Methylcyclohexane

Endpoint summary

Additional information

Valid short term tests with species of 3 trophic levels are available performed by Japanese authorities (MOE, 2008). Aquatic invertebrates and algae were the most sensitive species, with acute toxicity values (EC50) of 0.326 mg/L and 0.134 mg/L, respectively. Short term effects of the substance towards fish was markedly higher, showing a LC50 (96h) of 2.07 mg/L.

 

The following table summarises available data for methylcyclohexane (CAS No. 108-87-2):

 

	Species	Type of test	Test duration	Water	Value	Effect value [mg/L]	Remarks
Fish	Oryzias latipes	semi-static	48h	freshwater	LC50	5.02 (n)	supporting study
Fish	Oryzias latipes	semi-static	96h	freshwater	LC50	2.07 (m)	key study
Invertebrate	Daphnia magna	semi-static	48h	freshwater	EC50	0.326 (m)	key study
Algae	Pseudokirchneriella subcapitata	static	72h	freshwater	EC50	0.134 (m)	key study
					NOEC	0.0221 (m)
MO	Activated sludge	static	14d	freshwater	NOEC	2.76 (m)	key study

(m) measured concentration

(n) nominal concentration

 

It should be noted that initial measured concentrations of methylcyclohexane compared to nominal concentrations were variable in all acute toxicity studies performed by MOE (2008).

 

The initial measured concentration compared to nominal concentration was as follows:

Fish: 28.5 - 37% of nominal

Daphnia: 78 - 91% of nominal

Algae: 58 - 76% of nominal

 

The variability in initial measured concentrations can be attributed to the performance of various substance preparation steps (e.g. preparation of a concentrated acetone stock solution, preparation of a series of stock solutions by adding solution I into acetone etc.) together with the high volatility of methylcyclohexane triggering considerably lower initial concentrations. Nevertheless, as time weighted concentrations were used for effect calculation and as a dose-response curve were achieved in all aquatic tests the test results are valid.

 

Furthermore, the test design had a large influence on the substance stability during the test. For example there was only little substance loss within 24 hours in the Fish and invertebrate test. These tests were performed in a semi-static test design. In contrast, methylcyclohexane was rapidly declining during the algae test, which was performed in static way. This indicates, that triggered by the high volatility of methylcyclohexane, an accumulation of the test substance in the head space has taken place. This is attributed to the OECD 201 test design itself, which is static on one hand and includes constant stirring (leading to a considerably faster accumulation in headspace than a static fish test for example) on the other hand.

 

Substance recovery rates within 24 hours when compared with initial measured concentration:

Fish: 82 -101%

Daphnia: 78 -91%

Algae: <1%

 

The OECD guidance document Number 23 on aquatic toxicity testing of difficult substances and mixtures states that: “Algal tests with very volatile substances are technically very difficult to perform satisfactorily and may as a consequence yield results that are difficult to interpret and of limited relevance to real-world conditions”. The validity of the algae study can be questioned given the rapid substance decline. However, it is highly unlikely that a new test with a static test design may improve the result. There is currently no official guideline available for algae flow through systems, which would be the best way to test highly volatile substances, such as methylcyclohexane. A reduction in headspace is not considered to be sufficient to prevent a substantial loss of methylcylcohexane due to the constant shaking of test flasks on one hand and the low test substance concentrations that are applied leading to an accumulation even in a reduced head space, on the other hand.

 

Furthermore, as the dissolved concentrations of the test substance were well coordinated with the nominal concentrations (R2 = 0.9885) and the test results revealed a significant “concentration-effect relationship, the test results of the algae test are considered to be reliable and to reflect more closely “real-world conditions” for this highly volatile chemical.

 

In addition, for the substance evaluation it is important to consider that due to the high volatility of methylcyclohexane the target compartment for environmental distribution will be air. Even if released to water, immediate losses via evaporation can be expected. As an approximation, if the Henry´s Law constant is greater than 100 Pa m3/mol, more than 50% of the substance could be lost from the water phase-in 3-4 hours (Mackay, 1992 cited in OECD 23). Considering the Henry´s Law Constant of Methylcyclohexane (> 34000 Pa m3/mol) the evaporation is assumed to occur even more rapidly.

 

Further literature data provide toxicity values much higher of that considered in this hazard assessment. Panigrahi and Konar (1989), for example, measured acute EC50 values for plankton, mollusc and chironomid larvae of 865 mg/L (Cyclops viridis), 1160 mg/L (Thiara tuberculata) and 1000 mg/L (chironomid larvae). These higher toxicity values are considered to be influenced by the volatile property of the test substance. Methylcyclohexane easily volatilizes from the test water, leading to lower toxicity values. Therefore, these values were not considered as reliable toxicity values for the hazard assessment of methylcyclohexane.

 

Given the available data, methylcyclohexane is considered to be acute very toxic to aquatic life.

 

Reliable chronic data on fish and invertebrates are not available for methylcyclohexane. However, as the risk assessment ratios indicate no risk for the environment, further long term studies with fish and aquatic invertebrates are therefore not provided and the NOEC from Pseudokirchneriella subcapitata was used for assessing the chronic toxicity of methylcyclohexane.

 

Since no studies on the toxicity to aquatic microorganisms are available for Methylcyclohexane, a ready biodegradability study is used to derive a NOEC for the toxicity to aquatic microorganisms. For Methylcyclohexane a biodegradation test according to OECD guideline 301 D is available (Simon, 2015).The test includes a toxicity control, which contains 2.73 mg/L (mean measured) Methylcyclohexane and 2 mg/L of the reference material (i.e. sodium benzoate). The toxicity control attained 32 % degradation after 14 days of incubation. Hence, the substance is not toxic to aquatic microorganisms in the toxicity control and the test item concentration of 2.725 mg/L can be used as NOEC.