Disodium oxybis[methylbenzenesulphonate]

Toxicity to aquatic algae and cyanobacteria

Description of key information

The acute toxicity of ditolylether disulfonic acid disodium salt, isomer mixture to alga (Desmodesmus subspicatus) was tested according to OECD guideline 201 ' Alga, Growth Inhibition Test'. During 72 hours exposure an ErC50 of 112.2 mg/L and ErC10 of 66.9 mg/L was measured and a NOEC of 50 mg/L was calculated.

Key value for chemical safety assessment

EC50 for freshwater algae:

112.2 mg/L

EC10 or NOEC for freshwater algae:

66.9 mg/L

Additional information

A study was performed to assess the adverse effects of ditolylether disulfonic acid disodium salt, isomer mixture on the growth rate (= rate of increase in cell density with time) of the planktonic freshwater algal species Desmodesmus subspicatus (former name: Scenedesmus subspicatus) over several generations.

The study was conducted in accordance with Commission Regulation (EC) No 761/2009 amending Regulation No 440/2008, Method C.3 ‘Freshwater Alga and Cyanobacteria, Growth inhibition test’ (2009) which is equivalent to OECD Guideline for Testing of Chemicals No. 201 (2006).

Exponentially growing algal cells were exposed for a period of 72 hours to a range of concentrations, nominally 6.3, 12.5, 25, 50 and 100 mg/L of ditolylether disulfonic acid disodium salt, isomer mixture dissolved in dilution water. Auxiliaries used to prepare the test media was a magnetic stirrer. The cell densities were measured at 24 hour intervals. Inhibition of the algal population was measured as reduction in growth rate (index r), relative to control cultures grown under identical conditions. Growth rates were also used to calculate a No Observed Effect Concentration and a Lowest Observed Effect Concentration according to Welch-t test for Inhomogeneous Variances with Bonferroni-Holm Adjustment.

An ErC50 (0-72 h) of 112.2 mg/L and an ErC 10 (0-72 h)of 66.9 mg/L were obtained. The NOEC was calculated to be 50.0 mg/L and the LOEC was found to be 100.0 mg/L.

The results are expressed in terms of nominal concentrations. Effective concentrations ranged from 91.7 % to 94.1 % of nominal values at 0 hours, and from 92.6 % to 94.5 % of nominal values at 72 hours.

 

As recommended in REACH guidance document R.10 (ECHA, 2008), an EC10 for a long-term test which is obtained using an appropriate statistical method (usually regression analysis) will be used preferentially. The NOEC is defined as "the highest concentration tested at which the substance is observed to have no statistically significant effect (p< 0.05) when compared with the control, within a stated exposure period" (OECD TG 211, 1998) or the test concentration immediately below the LOEC, which when compared with the control has no statistically significant effect (p< 0.05) within a stated period (OECD TG 211, 1998). There has to be a concentration-effect relationship. In the past, the NOEC was mainly derived on the basis of ANOVA (analysis of variance) and a subordinate test (e.g. Dunett's) or determined directly from the concentration-effect curve by consideration of the deviation of the control (e.g. 10%). In older investigations, it may be difficult to find out how the NOEC was generated unless test reports or raw data are available. There has been a recommendation within OECD in 1996 to phase out the use of the NOEC, in particular as it can correspond to large and potentially biologically important magnitudes of effect. The preconditions for the use of ANOVA have to be fulfilled (normal distribution, homogeneous variances). The advantage of regression method for the estimation of ECx is that information from the whole concentration-effect relationship is taken into account and that confidence intervals can be calculated. These methods result in an ECx, where x is a low effect percentile (e.g. 5-20%).

In conclusion, it is not entirely clear as to whether the EC10 or NOEC from the algal study should be used as the main result, in the context of PNEC derivation. In this case, due to the shallow dose-response relationship seen in the study with Desmodesmus subspicatus, it is considered appropriate to use ErC10 as the primary result of the study.