa.gov/ngispgm3/iris/.

The RfD has the unit milligram of a daily intake of the toxic element, which is absorbed
in the body, divided by body weight (Table 4.3) (Olsen et al., 2000).
Baird et al. (1996) analyzed the RfDs reported in the USEPA’s Integrated Risk
Information System database (IRIS, 1996). Of 231 RfDs evaluated, 56% were below
the 5th percentile in corresponding uncertainty distributions, 44% were between the
5th and 15th percentiles and 3% were above the 15th percentile. Such a conserva-
tively biased approach is consistent with the objectives of many risk-screening
assessments (Krewitt et al., 2002).
In carcinogenic and noncarcinogenic effects, it can be expected that some of the
available data for existing substances have been derived from studies conducted in
vitro — the basic (and perhaps additional) studies on genotoxicity, for example. In
vitro studies may yield data on, for instance, metabolism and/or mechanism of
actions (including studies in cell cultures from different species), dermal absorption
(which may also be for different species) and various aspects of toxicity (e.g., test
for cytotoxicity in different types of cells, macromolecule bonding studies, tests
using embryo culture systems, and sperm mobility tests).

4.6.1.3 Epidemiological Information

Some data can be obtained from clinical studies of humans who have inadvertently
been exposed to a suspected toxicant; thus, a source of information relating exposure
to risk is obtained from epidemiological studies. Epidemiology is the study of the
incidence rate of diseases in real populations. When attempting to find correlations
between elevated rates of incidence of a particular disease in certain groups of people
and some measure of their exposure to various environmental factors, an epidemi-
ologist tries to show in a quantitative way the relationship between exposure and
risk. Such data should be used to complement animal data, clinical data and scientific
analyses of the characteristics of the substances in question.

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