Methods for Deriving Pesticide Aquatic Life Criteria - Reviews of Environmental Contamination and Toxicology - page 74

factor can be as high as 1000. The USEPA document (Nabholz 1991) recommends
factors ranging from 1 to 1000, depending on the amount and type of data available.
Factor size also depends on uncertainties arising from making extrapolations from
acute-to-chronic, laboratory to field, and from small (i.e., n = 1) acute data sets.
If data are available, an acute-to-chronic factor may be calculated, rather than using
a default value of 10.
In the UK, factors are used to deal with uncertainty arising from extrapolations
such as one species to another, short to long exposure times, acute-to-chronic
effects, chronic to ecosystem effects, and effects in one ecosystem to those in
another (Zabel and Cole 1999). Factor size depends on the quantity of data and
whether data are available for sensitive species. An additional factor may be applied
if a substance is bioaccumulative (usually substances with MW < 700 and BCF >
100 or log K ow > 3). Factors range from 1 to 1000, and the degree to which they are
applied relies heavily on expert judgment.
According to the OECD guidelines (1995), AFs are empirically derived; that is,
they have no theoretical basis. In this methodology, a factor of 10 is applied for
each of three possible extrapolation steps: (1) laboratory-derived NOEC to those in
the field; (2) short to long exposure times; and (3) acute-to-chronic effects.
Alternatively, the OECD (1995) provides for use of AFs presented in the EU risk
assessment methodology (ECB 2003).
ACRs are used in the USEPA methodology (1985) to derive chronic criteria
when chronic data are lacking. ACRs are calculated from chronic data, for which
at least one corresponding acute value is available (from the same study, or from a
different study using the same dilution water). Species mean ACRs are calculated
as the geometric mean of all available ACRs, for that species. To calculate the final
ACR, one of four methods is used: (1) for materials for which the species ACRs
covary with the SMAV, the ACR is calculated using only species whose SMAVs are
close to the FAV (“close” is not defined); (2) if there is no covariance, and all of the
ACRs for a set of species are within a factor of 10, then the final ACR is calculated
as the geometric mean of all species ACRs, including both freshwater and saltwater
species; (3) for acute tests with shellfish embryos and larvae, a final ACR of 2.0 is
used, which makes the FCV equal to the criterion maximum concentration (CMC);
and (4) for species with mean ACRs less than 2.0, a final ACR of 2.0 is used to
provide for possible acclimation of test species to the toxicant. If a final ACR cannot
be determined by any of these methods, then it is likely that neither the final ACR
nor the FCV can be calculated.
Factors used in deriving SAVs in the Great Lakes guidance range from 4.3 to
21.9, depending on how many of the minimum Tier I data requirements are met.
For example, if seven toxicity values from different families are available, then the
factor is 4.3, but if only one value is available, then the factor is 21.9. According to
Pepin (2005, personal communication), these factors are based on a USEPA study
by Host et al. (1995), which presents several methods for deriving factors to use for
data sets that are smaller than the minimum eight values. An SACR is derived by
using any available measured ACRs plus enough assumed ACRs of 18 (default
value adopted for the Great Lakes methodology) to give a total of 3 ACRs.