1-ethoxypropan-2-ol

Toxicity to aquatic algae and cyanobacteria

Description of key information

Measured data: EC50 (72hr)>100mg/L; NOEC (72hr)>=100mg/L

QSAR: EC50 =870mg/l, NOEC=150mg/

Weight of evidence data on read across substances (EC50 values):

Data for methoxypropanol (PGME): Measured data  (7d): >1000mg/l, QSAR prediction(96hr): 676mg/l

Data for propoxypropanol (PGPE): Measured data  (96hr): 3440mg/l, QSAR prediction(96hr): 215mg/l

Weight of evidence data on read across substances (NOEC/EC10 values):

Data for methoxypropanol (PGME): Measured data  (7d): >=1000mg/l, QSAR prediction(96hr): 171mg/l

Data for propoxypropanol (PGPE): Measured data  (96hr): ~500mg/l, QSAR prediction(96hr): 63mg/l

Key value for chemical safety assessment

EC50 for freshwater algae:

1 900 mg/L

EC10 or NOEC for freshwater algae:

500 mg/L

Additional information

There is no data available on this substance but only a limit study carried out for classification and labelling purposes. There is also measured data on a number of closely related substances and there is a reliable QSAR that can also be used and together these allow interpolation to be used to predict with some confidence the likely toxicity to algae of this substance.

In a guideline and GLP 72-hour aquatic toxicity study (static) in green algae, a limit concentration of 100 mg/L ethoxypropanol did not affect the rate of growth or total biomass.  The EC50 is therefore considered to be > 100 mg/L, and the NOEC is considered to be >= 100 mg/L under the conditions of this test. However, whilst it is the only available algal toxicity test on the substance itself, as a limit test, it is not useful for assessing the starting point for PNEC derivation.

In a US EPA guideline GLP study, propylene glycol propyl ether (PnP) was tested for its phytotoxicity to a freshwater green alga, Selenastrum capricornutum Printz, over a four day period. Nominal test concentrations were set at 0, 10, 50, 100, 500, and 1000 mg/l in triplicate with an initial cell density of ~10,000 cells/ml. The 2, 3 and 4 -day EC50 values (and their 95% confidence intervals) for algal growth were 5583 (556.4 -10610.9), 3440 (0.0 -8136.7) and 1466 (0.0 -3115.4) mg/l, respectively. The no observed effect concentration (NOEC) for algal growth could not be statistically determined because the analysis of variance was not significant. However, based on the data and visual interpretation of the growth curves, the lowest observed effect concentration (LOEC) is 1000 mg/l. The NOEC is, therefore, interpreted to be 500 mg/l.

In a study designed to determine the growth inhibition potential of methoxypropanol, the green algae Selenastrum Capricornutum was exposed to the substance for a period of 7 days. After this period, cell counts were slightly inhibited (<=7%) compared to the control at test concentrations 63, 125, 250 and 1000mg/l but not at 500mg/l. Total cell volume was significantly different from the control after 7 days. The 7-day EC50 and EC10 values were greater than 1000mg/l, using growth end points.

By interpolation from the results with PnP and methoxypropanol, it is possible to predict that the EC50 for ethoxypropanol is likely to be >1000mg/l and the EC50 in the range 500 -1000mg/l. The grouping/interpolation approach is fully justified in a document attached to chapter 13 of the lead company registration dossier. The approach fully meets the requirements of annex XI.

This conclusion is supported by the results from a QSAR (US EPA ECOSAR software (v1.11) that predicts the 96 -hr algae EC50 for ethoxypropanol to be 870 mg/L, based on structure activity relationship models for neutral organic compounds. Estimates for the surrogate materials using the same QSAR were 1700mg/l for methoxypropanol and 450mg/ml for propylene glycol butyl ether, suggesting that the model is consistent with and likely more conservative than experimental results. Predictions for the ChV (NOEC) values were 87, 150 and 260mg/l for the methoxy, ethoxy and propoxypropanols respectively. The mode of action of these substances is likely to be non-polar narcosis (see chapter 6.1 overall summary for further information.) This means that toxicity is likely to be linked with the partition coefficient. This would suggest increasing toxicity with increasing length of the alkyl group and decreasing toxicity with increasing number of PO units in the molecule. This is reflected in the QSAR results and the measured values available are consistent with this trend. The QSAR can be considered highly reliable in predicting relative toxicity. Quantitatively, the available data suggests that the QSAR over-predicts the toxicity of at least 5. Such a factor based on the QSARs would lead to a conservative conclusion of an EC50 of ~1900mg/l and a NOEC of ~500mg/l, which is entirely consistent with the measured data and corresponds to the measured data for propoxypropanol.

Based on the overall available data, values of 1900mg/l and 500mg/l from propoxypropanol are taken conservatively for the EC50 and NOEC respectively for the toxicity of ethoxypropanol to algae.

An extensive justification for read across is contained in the read across justification attached to chapter 13 of this dossier.