tert-butyl methyl ether

Endpoint summary

Additional information

There is a complete set of data available for MTBE, including short-term tests on fish, invertebrates and algae together with some long-term studies.

There are several acute and chronic guideline studies available for freshwater and marine fish. For freshwater fish the most critical effect concentration was a 96-h LC50 value of 672 mg/l in fathead minnow (Pimephales promelas) (Geiger et al., 1988). For marine fish the most critical effect concentration was a 96-h LC50 of 574 mg/l in inland silverside (Menidia beryllina) (BenKinney et al., 1994).

One chronic guideline test is available, an ELS test with eggs and larvae/fry from fathead minnow (Pimephales promelas). The 31-d NOEC is 299 mg/l (ENSR, 1999b). A non-guideline study by Moreels et al. (2000a) on an ELS with eggs from the African catfish (Clarias gariepinus) is not deemed reliable or relevant for hazard or risk assessment purposes due to some major deficiencies in the study. In particular, average exposure concentrations have been incorrectly calculated.

A fish sexual development toxicity assay in accordance with the OECD 234 guideline is also underway the results of which will be included in the dossier when available.

Two reliable short term reproduction assays are available on the toxicity of the test substance to zebrafish (Danio rerio) and the fathead minnow (Pimephales promelas). Both studies were performed under GLP conditions and in accordance with test guidelines OECD 229 and U.S. EPA OPPTS 890.1350.

The purpose of the studies was to investigate the potential for MTBE to interact with the endocrine system in fish. For the first study (Wildlife InternationalLtd, 2012) an exceptionally high exposure concentration of 147 mg/L was included in the study so that a comparison could be made with a study reported in the literature by Moreels et al. (2006b). Under usual circumstances, testing according to OECD 229 and U.S. EPA OPPTS 890.1350 guidelines would not include such high exposure concentrations.

In zebrafish (Wildlife International Ltd, 2012), a reduction in fecundity was accompanied by a significant increase in the incidence of oocyte atresia along with a significant increase in the accumulation of oocyte debris in the oviduct at 147 mg/L; consequently the NOEC for reproduction was determined to be 3.04 mg/L, the next exposure concentration tested. In fathead minnows a NOEC for reproduction was determined to be 62 mg/L, the highest exposure concentration tested. 

The fish endocrine screening studies are intended to be used for qualitative risk assessment purposes, such as to determine potential for endocrine interaction and further testing. As the OECD Fish Toxicity Framework (OECD, 2012) considers that dose spacing greater than a factor of around 3 should be considered for qualitative rather than quantitative risk assessment purposes, the NOEC for fish is considered as 62 mg/L rather than 3.04 mg/L. Considering the NOEC of 3.04 as not sufficiently precise an indication of toxicity to fish be relevant for quantitative risk assessment purposes is further supported by the next exposure concentration of 147 mg/L greatly exceeding the max. concentration for testing under the OECD 229 guideline of 10 mg/L. The U.S. EPA considers its corresponding OPPTS 890.1350 guideline with a max. concentration for testing of 100 mg/L as not intended to provide quantitative risk assessment data (U.S. EPA, 2008).

There are several acute and chronic guideline studies available for aquatic invertebrates. For freshwater invertebrates the most critical effect concentration was a 48-h EC50 value of 472 mg/l inDaphnia magna(Wildlife International Ltd., 1999c). Results from several studies indicate that the mysid shrimp is the most sensitive marine species (BenKinney et al., 1994; T. R. Wilbury Laboratories, Inc., 1994; Rausina et al., 2002), but the results from the different studies with the mysid shrimp are inconsistent (LC50 values in a range 44 – 200 mg/l). Because no technical reason is apparent to explain these differences, and since results from the three studies are less than one order of magnitude apart, the results were harmonised by a geometric mean. The resultant 96-h LC50 of 106 mg/l for mysid shrimp (Americamysis bahia) will be used in the assessment. Two chronic tests are available, for both freshwater and marine species. The 21-d NOEC in Daphnia magna is 51 mg/l (Wildlife International Ltd., 1999g) and the 28-d NOEC in mysid shrimp (Americamysis bahia) is 26 mg/l (Wildlife International Ltd., 1999h).

The test results on algae differ considerably from each other. The most critical effect concentrations are an IC50 of 491 and an IC20 of 103 mg/l for Pseudokirchnerialla subcapitata (ENSR, 1999g).

Two studies on the toxicity of MTBE to micro-organisms have been reported. Both have been performed with Pseudomonas putida, but they differ in duration and methodology. The most critical value is an 18-h cell multiplication inhibition EC10 value 710 mg/l (Hüls AG, 1991d).

Studies with sediment and terrestrial organisms are not available for MTBE. However since it has a low log Kow (1.06) and relatively high vapor pressure (330 hPa at 25 ˚C), it is expected that MTBE will partition to the air compartment to a significant extent. Therefore, direct and indirect exposure of soil and sediment is not expected as was demonstrated in the exposure assessment.

Data are not available to assess the toxicity of MTBE to birds. However, there is a large mammalian dataset available (see Section 5) that indicates MTBE poses a low level of hazard, and as its log Kow is low (1.06), it is not expected to bioconcentrate to any significant extent or accumulate in biota. An assessment of secondary poisoning is therefore not required.