The EPA method uses default values that are overly
conservative. For example, the 100 mg/day value for adult
soil ingestion shown in Table 2 is approximately at the 95th
percentile of adult soil ingestion rates--only a handful of
individuals in a population of 100 would be expected to
consume 100 mg/day or more of soil. EPA also assumes that
the individual is exposed to the soil 350 days each year for 30
years, another unlikely scenario. Finally, the concentration of
the chemical in the soil, 361 mg/kg, is also an upper-bound
value. A lower value should be used instead because, in this
example, the average concentration is only 40 mg/kg.
Example: EPA Superfund Exposure Assessment Provides an Estimate 1,167 Times
Greater Than a More Reasonable Estimate of the Risk-
MORE REASONABLE OVERESTIMATION
EXPOSURE FACTOR SINGLE-POINT EPA METHOD2/ CAUSED BY EPA
Chemical concentration 40 mg/kg 361 mg/kg 9.03 times
Rate of soil ingestion 25 mg/dayW 100 mg/day 4.00 times
Frequency of exposure 35 days/yr- 350 days/yr 10 times
Duration of exposure 9 years# 30 years 3.33 times
Calculated exposure 0.00000018 0.00021 mg/kg per day 1,167 times
mg/kg per day
I/ This is only an example. In this situation, EPA's approach provides an exposure estimate that is 1,167 times higher
than a more reasonable single-point estimate. While in some cases EPA's overestimate will be smaller, in many cases
it will be larger.
2/ Environmental Protection Agency, "Human Health Evaluation Manual, Supplemental Guidance: 'Standard Default
Exposure Factors," OSWER Directive 9285.6-03, at 15 (March 25, 1991).
3/ Agency for Toxic Substances and Disease Registry (ATSDR'), "Public Health Assessment Guidance Manual,"
at D-5 (March 1992).
4/ Value will vay, depending on conditions at site.
5/ National median time at one residence, as used by EPA and ATSDR.