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Alpha-methylstyrene is highly volatile from aqueous solution (Henry's Law constant calculated using EPIWIN v3.11, HENRYWIN v3.10: 438.63 Pa x m³/mole and calculation according to the equation H=vp/ws: 282 Pa x m³/mole at 25°C each). The Henry's Law constant calculated according to the equation H=vapor pressure/water solubility and using reliable experimental derived values also indicates that alpha-methylstyrene is highly volatile from aqueous solution (H=258.32 Pa x m³/mole at 25°C).

The environmental distribution of alpha-methylstyrene according to the fugacity model Mackay Level I v3.00 reveals that in the equilibrium state the substance is distributed to 98.4 % into the atmosphere (cf. chapter 5.4.3), whereas only minor amounts of the substance will be transferred to water (1.1 %), sediment (0 %) and soil (0 %). Furthermore, SIMPLEBOX results indicate that the main removal pathway in sewage treatment plants (STP) is volatilisation. It could be demonstrated by the exposure estimation specified in the quantitative environmental exposure assessment using the SIMPLETREAT model (cf. Quant_Env_ERA_AMS-CSR; attached document in section 13) that the major part of alpha-methylstyrene introduced into STP is directed to air.

The assumption, that the atmosphere has to be regarded as the dominant environmental target compartment of alpha-methylstyrene is underlined by monitoring data from surface water, sediment and air in Japan (1997-2005, cited in CHRIP database, cf. chapter 5.5.1). Whereas the substance was detected in none of more than 100 surface water (detection limit: 0.009 - 4 µg/L) or bottom sediments samples (detection limit: 0.0007 - 0.01 µg/g dw), it was found in 20 out of 26 air samples (detection limit: 1.9 ppb). Furthermore, alpha-methylstyrene was not detected in the sludge from the wastewater treatment plant of the Emscher Genossenschaft receiving the wastewater of INEOS Phenol (one sample collected 2010; limit of determination: 0.5 mg/kg dw).

Once introduced in the atmosphere, alpha-methylstyrene is rapidly degraded by OH-radicals.

The rates of the reactions of OH radicals with styrene, alpha-methylstyrene, and beta-methylstyrene were measured by irradiating mixtures of these aromatic olefins and NO in an environmental chamber at 298K. Based on the experimentally determined reaction rate constant k(OH) =5.3 +/- 0.6 x 10E-11 cm³/molecule x sec (25°C) and assuming a tropospheric OH radical concentration of 5 x 10E5 radicals/cm³ (24 h day) a half-life of 7.27 h can be estimated for alpha-methylstyrene. Based on the results, alpha-methylstyrene present in air will be relative quickly photodegraded by reaction with OH radicals. Acetophenone and formaldehyde were observed as main breakdown products which in turn undergo further decomposition. Similar results were obtained using the generally accepted calculation model EPIWIN v3.11, AOPWIN v1.91 (t1/2=7.2 h). The reaction of alpha-methylstyrene with ozone is a further elimination pathway (t1/2=2 h, calculated via EPIWIN). In addition, due to the spectral properties of alpha-methylstyrene (absorption of light in the environmental UV spectrum) direct photolysis is to be expected.

In a study conducted by CITI (Japan) according to OECD Guideline 111 (Hydrolysis as a Function of pH), alpha-methylstyrene proved to be stable to hydrolysis at pH 4, 7, and 9 at 25°C.

For the the ready and inherent biodegradability of alpha-methylstyrene, reliable results from studies conducted according to OECD guidelines (OECD 301C, 301D, 301F and 302C) are available.

The ready biodegradation of alpha-methylstyrene was investigated in a study conducted according to OECD Guideline 301 D (Closed Bottle Test) employing a mixed microbial inoculum obtained from the effluent of a wastewater treatment plant and soil. The test substance proved to be not inhibitory at the concentration tested. In this close-bottle test 56 % biodegradation were obtained within 21 days, which is very close to the 60 % threshold for ready biodegradability. Regarding this test, a number of uncertainties have to be taken into account. During application of the volatile AMS to the medium in the Closed Bottle test, the spiked medium was left at least for some time in an „open system“ and so a part of the applied amount of AMS might evaporate from the medium before transferred into the test vessels or at least before the vessels were gas-tight sealed. In this case, the actual loading would have been below the nominal one and a part of the nominal concentration would have been not available for microbial biodegradation.

The latter uncertainty may also be related to the results of a study conducted according to OECD Guideline 301 F, in which alpha-methylstyrene underwent 8 % and 21 % biodegradation after 28 d at 100 and 20 mg/L, respectively, under the specific test conditions. The part of applied test substance, which might have evaporated from the test solution, would have been no longer available to degradation by the inoculum, leading to an underestimation of the degradation rate. This is supported by the higher percentage observed with the lowest nominal concentration, suggesting that the actual level of dissolved substance (and consequently actual ThOD) is closest to 20 than 100 mg/L, and probably lower than 20 mg/L.

In the study conducted according to OECD Guideline 301C (Ready Biodegradability: Modified MITI Test (I)), alpha-methylstyrene was not degraded under the test conditions employed (0% biodegradation after 14 d; initial concentration: 100 mg/l). The test method was identified by OECD as being in principal appropriate for volatile substances and an ‘improved type of study design for volatile substances’ was applied for testing AMS (not further details specified). The reason for the significant difference compared to the results of all other studies available on biodegradability can not be explained by the available references. However, it seems questionable, whether the applied MITI-I method is the best choice for testing ready biodegradation of substances exhibiting both, high volatility as well as limited water solubility.

No definitive proof of ready biodegradability has been obtained from the experimental study results available for AMS on this endpoint, when strictly applying the criteria of the OECD 301 guidelines. However, from the diversity of degradation rates it can be concluded that the substance, while being in principal not recalcitrant to biodegradability, requires a specific experimental design taking into account its significant volatility as well as limited water solubility (100-116 mg/L in pure water, i.e. lower in test medium that contains salts). The potential for biodegradability is supported by inherent test biodegradation, and ability of isolated strains to grow on AMS as sole source of carbon and energy, as shown hereafter.

Alpha-methylstyrene proved to be inherently biodegradable in a study conducted according to OECD Guideline 302 C (Modified MITI Test (II)) using municipal activated sludge as inoculum. A biodegradation rate of 56% was determined after 28 d incubation by measurement of oxygen consumption. Based on the results and the validity criteria of the guideline the test substance can be assumed to be not inhibitory to the inoculum, providing evidence of biodegradability up to 70 % in 28 days, as shown in one of test duplicates. From a long lag phase (minimum 10 days) in the other duplicate it can be concluded that biodegradation of the AMS-molecule requires the presence of specific bacterial species that may be relatively infrequent, or as seen in other tests, that actual substance concentrations achieved were irregular in different replicates. In other tests it was shown that various bacterial strains, mostly isolated from contaminated sites, were able to grow on alpha-methylstyrene and use the test substance as sole carbon and energy source.

On another hand, the ready biodegradability of alpha-methylstyrene would be supported by the structure analogue substances styrene, acetophenone and cumene biodegradation results.

Styrene is structurally closely related to alpha-methylstyrene, the only difference is the presence of a methyl group in the latter. Therefore, it is expected that the same enzyme causes the initial step of biodegradation. When the methyl group is removed, the same metabolism pathway is expected for both styrene and alpha-methylstyrene. As revealed in several valid standard or near-standard tests, styrene proved to be readily biodegradable under aerobic conditions meeting the 10-day window criterion.

Acetophenone is structurally closely related to alpha-methylstyrene, the only difference is the presence of an oxygen atom instead of the methylene group. This gives acetophenone lower volatility and higher water solubility., Acetophenone was shown to be readily biodegradable (64,7 % after 14 d in OECD 301C), and this was confirmed by water simulation test giving a DT50 from 6 to 8 days.

Cumene is structurally closely related to alpha-methylstyrene, the only difference is a double bond in the side chain in the latter. Cumene was shown to undergo >60 % biodegradation after 10 d in a test according to US-APHA guideline.

In Annex IX of Regulation (EC) No 1907/2006, it is laid down that biodegradation simulation testing in surface water and/or sediment as well as soil shall be proposed by the registrant if the chemical safety assessment according to Annex I indicates the need to investigate further the degradation of the substance and its degradation products in appropriate media. Experimental testing need not be conducted if direct and indirect exposure of surface water, sediment and/ or soil is unlikely or technically not feasible. Alpha-methylstyrene is a highly volatile substance, for which the atmosphere is considered to be the dominant environmental target compartment, whereas hydrosphere and terrestrial compartment are of minor importance. This is underlined by monitoring data from waste water treatment plants (wwtp) in Germany as well as from surface water, sediment and air in Japan, where the substance was detected in none of more than 100 surface water, bottom sediments and wwtp samples while it was found in 20 out of 26 air samples (see below). Moreover, biodegradation testing in surface water, sediment and/or soil according to generally accepted OECD standard guidelines is technically not feasible. Due to its high volatility alpha-methylstyrene clearly does not fall into the applicability domain of either of the OECD methods. Therefore, according to Annex IX of Regulation (EC) No 1907/2006, biodegradation simulation tests in surface water, sediment and/or soil are considered to be scientifically unjustified for alpha-methylstyrene.

In a bioaccumulation study conducted according to OECD Guideline 305 C, fish were exposed to alpha-methylstyrene concentrations of 0.03 and 0.3 mg/L at 25°C for 56 d under flow through conditions. BCF values of 12 -113 (0.03 mg/L) and 15 -140 (0.3 mg/L) were determined. A BCF= 95 was calculated via EPIWIN. These results indicate a low potential for accumulation of alpha-methylstyrene in fish.

The soil adsorption coefficient of alpha-methylstyrene was investigated in a study conducted according to OECD Guideline 121 and EU Method C.19. The soil adsorption coefficient log Koc was determined to be 2.8 (Koc=692). The calculation (EPIWIN v3.11, PCKOC v1.66) of the soil sorption coefficient Koc of alpha-methylstyrene yielded a Koc=817 (log Koc=2.9). Based on the results and taking into account the classification scheme of Blume and Ahlsdorf (1993), alpha-methylstyrene can be regarded as a substance with high sorption potential onto soil organic matter. However, it has to be underlined again that environmental distribution of alpha-methylstyrene according to the fugacity model Mackay Level I v3.00 reveals that in the equilibrium state the substance is distributed to 98.4 % into the atmosphere (cf. chapter 5.4.3).

Alpha-methylstyrene was not detected in the sludge from the wastewater treatment plant of the Emscher Genossenschaft receiving the wastewater of INEOS Phenol (one sample collected 2010; limit of determination: 0.5 mg/kg dw). Alpha-methylstyrene was monitored in Japan in surface water, sediment and air in Japan during the years 1977-2005. Whereas the substance was detected in none of more than 100 samples from surface water (detection limit: 0.009 - 4 µg/L) or bottom sediment (detection limit: 0.0007 - 0.01 µg/g dw), it was found in 20 out of 26 air samples (detection limit: 1.9 ppb).