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PBT assessment

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PBT assessment: overall result

PBT status:
the substance is not PBT / vPvB

Persistence Assessment

The persistence assessment has been done using the following data.


Ready biodegradability study: the substance is not considered ready biodegradable under the condition applied in a Closed Bottle test, having not reached the pass level (60% biodegradation) in the 10-day window. However the substance showed a maximum value of 2,7 % of biodegradation under this period leading to consider that some initial biotic degradation started during the test.


Hydrolysis versus pH:

the substance is not considered to hydrolyse. A fully experimental test was not possible due to the lack of a suitable analytical method.

The persistence in the water compartment cannot be totally excluded on the basis of the over-mentioned criteria even if a preliminary biodegradation has been observed. No degradation products are expected as no hydrolytic behaviour has been recorded experimentally.



Methods for the experimental measurement of adsorption/desorption property for this substance are technically not feasible. On the basis of the very low water solubility and its chemical nature, the substance is expected to have a high ability to absorb to soil. Estimated value of Log Koc (ca. 9) supports the qualitative evaluation of the adsorption/desorption property.



A theoretical assessment of the equilibrium partitioning coefficient air-water was not possible and not reliable on the basis of lack of the vapour pressure results.

The partitioning in the atmospheric compartment is deemed negligible on the basis of the physico-chemical properties and the very low volatilisation capability.

No direct half-life values are available at this level of testing and therefore no direct comparison with P criteria described in annex XIII and Chapter R11 of the Guidance on Information requirements and Chemical Safety assessment is possible.

Furthermore no environmental monitoring data are available.


On the basis of the previous discussion the substance is considered to be Persistent/very Persistent.

 Bioaccumulation Assessment

The standard bioaccumulation assessment was not performed for the following reason:

Partition Coefficient Log Po/w test: not available as no suitable analytical method was available due to the peculiar chemical structure of the substance its unknown composition

Bioaccumulation test (BCF data)or other accumulation. and/or toxico-kinetics studies are not available as no suitable analytical method is available due to the peculiar chemical structure of the substance and its unknown composition.

Therefore neither direct comparison with Annex XIII criteria can be done with the available data nor screening criteria for B or vB can be assigned.

On the other end the octanol solubility test performed gave a value of > 30 mg/ml suggesting a good affinity to lipids that may suggest a possible bioaccumulation in organisms. In order to clarify the bioaccumulation potential of this substance the “molecular size and molecular weight approach” was here followed as also suggested by related guidance on PBT assessment.


A molecular modelling calculation was therefore performed applied on the following hypothetical worst case chemical structure. The substance is an UVCB one, a complex reaction product, then its exact composition is unknown. The structure below is the smallest molecule arising from the chemical synthesis, then it represents the worst case for the assessment of the bioaccumulation potential.

A calculation of the molecular length and diameter has been done by Prof. Alessia Bacchi, Department of Chemistry,,.

Two possible isomers have been considered: cis/trans referring to the cis/trans configuration of the central maleic/fumaric mojety as showed in the structure. In each case the most extended conformation has been built as a starting model in order to give an estimate of the maximum possible molecular length. The energies of the two structures have been minimized by molecular mechanics using the Universal Force Field.Molecular dimensions (length and diameter) has been measured for the two minimized structures as distances between the two extreme atoms along the directions of maximum length and along the diameter in the central section of the molecule.

The following table shows the obtained results:




Length of the substance

(MML=Maximum Molecular Length) (nm)



Average maximum diameter

(Dmax aver) (nm)



Molecular weight



These values were used in the Bioaccumulation assessment following criteria set down by the PBT Assessment guidance (Chapter R11).

First of all the Molecular Weight (MW) of813.25is greater than the threshold level of 700 given in the guidance and therefore the substance is not considered a vB substance, as it is generally consider that it’s unlikely to have a BCF > 5000. It is, in fact, accepted (EU TGD, EC 2003) that molecules with a MW higher than 700 are less likely to be absorbed and bioconcentrate. It is as well accepted that the MW alone is not sufficient to guarantee a complete B assessment and a weigh of evidence approach using some other indicators would be more appropriate. Such indicators are normally calculated from molecular modelling approaches and information on molecular size can strengthen the evidence for a limited bioaccumulation potential of the substance.

In our case further two indicators are available could be used to build up a good rationale for the bioaccumulation assessment: length and diameter of the molecule. The diameter of the molecule is here not of help for the fact that the structure is distributed along a chain of atoms.


The length of the molecule is indeed very indicative. The length (5.7 – 6.0) is largely exceeding the value of 4.3 nm given by the guidance

It is well know and experimented that molecules having a length greater that 4.3 nm would not pass biological membranes at all, either in the gill of in the gut, based on a series of bioaccumulation and bioconcentration studies with linear and cyclic polidimethylsiloxanes (PDMS or silicones) varying in chain length. The passage of these molecules through biological membranes is very unlikely since it would mean that the entire interior of the lipid membrane would be disturbed. Furthermore Molecular Weight did not explain alone the reduced uptake since one of the substance with MW of 1.050 was found in the fish. Additionally the cross diameter itself could not explain the reduce uptake since those were smaller or equal to those of the substances that did bioaccumulate strongly in such studies. Therefore the main indicator for bioaccumulation is the length of the substance; better if associated with Molecular Weight.

The assumption that the substance under this registration is not a bioccumulative one is also supported by the fact that it is consider not to be bioavailable in the toxicity studies. In the combined OECD 422 study the substance did not induce any systemic effects both for toxicity and reproductive end-points after gastric gavage. The adverse effects seen in lungs is considered a local effect possibly consequence of an aspiration pneumonitis with equivocal mechanism of action.


On the basis of the previous discussion the substance is not considered to bioaccumulate in the aquatic organism as it is not able to pass the biological membranes.


Toxicity Assessment

The toxicity assessment has been performed using the following data/information:

a) evidence of classification from mammalians studies

b) classification for carcinogenesis, mutagenesis or reproductive toxicity

c) Acute eco-toxicological studies performed at the solubility levels


a) there is no evidence of classification from mammals studies as T or Xn R48 end-points

b) the substance is not a CMR

c) acute effects and classification

- Acute toxicity in fish :       LC50 > 100 mg/L, NOEC > 100 mg/L

- Acute toxicity in daphnia : 48 h EC50 > 100 mg/L; 48 h NOEC > 100 mg/L,

- Acute toxicity in algae :     EC 50 > 100 mg/L (mass and growth); NOEC 50 > 100 mg/L (mass and growth)


As already reported, the eco-toxicological acute studies were performed using the nominal concentration as no suitable analytical method was available and applicable to such studies. It is here assumed that the environmental species used in such studies (fish, daphnia and algae) were exposed during the studies to a real dose of 2.17 mg/L which is the value calculated in the water solubility study. Such concentration is considered realistic as is the higher solubility concentration that can be present in the environment in case of release of the substance in it. Therefore the previous results can be converted as follows:


- Acute toxicity in fish :       LC50 > 2.17 mg/L, NOEC > 2.17 mg/L

- Acute toxicity in daphnia : 48 h EC50 > 2.17 mg/L, 48 h NOEC > 2.17 mg/L,

- Acute toxicity in algae :     EC 50 > 2.17 mg/L (mass and growth); NOEC 50 > 2.17 mg/L (mass and growth)


All acute data from eco-toxicological experimental studies are not lower than 0.01 mg/L but largely higher (the lowest is 48 h EC50 > 2.17 mg/L) not leading to any classification due to testing at the solubility value and lack of any effects at such concentration.

In conclusion, base on the over-mentioned, data the substance is not considered to cover the criteria of the T assessment.


1.1.2                       Summary and overall Conclusions on PBT or vPvB Properties

The assessment of PBT/vPvB properties has been done on the basis of the data available from the testing programme as well as from some calculated approaches in order to evaluate the Bioccumulation potential and referring to the principles laid down in the “Guidance on information requirements and chemical safety assessment Parc C: PBT Assessment and Chapter 11: PBT Assessment”.

An overall estimation of the available data/information discussed in previous paragraphs lead to consider that they do not allow a direct comparison with all the criteria in Annex XIII but nevertheless indicates that the substance do not have these properties and the substance is not considered to be a PBT or vPvB. Additionally the substance is not absorbed in the organism as it was seen in the combined OECD 422 study (Toxicity and reproductive end-points) and it hence not considered to biomagnify along the food chain on the basis of secondary poisoning argumentation as discussed in the specific paragraph.

Conclusion : the substance is not a PBT or vPvB chemical.