4-methylstyrene

Endpoint summary

Description of key information

Based on the available weight of evidence from QSAR and experimental studies of test substance, p-methylstyrene can be expected to fulfil the requirements for classification as readily biodegradable in water, sediment and soil.

Additional information

The biodegradation potential of p-methylstyrene (PMS) is evaluated based on modelling as well as experimental data available for the test substance.

Based on the results of the studies available with the test substance, p-methylstyrene and the strict interpretation of the REACH guidelines for biodegradation, PMS and VT are not considered to be readily biodegradable, as preconditioning of the microorganisms is required. However it has to be acknowledged that these substances as well as other similar substances (such as ethylbenzene, xylenes and styrene) act as metabolisable substrate for microorganisms, which adapt to use them as a food source. Once adaptation occurs their removal is rapid. However, on the contrary these other structurally similar substances (i.e., ethylbenzene, xylenes and styrene) have all been shown to be classifiable as biodegradable in water and soil even though preconditioning of microbial populations is still required. Also, their pathways for biodegradation have been also elucidated.

Therefore, the available biodegradation data for one of the similar substance (styrene CAS 100-42-5), which is structurally closer to PMS and VT, as well for which there is a UK HSE evaluation report concluding on its biodegradation potential in the environmental, has been further evaluated.

The rationale for read across between these three substances has been presented below:

Rationale for read across between styrene and PMS/VT.

The chemical structure of the three molecules is essentially similar with differences being limited to minor variations in the number of hydrogen atoms in substitution groups in the case of VT and the addition of a second CH group attached in the para position in the case of PMS. The molecular formula for styrene is C8H8, for VT and PMS C8H10. The molecular weights are 104.15 and 118.18. The experimental value of the logPow values for styrene is 3.02, for VT 3.46 and for PMS 3.35. None of the three materials are bioaccumulative in experimental determinations although all have logPow values in excess of 3.0.

Modelling of biodegradation using the US EPA EPI Suite BioWin model gives the following results.

Biowin Model.	Biodegradation prediction timeframe
	Styrene CAS 100-42-5	VT CAS 25013-15-4	PMS CAS 622-97-9
BIOWIN 1	Fast	Fast	Fast
BIOWIN 2	Fast	Fast	Fast
BIOWIN 3	Weeks	Weeks	Weeks
BIOWIN 4	Days/weeks	Days/weeks	Days/weeks
BIOWIN 5	Not fast	Not fast	Not fast
BIOWIN 6	Fast	Not fast	Not fast
BIOWIN 7	Not fast	Not fast	Not fast
Summary prediction of readily biodegradation	NO	NO	NO

In summary it is evident that the predicted biodegradation predictions for the three compounds is identical with the exception that styrene in MITI model (BIOWIN 6) is predicted to degrade quickly whereas VT/PMS is predicted to degrade slower.

Modelling results for persistence in sewage treatment plants are shown in the table below.

STP fugacity model	Percentage distribution
	Styrene CAS 100-42-5	VT CAS 25013-15-4	PMS CAS 622-97-9
Primary sludge	2.28	6.36	5.10
Waste sludge	1.39	2.85	2.17
Primary volatilization	1.17	1.08	1.12
Cycling volatilization	1.56	1.51	1.41
Aeration (gas release)	47.59	42.91	48.79
Primary biodegradation	0.02	0.03	0.03
Settling biodegradation	Zero	0.01	Zero
Aeration biodegradation	0.04	0.07	0.06
Final water effluent	45.94	45.18	41.32
Total removed	54.06	54.82	58.68

The results of this evaluation show that styrene is comparable to VT and PMS.

 

Level III fugacity modelling for environmental persistence and removal results are shown in table below.

Level III fugacity model	Percentage distribution
	Styrene CAS 100-42-5		VT CAS 25013-15-4		PMS CAS 622-97-9
	Reaction	Advection	Reaction	Advection	Reaction	Advection
Air	63.7	3.01	59.6	1.23	58.3	4.25
Water	12.4	6.46	12.4	6.46	12.3	6.41
Soil	14.4	Zero	20.2	Zero	18.7	Zero
Sediment	0.0211	0.00197	0.0359	0.00335	0.0364	0.0034
Persistence time	218 hours		277 hours		264 hours
Reaction time	241 hours		300 hours		296 hours
Advection time	2300 hours		3600 hours		2480 hours
% Reacted	90.5		92.3		89.3
% Advected	9.48		7.69		10.7
Half lives
Air	3.28 hours		1.431 hours		5.051
Water	360 hours		360 hours		360 hours
Soil	720 hours		720 hours		720 hours
Sediment	3240 hours		3240 hours		3240 hours

The results of this evaluation show that styrene is comparable to VT and PMS.

Environmental evaluation of styrene – EU Risk Assessment Report volume 27 EUR 20541[1].

As per the environmental evaluation of styrene by UK HSE in 2002 (EU Risk Assessment Report volume 27 EUR 20541) the main routes for removal of styrene from the environment are photo oxidation, volatilisation and biotransformation. Styrene like VT and PMS is very susceptible to photo degradation by hydroxyl radicals and ozone in the atmosphere but is not susceptible to photolysis. Photo degradation calculated by modelling indicates that degradation is extremely rapid with DT50 values measured in hours. All three substances are volatile therefore persistence in soil and water is not anticipated.

Biodegradation by aquatic and soil microbes have been extensively studied for styrene. It displays similar properties to VT/PMS in that the microbial population requires acclimatisation to the material before biodegradation is possible. Once acclimatisation is complete biodegradation is rapid. For styrene the route of biodegradation has been elucidated and the pathway is thought to proceed by two different mechanisms. In the first oxidation to phenylethanol and phenylacetic acid occurred. However the second route appeared to be via spontaneous polymerisation to low molecular weight oligomers which are subsequently degraded. As styrene can and undergo explosive polymerisation without an inhibitor the authors of the study attributed degradation of styrene to removal of the inhibitor by microbial action. It is therefore highly probable that VT and PMS are also subject to the same degradation mechanistic routes. The requirement for adaptation of bacterial populations was investigated in a study on biodegradation in soil where pre-acclimatised bacterial from a landfill site achieved a more rapid rate of biodegradation than bacteria present in natural top soil. However, it was also clear from this study that acclimatisation by microbial populations in natural top soil was rapid with differences recorded in removal after a 16 week period of less than 10% of the applied dose.

The results of ready biodegradation studies reported in this review show that following adaptation to the food source removal was comparatively rapid. This is very similar to the results seen in the VT/PMS studies. Closed bottle tests to assess biodegradation also demonstrated that styrene was readily biodegradable in both freshwater and sea water. It is therefore highly probable that VT/PMS would also be biodegradable under these conditions. However, the report notes that biodegradation in freshwater tends to proceed more rapidly than in sea water. Of particular interest is the OECD 302C (MITI test) result which failed to confirm the modelling result that styrene would biodegrade fast. The results of this study show that the material is inherently biodegradable rather than readily biodegradable and it is possible that the modelling produced a result that was borderline. In point of fact this model (BIOWIN 6) predicts VT/PMS to be inherently biodegradable but not readily biodegradable therefore it is quite possible that the three materials are comparable in terms of biodegradability as measured by this test.

Bioaccumulation experiments conducted with styrene indicate that the potential for bioaccumulation is substantially less than that predicted based on the log Pow. The experimental data available for VT and PMS is better than that available for styrene. This stated clearly demonstrates that neither material bioaccumulates in aquatic species.

Based on the available data and the conclusions of the HSE EU report, it is possible to state that styrene meets the conditions for readily biodegradable. In addition the opening statement in the EU report states that sufficient work has been submitted and evaluated on which a risk assessment can be generated for styrene.

Overall, on the basis of the structural and behavioural similarity of styrene to VT and PMS it is possible to read across between the substances for the purposes of classification. There is considerably more data available on styrene, based on which the biodegradation characteristics of the PMS and VT can be predicted. Also, it is evident from modelling that the three substances should behave in identical manner therefore it is reasonable to predict that the findings for styrene will also apply to VT and PMS. On this basis it can be predicted that both substances, PMS and VT will also fulfil the requirements for classification as readily biodegradable in water, sediment and soil.

[1] https://echa.europa.eu/documents/10162/a05e9fc2-eaf7-448e-b9b2-d224d28173c0