Hydrocarbons, C4, steam-cracker distillate

Endpoint summary

Additional information

As 1,3-butadiene is a gas at standard temperature and pressure, conducting a standard ready biodegradability test is technically difficult and, based on exposure estimates, the test may not be relevant.

An OECD TG 301D closed bottle test (MITI, 2001) reported 0 - 4% biodegradation over 28 days, though we have been unable to obtain a copy of the original report in order to assess its reliability. In addition, various researchers have isolated microorganisms that are capable of metabolising 1,3 -butadiene. One group (Hou et al. 1983) demonstrated that microorganisms can biodegrade 1,3-butadiene under aerobic conditions, however, the study did not follow a standard test guideline for ready biodegradability. This results add to the weight of evidence that 1,3-butadiene has the potential to biodegrade if released to the environment. In the absence of a standard study, a QSAR model has been used to predict the biodegradability and results from the Biowin model indicate that 1,3-butadiene would not be expected to be persistent.

Ready biodegradation of 1,3-butadiene has not been demonstrated and for the purposes of the risk assessment ready biodegradability will not be assumed. This was also the approach followed in the EU RAR for 1,3-butadiene (2002), which concluded that there were insufficient data to determine whether or not 1,3-butadiene would be classed as readily biodegradable. The EU RAR thus assumed that no biodegradation of 1,3-butadiene would occur (a biodegradation rate of 0).

One of the screening criteria for persistence in the PBT assessment is that if a substance is considered to be readily biodegradable the substance is screened as being not P and not vP. For the purposes of this assessment, 1,3- butadiene is not considered to be readily biodegradable. However, EPIWIN predicts rapid volatilisation from water (half life of 45 minutes in a river, 2.9 days in a lake) and degradation of 1,3-butadiene is expected to be rapid once in the atmosphere. According to Atkinson (1985), the atmospheric photodegradation half-life for buta-1,3-diene was estimated as 0.24 days. This value was based on a rate constant derived from a range of experimental results assessed to be valid and the concentration of hydroxyl radicals recommended in the Technical Guidance Document (2003). Taken together these data suggest that 1,3-butadiene will not persist in the environment.

Although measured data are not available, the modelled data indicate that the half life of 1,3-butadiene in water is expected to be much lower than the Annex XIII P criterion of 40 days. Therefore, based on the criteria for persistence stipulated in REACH Annex XIII 1,3-butadiene is not considered to be persistent (P) or very persistent (vP). However we must stress that this is on the basis of a modelled data due to lack of experimental data to fulfil this endpoint requirement.