Registration Dossier

Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

The toxicity of the substance to algae was measured during a period of 72 hours and the growth inhibition showed an ErC10 of 0.02 mg/L and an ErC50 of 0.035 mg/L. 

Key value for chemical safety assessment

EC50 for freshwater algae:
0.035 mg/L
EC10 or NOEC for freshwater algae:
0.02 mg/L

Additional information

The toxicity of the substance to algae was assessed in a 72h-growth inhibition test according to the OECD 201 (2006). Desmodesmus subspicatus was used as test species and the nominal concentrations tested were 0.05, 0.1, 0.2, 0.4 and 0.8 mg/L. The criteria for adverse effects used in the present study were item-induced inhibition of yield and growth rate of the algal population determined by measuring the cell density in a microcell counter. Cell densities were measured at 24h intervals. Since the deviation from the initial concentration is not within the range of +/- 20%, analysis on the results are based on the geometric mean concentration during exposure. The 72h-EC10 for growth was determined to be 0.02 mg/L and the 72h-EC50 for growth was determined to be 0.035 mg/L. The NOEC was found to be 0.01 mg/L.

The study report also gives a NOEC but clearly states that this value is invalid as it could not be mathematically derived using valid statistical software (ToxRatPro Version 2.10 (released 19.02.2009)). In fact, it was only read from the table which gives concentrations tested (Table 8, p. 27). At the NOEC concentration there was even a slight increase in algal growth instead of an inhibition. The use of ErC10 instead of NOEC would, therefore, be preferred.

The use of EC10 instead of NOEC is also supported by ECHA Guidance Document R.10 (May 2008):

- In general, this Guidance Document always asks to use “EC10 or NOEC” which underlines that, basically, both values may be used for the assessment.

- On page 8 it is highlighted that “The value of the NOEC is limited to being one of the tested concentrations (i.e. if different values were chosen for the tested concentrations, the value of the NOEC would be different).” However, the concentration space is very close in the present study: 0.05, 0.1, 0.2, 0.4 nd 0.8 mg/L and only an EC10 at 0.2 mg/L could be mathematically determined.

- Furthermore, R.10 on page 8 highlights “If the power [of a test] is high, it may occur that biologically unimportant differences are found to be statistically significantly different.” The statistical power of the available GLP and OECD 201 study is high but a NOEC could, nevertheless, even not be calculated.

- EC10 and NOEC both use the same assessment factors to derive the PNECs (e.g. in Table R.10-4).

- Page 11 (Table R.10-1) section “long-term studies” gives: “An EC10 for a long-term test which is obtained using an appropriate statistical method (usually regression analysis) will be used preferentially.”

Furthermore, CLP Regulation (EC) No 1272/2008 (31.12.2008), section 4.1.2.7. Aquatic toxicity, subsection 4.1.2.7.2., gives: “For determining chronic aquatic toxicity for classification purposes data generated according to the standardised test methods referred to in Article 8(3) shall be accepted, as well as results obtained from other validated and internationally accepted test methods. The NOECS or other equivalent L(E)Cx (e.g. EC10) shall be used.” This statement is important and has to be kept in mind as most tables only give NOEC and not additionally ECx to derive the environmental classification of a substance.

Finally, OECD 201 (23 March 2006) “Freshwater Alga and Cyanobacteria, Growth Inhibition Test”, which is the base for determining NOEC or ErCx of the present algal study, in its section on statistical procedures, No. 58, gives: “For determining chronic aquatic toxicity for classification purposes data generated according to the standardised test methods referred to in Article 8(3) shall be accepted, as well as results obtained from other validated and internationally accepted test methods. The NOECS or other equivalent L(E)Cx (e.g. EC10) shall be used.”

And Annex 1 of OECD 201 defines LOEC as: “Lowest Observed Effect Concentration (LOEC) is the lowest tested concentration at which the substance is observed to have a statistically significant reducing effect on growth (at p < 0.05) when compared with the control, within a given exposure time. However, all test concentrations above the LOEC must have a harmful effect equal to or greater than those observed at the LOEC. When these two conditions cannot be satisfied, a full explanation must be given for how the LOEC (and hence the NOEC) has been selected.”

And last but not least, taking into account that technical advancement has led to low variance in control as well as in test concentrations studies need to be critically evaluated if statistically significant deviations are also biologically relevant. Small deviations might result in statistical significance but positive biological significance at low inhibition levels is questionable. USEPA, e.g., gives that NOEC shall be determined at 10-30% inhibition but not at lower inhibitions.

Concluding, for ditolyl ether there is a GLP OECD 201 study on algal growth inhibition for which only an ErC10 can mathematically be determined. The NOEC was given to be invalid in the study. However, ECHA Guidance R.10 gives that ErC10 can be used instead of NOEC (for PNEC derivation). Furthermore, there is an indication that NOEC might even be critical for regulatory purposes from due to conflict between statistical and biological relevance. Therefore, only ErC10 is valid and shall be used for regulatory purposes of ditolyl ether.