tert-butyl methyl ether

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

5.1 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

47.2 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.26 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

71 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

23 mg/kg sediment dw

Extrapolation method:

sensitivity distribution

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

1.17 mg/kg sediment dw

Extrapolation method:

sensitivity distribution

Hazard for air

Air

Hazard assessment conclusion:

hazard related to composition of atmosphere identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

1.56 mg/kg soil dw

Assessment factor:

100

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

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).

Based on a short-term reproduction assay with fathead minnow, the 21-day NOEC is 62 mg/l (Wildlife International Ltd., 2013). This NOEC is deemed rather conservative given that it is the highest concentration tested and exceeds the max. recommended exposure concentration in the OECD 229 test guideline of 10 mg/L. By comparison a guideline ELS test with eggs and larvae/fry from fathead minnow gives a 31-d NOEC of 299 mg/l (ENSR, 1999b).

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 in Daphnia 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).

Three studies of acute toxicity to earthworms are available: each includes testing exposure by contact with wetted filter paper and exposure via soil. The lowest EC50 value arising from these studies was a 72h EC50 of 826 mg MTBE/kg dry soil. Two studies of short-term toxicity to higher plants are available: unfortunately both monitored seed germination and post-emergence growth over only 5 days (much shorter than the relevant OECD test guideline requires) and neither study is considered reliable for use in assessment.

One reliable study of long-term toxicity to terrestrial organisms is available. For the most sensitive of 3 Collembola (springtail) species tested, a 35-day NOEC for reproduction of 156 mg/kg dry soil was determined in sandy loam soil (other NOECs determined across the 3 species and in different soils were 305-620 mg MTBE/kg dry soil) The long-term (reproduction) NOEC value of 156 mg/kg soil dry weight was taken forward for use in assessment.

Studies with sediment 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 sediment (and of soil) 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.

Conclusion on classification

There is a complete set of data available for this substance. Besides short-term tests on fish, invertebrates and algae also long-term studies are available. There are several guideline studies for freshwater and marine fish available.

 

For freshwater fish the most critical effect concentration was a 96-h LC50 value of 672 mg/l in fathead minnow (Pimephales promelas). The most critical effect concentration in marine fish was a 96-h LC50 of 574 mg/l in inland silverside (Menidia beryllina). Based on a short term reproduction assay with fathead minnow, the 21 day NOEC is 62mg/l (Wildlife International Ltd, 2013). This NOEC is deemed rather conservative given that it is the highest concentration tested and exceeds the maximum recommended exposure in OECD 229 test guideline of 10mg/l.

 

For freshwater invertebrates the most critical effect concentration was a 48-h EC50 value of 472 mg/l in Daphnia magna. The most critical effect concentration in marine invertebrates was a 96-h LC50 of 44 mg/l in mysid shrimp (Americamysis bahia). This is the only short term LC50 to be below 100 mg/l. Further, a 96 hour LC50 of 44 mg/l (nominal conc.) is considered to be incompatible with an observed 28 day NOEC of 26 mg/l in the same species. Therefore the 96 h LC50 value of 44 mg/l for the mysid shrimp (Americamysis bahia) is declared to be not reliable. Two chronic tests are available, for both freshwater and marine species. The 21-d NOEC in Daphnia magnais 51 mg/l and the above mentioned 28-day NOEC in mysid shrimp (Americamysis bahia) is 26 mg/l. The test results on algae differ considerably from each other. The most critical effect concentrations are an IC50 of 491 mg/l and an IC20 of 103 mg/l in Pseudokirchneriella subcapitata.

 

Only two studies on the toxicity of this substance to microorganisms have been reported. Both of them have been performed with Pseudomonas putida, but they differ in duration and methodology. The most critical value is 710 mg/l from the test lasting 18 hours and measuring cell multiplication inhibition.

For terrestrial organisms a long-term NOEC value for soil arthropods is available: the 35-day NOEC for reproduction of Collembola (springtails) of 156 mg/kg soil dry weight is used for assessment.

No studies with sediment organisms are available, however as the log Kow is very low (1.06) direct and indirect exposure of this compartment is not expected, as was demonstrated by the exposure assessment.

 

No data on bird toxicity is available, however a large mammalian dataset is available and as the log Kow is very low (1.06) secondary poisoning is not expected.

 

Based on all available environmental fate and ecotoxicological data for daphnids, fish and algae, this substance does not need to be classified according to the EU CLP (Regulation (EC) No. 1272/2008). All reliable 48-h EC50s and 96-h LC50s are larger than 100 mg/l and all reliable chronic NOEC values are greater than 10 mg/l. And although this substance is not readily biodegradable, the substance is not bioaccumulative, which leads to non-classification.