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EC number: 202-705-0
CAS number: 98-83-9
Reliable results of guideline studies (OECD 301C, OECD 301D, OECD
301F and OECD 302C) investigating the ready and inherent biodegradation
of alpha-methylstyrene are available.
The ready biodegradation of alpha-methylstyrene was investigated
in a study conducted according to OECD Guideline 301 D (Closed Bottle
Test; 1992) and EU Method C.4-E (2008) over a period of 28 days. In
deviation from OECD Guideline 301D a mixed inoculum obtained from the
effluent of a wastewater treatment plant and soil was used in the test.
The biodegradation rate was determined by measurement of oxygen
consumption. Inoculum blank, procedural/functional control with the
reference substance sodium benzoate, and toxicity control using 1.13
mg/L test substance and 3.039 mg/L reference compound were performed.
The functional control reached the pass level >60% after 14 d. In the
toxicity control containing both test and reference item 63 %
biodegradation based on oxygen consumption occurred within 21 d (78 %
after 78 d) thus indicating that the test substance was 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 chosen test conditions. The
part of applied test substance, which might have evaporated from the
test solution, would have been no longer available to the degrading
inoculum, leading to an underestimation of the degradation rate. This is
supported by the higher percentage observed with the lowest nominal
concentration, which suggests 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 (no 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.
From the experimental study results available for AMS on the
endpoint ready biodegradability, no definitive proof of ready
biodegradability has been obtained, when strictly applying the criteria
of the OECD 301 guidelines. However, from the diversity of degradation
rates it can be furthermore 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). This 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
The inherent biodegradation of alpha-methylstyrene was
investigated in a study conducted according to OECD Guideline 302 C
(Modified MITI Test (II)) over a period of 28 days and using municipal
activated sludge as inoculum. The biodegradation rate was determined by
measurement of oxygen consumption. Inoculum blank, procedural/functional
control with the reference substance sodium benzoate, and toxicity
control using 31.8 mg/L test item and 100 mg/L reference compound were
performed. Alpha-methylstyrene proved to be inherently biodegradable
under the test conditions employed (56% biodegradation after 28 d, as
mean of duplicates). The functional control reached the pass level >60%
after 14 d. In the toxicity control containing both test and reference
item ca. 80% biodegradation occurred within 14 d. Based on the results
and the validity criteria of the guideline (>=25% biodegradation based
on ThOD) the test substance can be assumed to be not inhibitory to the
inoculum. Furthermore, this study provides evidence of biodegradability
up to 70 % in 28 days, as shown in one of test duplicates. Obviously, no
molecular configuration is present in the substance that is resistant to
the biodegradation process. On the other hand, the second replicate
shows a long lag phase (minimum 10 days). 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
In other tests it was shown that various bacterial strains, mostly
isolated from contaminated sites, were able to grow on
alpha-methylstyrene and the test substance was used as sole carbon and
energy source: e.g. Bacillus cereus 3, Pseudomonas aeruginosa 21, BS176
and DS13, Pseudomonas convexa S107B1, Pseudomonas putida MST. Several
intermediates were isolated after incubation: 2 -phenyl-2-propen-1-ol, 3
-isopropylcatechol, 1,2 -dihydroxy-3-isopropenyl-3-cyclohexane, 2
-phenylpropenoic acid, and acetophenone.
On another hand, the ready biodegradability of alpha-methylstyrene
is supported by the structure analogue substances styrene, acetophenone
and cumene biodegradation results.
Styrene is structurally closely related toalpha-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 andalpha-methylstyrene.As revealed
in several tests on ready biodegradability, styrene proved to be readily
biodegradable under aerobic conditions. There are sufficient results
from standard or near-standard tests to consider styrene to be readily
biodegradable and meeting the 10-day window criterion.
Acetophenoneis structurally closely related toalpha-methylstyrene.
The only difference is the presence of an oxygen atom instead of the
methylene group, causing 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 toalpha-methylstyrenewith a
double bond in the side chain being the only difference in the
latter.Cumene was shown to undergo >60 % biodegradation after 10 d in a
test according to US-APHA guideline.
Biodegradation in water and sediment (simulation test) /
biodegradation in soil:
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 fugacity model Mackay Level I v3.00
reveals that in the equilibrium state the substance is distributed to
98.4 % into the atmosphere. In air, alpha-methylstyrene is rapidly
degraded by OH-radicals with an estimated half-life of 7.27 hours.
Therefore, it can be concluded that the dominant environmental
target compartment of alpha-methylstyrene is the atmosphere, while
hydrosphere and terrestrial compartment are of minor importance. This 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 or
bottom sediments samples (detection limit: 0.009 - 4 µg/L, resp. 0.0007
- 0.01 µg/g dw), it was found in 20 out of 26 air samples (detection
limit: 1.9 ppb).
Moreover, biodegradation testing in surface water, sediment and/or
soil according to generally accepted OECD standard guidelines is
technically not feasible due to the substance properties of AMS.
Alpha-methylstyrene, due to its Henrys law constant of >258 Pa x m3/mol
(at 25°C), clearly does not fall into the applicability domain of either
of the OECD methods.
Due to these reasons and 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
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