Assessment of a New Approach Method for Grouped Chemical Hazard Estimation: The Toxicity-Normalized Species Sensitivity Distribution (SSDn)

New approach
methods are being developed to address the challenges of reducing
animal testing and assessing risks to the diversity of species in
aquatic environments for the multitude of chemicals with minimal toxicity
data. The toxicity-normalized species sensitivity distribution (SSDn)
approach is a novel method for developing compound-specific hazard
concentrations using data for toxicologically similar chemicals. This
approach first develops an SSDn composed of acute toxicity values
for multiple related chemicals that have been normalized by the sensitivity
of a common species tested with each compound. A toxicity-normalized
hazard concentration (HC5n) is then computed from the fifth percentile
of the SSDn. Chemical-specific HC5 values are determined by back-calculating
the HC5n using the chemical-specific sensitivity of the normalization
species. A comparison of the SSDn approach with the single-chemical
SSD method was conducted by using data for nine transition metals
to generate and compare HC5 values between the two methods. We identified
several guiding principles for this method that, when applied, resulted
in accurate HC5 values based on comparisons with results from single-metal
SSDs. The SSDn approach shows promise for developing statistically
robust hazard concentrations when adequate taxonomic representation
is not available for a single chemical.