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additional factor of 10. This is consistent with what is done in deriving
human health RfD values.
f. In cases where NOAELs are available as a dietary concentration (e.g.,
milligram contaminant per kilogram food), a consumption rate for marine
birds or marine mammals may be estimated based on various food intake
summaries (e.g., USEPA's Wildlife Exposure Factors Handbook
(USEPA/Office of Research and Development 1993)) and a corresponding
NOAEL may be calculated. This consumption rate is expressed as a
percentage of the animal's body weight on a wet weight basis or in units of
kilogram of food (wet weight) per kilogram of body weight per day
(kg/kg/day).
g. Some NOAEL values may be over conservative because they provide
information on which dose produces no effect, but not how much higher the
concentration has to be to produce an effect. Where the lowest NOAEL
available in the literature is so low that background concentrations will
produce a dose that exceeds it, reject the lowest NOAEL and use the next
highest NOAEL.
California EPA approach
The following description is adapted from the California EPA approach for
calculating toxicity factors (adapted from California EPA 1996).
a. Use toxicity data for representative species and members of the same
taxonomic family in estimating toxicity to representative species.
b. If data are lacking or judged inappropriate, use data for surrogate species
following application of one or more uncertainty factors (UFs). These UFs
may be based on data when available or, in the absence of data, on the
default values provided below.
(1) Apply a UF of 500 to adjust from less sensitive end points, such as
mortality, to a chronic NOAEL (e.g., LD50 to NOAEL Chronic).
(2) Apply a UF of 10 to adjust from an acute LOAEL to a chronic NOAEL
(e.g., LOAEL Acute to NOAEL Chronic).
(3) Apply a UF of 5 to adjust from LOAEL to NOAEL.
(4) Apply a UF of 1 for interspecies extrapolations within the same
taxonomic family (e.g., beagle to fox - canidae to canidae).
(5) Apply a UF of 5 for interspecies extrapolation within the same
taxonomic order.
(6) Apply a UF of 10 for interspecies extrapolation between taxonomic
orders.
66
Chapter 3 Ecological Exposure Assessment

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