Advancing Fifth Percentile Hazard Concentration Estimation Using Toxicity-Normalized Species Sensitivity Distributions

The species sensitivity distribution (SSD) is an internationally
accepted approach to hazard estimation using the probability distribution
of toxicity values that is representative of the sensitivity of a
group of species to a chemical. Application of SSDs in ecological
risk assessment has been limited by insufficient taxonomic diversity
of species to estimate a statistically robust fifth percentile hazard
concentration (HC5). We used the toxicity-normalized SSD (SSDn) approach,
(Lambert, F. N.; Raimondo, S.; Barron, M. G. Environ. Sci.
Technol.2022,56, 8278–8289),
modified to include all possible normalizing species, to estimate
HC5 values for acute toxicity data for groups of carbamate and organophosphorous
insecticides. We computed mean and variance of single chemical HC5
values for each chemical using leave-one-out (LOO) variance estimation
and compared them to SSDn and conventionally estimated HC5 values.
SSDn-estimated HC5 values showed low uncertainty and high accuracy
compared to single-chemical SSDs when including all possible combinations
of normalizing species within the chemical-taxa grouping (carbamate-all
species, carbamate-fish, organophosphate-fish, and organophosphate-invertebrate).
The SSDn approach is recommended for estimating HC5 values for compounds
with insufficient species diversity for HC5 computation or high uncertainty
in estimated single-chemical HC5 values. Furthermore, the LOO variance
approach provides SSD practitioners with a simple computational method
to estimate confidence intervals around an HC5 estimate that is nearly
identical to the conventionally estimated HC5.