PRINCIPLES OF TOXICOLOGY

488 EXAMPLE OF RISK ASSESSMENT APPLICATIONS
TABLE 19.5 Arsenic Risk Assessment Example: Typical USEPA Reasonable
Maximum Exposure Soil Exposure Parameters a
Exposure Parameter Value Reference
ABSgi 0.28 Freeman et al. (1995)
ABSsk 0.001 USEPA (1995)
AF 0.2 mg/cm 2
USEPA (1997)
ATnc
8760 days (adult);
2190 days (child)
USEPA (1991)
ATc 25,550 days USEPA (1991)
BW 70 kg (adult);
15 kg (child)
USEPA (1991)
CA 1.8 × 10 –7 mg/m 3
Modeled air concentration
CS 90 mg/kg Site-specific average arsenic
concentration in soil
ED 24 years (adult);
6 years (child)
USEPA (1991)
EF 350 days/year USEPA (1991)
IR 100 mg/day (adult)
200 mg/day (child)
USEPA (1991)
SA 2900 (adult)
1000 (child)
USEPA (1997)
VR 20 (adult)
10 (child)
USEPA (1997)
a
Note: USEPA typically assumes 80–100 percent bioavailability for arsenic in soil. Therefore, the
USEPA default value for ABSgi is 0.8–1.
The calculated HI is rounded to one significant figure. Because the HI does not exceed one,
arsenic exposure would be unlikely to cause noncancer effects. However, even if the HI value
slightly exceeded one, this is would be unlikely to be of significant health consequence. This is
particularly the case since the oral RfD for arsenic is based on a no-observed-adverse-effect level
(NOAEL) in humans of 8 × 10 –4 mg/kg⋅day. As stated by the USEPA, a case can be made for
setting the oral RfD as high as the NOAEL. The USEPA adjusted the NOAEL downward using
an uncertainty factor of 3 to account for uncertainty associated with an incomplete database
regarding the noncarcinogenic effects of arsenic.
Note that if calculated for the adult, the HI for exposure to arsenic in soil would be lower
because a child is exposed to more soil than an adult when dose is calculated on the basis of body
weight.
TABLE 19.6 Arsenic Risk Assessment Example: Calculated Daily Exposure (in mg/kg) to Arsenic in
Residential Soil
Exposure Pathway ADI a
Ingestion 3.22 × 10 –4
Skin absorption *1.15 × 10 –6
Inhalation 1.06 × 10 –7
a Average daily intake.
b Lifetime average daily intake.
c Expressed as an absorbed dose rather than a daily intake.
Child Resident Adult Resident
LADI b
2.76 × 10 –5
*9.86 × 10 –8
9.05 × 10 –9
3.45 × 10 –5
∗7.15 × 10 –7
3.46 × 10 –8
ADI LADI
1.18 × 10 –5
*2.45 × 10 –7
1.19 × 10 –8