Isoprene

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.04 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.058 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.004 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.2 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.767 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.077 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.553 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Measured ecotoxicity results are available for isoprene for acute toxicity to fish and Daphnia, and algal growth inhibition. The results are taken from GLP-compliant OECD guideline studies (HLS 2004) and are considered to be Klimisch 1 key studies.

Ecotoxicity data have been identified for isoprene from several regulatory sources, including the OECD SIDS, the Japanese government website, the European Joint Research Committee and the US EPA’s High Production Volume program. Experimental data from regulatory sources is included for acute toxicity to fish and Daphnia, chronic toxicity to Daphnia and algal growth inhibition. Although listed as Klimisch 4 because the original studies have not been reviewed, data taken from regulatory sources are considered sufficiently robust to be included in the development of the PNEC.

Data for the chronic toxicity to fish have been derived from the OECD toolbox, which is listed in the Chapter R6 guidance document, QSARs and grouping of chemicals, and is considered to be a reliable source of data. The OECD Toolbox prediction was based on a dataset comprised from the NOEC descriptors, where the estimation method takes the average value from the 5 nearest neighbours. Isoprene is considered to be ‘in domain’ for the OECD toolbox, with the classification of Class 1 (narcosis or baseline toxicity) using the acute aquatic toxicity classification by Verhaar (Modified). The log partition coefficient for isoprene of 2.42-2.58 falls within the specified domain of = 2.01 and = 3.11.

QSAR data also have been included for all of the ecotoxicity endpoints for isoprene. Due to the practical difficulties associated with the ecotoxicity testing of highly volatile substances (i.e. maintaining exposure concentrations), the use of QSAR toxicity estimates is considered an appropriate alternative. The ECOSAR model is a reliable and appropriate QSAR model to apply to isoprene as it is based on a related chemical dataset that calculates the toxicity of neutral organic hydrocarbons. Isoprene falls within the applicability domain of the ECOSAR QSAR model as it is organic with a molecular weight of <1000 g/mol, with log Kow in the range of -3 to 8. The use of QSAR models for the ecotoxicity endpoints is in-line with the approach in the OECD SIDS for isoprene.

The 21 day NOEC for isoprene toxicity to Daphnia of 0.402 mg/L has been taken as the ecotoxicity reference value as it is the lowest result. Although the original study has not been reviewed, the result has been taken from a regulatory source and is considered sufficient reliable for use as the driving value for the PNECs. Furthermore, the result is included in the OECD toolbox, where it is considered to be “high quality”. Comparison of the available data from two separate ecotoxicity testing programs (HLS 2004 and NITE/MITI 2010), both covering acute toxicity to Daphnia magna and algal growth inhibition but with different species for acute toxicity to fish (Oncorhynchus mykiss and Oryzias latipes respectively), indicated that invertebrates showed the most sensitivity to acute isoprene exposure. The available data indicates that this trend in decreasing sensitivity from invertebrates to fish then algae is also expected for chronic toxicity. Therefore derivation of the PNEC for freshwater and marine water on the basis of the chronic Daphnia data is considered to be robust.

The MITI value is considered to represent a realistic worst-case result for the aquatic toxicity of isoprene, with the result providing a conservative indication of aquatic toxicity. The result is lower than the QSAR estimates for isoprene, as well as measured data for structurally similar C6 substances, 1,4-hexadiene and 2-methyl-1-propene. Comparison of these substances to isoprene is considered to be robust on the basis that the substances are structurally similar, have similar behaviour in the environment and similar toxicity to aquatic organisms.1,4-hexadiene is a C6 unsaturated straight chain substance, with a log partition coefficient of 2.9 and a water solubility of 123 mg/L (QSAR estimates). 2-methyl-1-pentene is a C6 unsaturated branched substance, with a log partition coefficient of 3.2 and a water solubility of 78 mg/L (QSAR estimates). These are similar to the physico-chemical properties of isoprene, a C5 unsaturated branched substance, though the water solubilities are lower than for isoprene (642 mg/L) and the log partition coefficients are slightly higher (2.42 for isoprene). The available ecotoxicity data for the C6 substances also indicates that they are expected to have similar aquatic toxicity profiles to isoprene.

The acute Daphnia data for isoprene (48 hour EC50 of 5.77 mg/L, Huntingdon Life Science 2004) is comparable but slightly higher than that of the C6 substances (48 hour EC50 of 4.9 mg/L for 1,4-hexadiene and 3.0 mg/L for 2-methyl-1-pentene, Kelley et al. 2015). The short-term toxicity to algae (72 hour EC50 based on growth rate) is 25 mg/L for 1,4-hexadiene and 12 mg/L for 2-methyl-1-pentene (Sutherland et al. 2015), which is comparable to the result of 15.3 mg/L for isoprene (Huntingdon Life Sciences 2004). The long-term toxicity to algae (72 hour NOEC based on growth rate) is 6.4 mg/L for 1,4-hexadiene and 7.4 mg/L for 2-methyl-1-propene (Sutherland et al. 2015), which is comparable to, but slightly higher than, the result of 6 mg/L for isoprene (Huntingdon Life Sciences 2004). However the data on the long-term toxicity to invertebrates for isoprene (21 day NOEC of 0.402 mg/L, MOE 2010) is an order of magnitude lower than (at least half) that of the C6 substances (7 day NOEC of =2.2 mg/L for 1,4-hexadiene and 1.1 mg/L for 2-methyl-1-pentene, Knarr et al. 2015), indicating that the MOE study provides a worst-case result for this endpoint.

 

The 21 day NOEC for Daphnia of 0.402 mg/L is the lowest result available for isoprene. The value is considered to be protective of the aquatic environment and is taken as the ecotoxicity reference value. Experimental data are available for acute toxicity to fish and Daphnia, chronic toxicity to Daphnia and algal growth inhibition with robust QSAR results for all ecotoxicity endpoints. As robust data are available for all three trophic levels for short- and long-term toxicity and aquatic invertebrates are the most sensitive species for acute and chronic endpoints, the use of an assessment factor of 10 is considered to be robust.

The freshwater PNEC has been derived using the ecotoxicity reference value with an assessment factor of 10. No marine data are available and therefore the marine water PNEC has been derived on the basis of the freshwater results with an assessment factor of 100. The freshwater PNEC is 0.0402 mg/L and the marine water PNEC is 0.00402 mg/L.

The PNECs for sediment, and soil were derived using equilibrium partitioning calculations.

Conclusion on classification

Data are available on the toxicity of isoprene to algae, Daphnia and fish. The lowest reported acute effect concentration is a 48-hour EC50 value of 5.77 mg/L for Daphnia. The lowest reported chronic effect concentration is a 21 day NOEC of 0.402 mg/L for Daphnia.

Isoprene has a harmonised classification of Chronic Category 3 under CLP. As isoprene is not readily biodegradable and on the basis of the available experimental data, isoprene would not be classified for acute hazards but would meet the criteria for classification as Chronic Category 2 under CLP. This has been added as a self-classification to the dossier.