Ethanethiol

Toxicity to microorganisms

Description of key information

Toxicity to microorganisms: 3 hour EC50 = 880.5 mg/l (OECD 209) read across from a related substance, propane-2-thiol.

Key value for chemical safety assessment

EC50 for microorganisms:

880.5 mg/L

Additional information

There are no reliable toxicity to microorganisms data available for ethanethiol. The key study for this endpoint has therefore been read across from the structurally analogous substance, propane-2-thiol (CAS 75-33-2). In a study conducted according to OECD guideline 209 and in compliance with GLP, the 3 hour EC₅₀for propane-2-thiol was 880.5 mg/L. The validity criteria of the study were met. The study is therefore considered Klimisch 1, and read-across is considered to be Klimisch 2 (Fiel, 2010).

Supporting information is available from two ready biodegradation studies with the registration substance itself. In a GLP-compliant manometric respirometry test, conducted according to OECD guideline 301F, the toxicity control attained 51% biodegradation in 29 days at a concentration of 50 mg/L, indicating that ethanethiol was not inhibitory to the sewage treatment microorganisms used in the test (Clarke, 2011). In a Closed Bottle Test, conducted according to OECD guideline 301D and in compliance with GLP, no inhibition was reported in the toxicity control at a concentration of 1.25 mg/L (Thiebaud, 1994).

Read-across justification

The registration substance is a member of a Category of low molecular weight aliphatic alkyl thiols. As shown in the table below, all of the substances for which measured data are available are hazardous to the aquatic environment although there is some variability in the reported biodegradability. Both the registration substance, ethanethiol, and the read-across substance, propane-2-thiol, would be expected to dissolve in culture medium at concentrations relevant to the OECD 209 test with activated sludge (8660 and 1900 mg/l, respectively).

	Ethanethiol	Propane-1-thiol	Propane-2-thiol	Butane-1-thiol	Butane-2-thiol	2-Methylpropane-2-thiol
Synonym	Ethylmercaptan	Propylmercaptan	Isopropylmercaptan	n-Butylmercaptan	s-Butylmercpatan	t-Butylmercaptan
CAS number	75-08-1	107-03-9	75-33-2	109-79-5	513-53-1	75-66-1
Molecular weight	62.13	76.16	76.16	90.18	90.18	90.18
SMILES	CCS	CCCS	CC(S)C	CCCCS	CC(S)CC	SC(C)(C)C
Physical state at STP	Liquid	Liquid	Liquid	Liquid	Liquid	Liquid
Vapour pressure (kPa)	58.9 at 20°C	20.5 at 25°C	33.1 at 25°C	6.06 at 25°C	10.8 at 25°C	24.0 at 25°C
n-octanol/water partition coefficient (log Kow)	1.5	1.8	1.7 (Calculated)	2.3	2.2 (Calculated)	2.1 (Calculated)
Water solubility (mg/l)	8860 at 20°C	1900 mg/l at 25°C	1900 mg/l at 25°C (r/a/ from propane-1-thiol)   4835 mg/l at 25°C (Calculated)	595 mg/l at 20°C	1320 mg/l at 20°C	1470 mg/l at 25°C
Biodegradation	Not readily biodegradable	Readily biodegradable	Not readily biodegradable	Readily biodegradable	No data	Not readily biodegradable
Acute toxicity to fish (96 h)	LC50= 2.4 mg/l	LC50= 1.3 mg/l	No data	No data		LC50= 34 mg/l
Acute toxicity to invertebrates (48 h)	EC50<0.1 mg/l	EC50= 0.07 mg/l	EC50= 0.25-0.5 mg/l	No data		EC50= 6.7 mg/l
Algal growth inhibition (72 h)	ErC50= 3.0 mg/l	No data	ErC50= 21.9 mg/l	No data		ErC50= 24 mg/l
Activated sludge respiration inhibition (3 h)	No data	No data	EC50= 880.5 mg/l	No data	No data	No data

Both substances would be expected to metabolise following common metabolic pathways (refer to IUCLID Section 7.1 and Section 1.4 of the Chemical Safety Report for further information). Although there is an approximately 10-fold difference in the reported acute algal inhibition EC₅₀values for the two substances, measured data for acute toxicity to Daphnia magna are within the same order of magnitude (IUCLID Section 6.1.3).

Neither substance is readily biodegradable (IUCLID Section 5.2) but a toxicity control included in the key biodegradation test with ethanethiol indicated that the substance was not inhibitory to microorganisms at the concentrations tested.

Since there is no direct release of ethanethiol to waste water treatment, read-across from propane-2-thiol is considered to be adequate to address this endpoint.