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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The HYDROTOPES Category comprises the following 6 substances:
STS - Sodium toluene 4-sulphonate (CAS 657-84-1, EC 211-522-5)
SXS - Sodium (xylenes and 4-ethylbenzene) sulphonate (EC 701-037-1)
NH4XS - Ammonium (xylenes and 4-ethylbenzene) sulphonate (EC 943-024-5)
SCS - Sodium p-cumenesulphonate (CAS 15763-76-5, EC 239-854-6)
KCS - Potassium p-cumenesulphonate (CAS 164524-02-1, EC 629-764-9)
NH4CS - Ammonium p-cumenesulphonate (CAS 680972-33-2, EC 811-484-5) 
In addition CaXS (Calcium Xylenesulphonate, CAS 28088-63-3, EC 248-829-9) was evaluated for complete the assessment despite it is not registered under REACH.

There are available studies on short term toxicity to algae performed on the three subgroups (toluene, xylene and cumene).
The results of the availbale studies are:
STS (1995, GLP): EC50 72 h = 150 mg/l (a.i.)
SXS (1993, no GLP): EC50 96 h > 230 mg/l (a.i.)
CaXS (1994, GLP): EC50 96 h = 758 mg/l (a.i.)
SCS (2020, GLP): EC50 72 h > 100 mg/l (a.i.)

The key study used for the assessment is the test performed on SCS (2020, GLP) with the 72 hour EC50 value > 100 mg/L.

The other available studies are: 
- reliable 1993 USEPA Guideline study (EPA OTS 797.1500 - Algal Toxicity). The freshwater Selenastrum sp. was exposed to a range of nominal concentrations of the test substance, sodium xylenesulphonate that reported a 96 -hour EC50 of >= 230 mg/L active substance based on cell density (with a 95% CL of 47 to 1200 mg/L). There was an apparent dose response and the 96-hour NOEC was 31 mg/L. The test substance was observed to be algistatic but not algicidal at the highest concentration tested (1000 mg/L active substance).
- a 1994 EPA Guideline study (EPA OTS 797.1500) with the test substance calcium xylenesulphonate was conducted under GLP requirements and with analytical confirmation reported a 96 hour EC50 of 758 mg/L active substance (with a 95% confidence limit of 592-924 mg/L) and a NOEC of 240 mg/L based on growth rate of Selenastrum sp.
- a 1995 EU Guideline study (EU C.3 Algal Inhibition Test) of sodium toluenesulphonate conducted under GLP requirements and with analytical confirmation reported a 72 -hour EbC50 of 150 mg/L active substance based on cell density and a 72 -hour ErC50 of 310 mg/L based on specific growth rate of Scenedesmus sp. The 72 -hour NOEC is 40 mg/L active substance based on biomass.
These studies indicate that of the Hydrotropes substances are comparable in term of effects on algae and show low toxicity.

Key value for chemical safety assessment

EC50 for freshwater algae:
100 mg/L
EC10 or NOEC for freshwater algae:
100 mg/L

Additional information