LINEAR ALKYLBENZENE SULFONATE (LAS) - UNEP Chemicals

OECD SIDS LINEAR ALKYLBENZENE SULFONATE (LAS)
Introduction
ANNEX 3 – DERIVATION OF THE HC5 VALUE
Linear Alkylbenzene Sulfonate (LAS)
Industry Coalition for the SIDS Assessment of LAS
Extrapolation procedures are commonly used to evaluate the available laboratory-generated singlespecies
toxicity test data. For data sets in which toxicity data are available for a reasonably large
number of species, the species sensitivity distribution approach is often used. In this approach, the
concentration protection of most single species (generally 5%, i.e., 95% of the species NOECs are
greater) is calculated. This value, called the HC5, is the lower 5 th percentile of a distribution of
single-species NOEC tests and thus is protective of the environment (Aldenberg and Slob 1993).
For the current evaluation of LAS data, the HC5 for aquatic species was calculated for LAS using
the available single-species chronic freshwater data including the data summarized by van de
Plassche et al. (1999).
Method
The van de Plassche et al. (1999) data were analyzed using several types of distributions as
described in Versteeg et al. (1999). The goodness-of-fit for each distribution was evaluated by the
one-sample Cramer-vom Mises statistical test, which was used to compare the relative goodness-offit
with the various distributions. Based on higher p-values, the log-logistic distribution was a better
fit to the data than the log normal distribution used by van de Plassche et al. (1999).
Consequently, the available chronic data and the van de Plassche et al data were plotted using loglogistic
distributions. The fitted NOEC distribution is shown by the solid line in the distribution
function. Lower 95% confidence limits on the fitted NOEC distribution function (dashed lines)
were calculated analogous to the methods of Aldenberg and Slob (1993), with the exception that
here the maximum likelihood estimators were used as opposed to moment estimators to measure
goodness-of-fit. The maximum likelihood estimators are generally less biased and have better
precision than moment estimators (Schafer and Sheffield 1973).
The Kolmogorov-Smirnov (K-S) statistical test was used to determine the goodness-of-fit and
calculate the HC5 (HC5, 50% confidence interval) value.
The available chronic toxicity data and the data of van de Plassche et al. (1999) are presented in
Table 12A of this SIAR. For Microcystis aeruginosa, dossier 4.3s, the NOEC was calculated by
dividing the EC50 value by 3. As documented below (data from BKH 1993), the average
EC50/NOEC ratio for LAS is 3, and thus this calculation of the NOEC value from the EC50 is
supported by a large database of information.
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