2,6-diisopropylnaphthalene

Endpoint summary

Additional information

General remarks

No aquatic toxicity studies concerning only 2,6-diisopropylnaphthalene (2,6-DIPN) are available. Therefore aquatic studies using Bis(isopropyl)naphthalene (DIPN) were used in a read-across approach in accordance to Regulation (EC) No 1907/2006 Annex XI, 1.5 for assessment of the aquatic toxicity of 2,6-DIPN. Bis(isopropyl)naphthalene (DIPN) is an isomeric mixture, which consists of seven isomers (1,3-, 1,4-, 1,5-, 1,6-, 2,6- and 2,7-DIPN).

Short-term toxicity to fish

Two appropriate studies performed with DIPN on short-term toxicity to fish have been identified. Both of them have shortcomings. Only one reported a LC50 which can be considered to be reliable (Kikushima et al., 2003). But for this study also, validity criteria are not fulfilled (use of vehicle, no information on analytical test concentration measurements).

For only one study the concentration used is in the range of the maximum water solubility of DIPN (Handley/Safepharm, 2003). This study has been selected as key study. It is a limit test with a concentration (0.5 mg/L) slightly above the water solubility of DIPN. Test substance concentrations were measured and demonstrated to decrease within the renewal periods (24 h) to about 0.098 to 0.135 mg/L which corresponds to the approximate level of water solubility (ca. 0.122 mg/L). There was no mortality observed at this concentration. Thus, the LC0 is ca. 0.24 mg/L (averaged geometric means of concentrations at the start and the end of renewal solutions from different time points). The LC50 (96 h) is reported as > 0.5 mg/L.

There were no effects observed at concentrations within the water solubility of DIPN.

Short-term toxicity to aquatic invertebrates

For assessment of the short-term toxicity to aquatic invertebrates, a study with Daphnia magna is available  performed with DIPN. The study from Aniol et al. (2012) used for assessment of acute toxicity towards aquatic invertebrates was performed under GLP conditions according to OECD Guideline 202 using five treatments from 1.0 - 25 mg/L (nominal loadings) prepared as water accommodated fractions. Twenty Daphnia divided into four replicates per treatment and control were tested in none aerated glass beakers. The test was conducted under strictly closed conditions, taking the volatility of the substance into account. A semi-static test design with a medium exchange after 24 hours was used. Due to the very low solubility of DIPN, no analytical determination using TOC was possible. All validity criteria stated in OECD Guideline 202 were fulfilled.

After 48 h the EL50 for DIPN was determined as 1.7 mg/L based on nominal loadings.

Long-term toxicity to aquatic invertebrates

Four tests on long-term toxicity of DIPN to aquatic invertebrates are available, two for the freshwater species Daphnia magna (WaBoLu/BGA, 1990 & Laus 2017) and two for the marine copepods Acartia tonsa (TNO, 1998a & 1998b).

One reproduction test with Daphnia magna was conducted according to OECD 211 (Laus, 2017). The Daphnids were exposed to the substance for 21 days in a semi-static test design. The test solutions were renewed three times per week. Based on Daphnia reproduction a NOEC (21 d) of 0.0118 mg/L (measured, TWA) was determined. This result is confirmed by the findings of a second chronic toxicity test on D. magna, conducted according to OECD 202, part 2 (adopted 1984) (WaBoLu/UBA 1990b). The determined NOEC (21 d) in this study was 0.013 mg/L (nominal).

The studies with the marine copepod Acartia tonsa (TNO, 1998a & 1998b) resulted in NOEC values of approx. 0.05 mg/L (measured) and 0.02 mg/L (measured), respectively.

In all four studies a decrease of test item was noted, which might lead to an underestimation of the toxicity.

Considering that the results of the two Daphnia reproduction studies with the isomeric mixture bis(isopropyl)naphthalene are on the borderline of meeting the T-criterion, two further Daphnia reproduction tests in a closed system under flow-through conditions were requested by ECHA, for the most critical isomers of the isomeric mixture (1,3-DIPN and 1,4-DIPN) with respect to PBT properties. These two studies were performed in order to reduce any volatilisation which was identified as a deficiency in the available studies. In the OECD 211 test with 1,3-DIPN the number of alive offspring produced by adults did not show any statistically significant decrease of reproduction up to the highest test concentration of 40 µg/L compared to the solvent control (Eurofins, 2020a). Based on the properties of the test item, the study was performed in a flow-through test design with closed bottles. The measured content of 1,3-DIPN in the replicates was between 42 and 86% of nominal with a mean of 65% of nominal. The measured content of 1,3-DIPN in the fresh stock solutions was between 116 and 136% of nominal and between 117 and 135% of nominal in the aged stock solutions. It is therefore proven that stock solutions remained stable between renewals. The toxicological endpoints were evaluated using nominal test item concentrations and the actual concentrations (based on the time-weighted arithmetic mean of the exposure concentrations). Therefore, the NOEC (21 d) was set to be ≥ 40.0 µg/L (nominal) equivalent to ≥ 24.4 µg/L (measured), including mortality of adults, reproduction and body length.

In the second test with 1,4-DIPN the number of alive offspring produced by adults showed statistically significant decrease of reproduction in the highest test item concentration of 125 µg/L (nominal) equivalent to 38.8 µg/L (measured) compared to the solvent control (Eurofins, 2020b). This study was performed in a flow-through design with closed vessels as well. The measured content of 1,4-Diisopropylnaphthalene was between 16 and 67% of nominal with a mean of 43% of nominal. The measured content of 1,4-Diisopropylnaphthalene was between 95 and 115% of nominal in the fresh stock solutions and between 101 and 119% of nominal in the aged stock solutions. It is therefore proven that stock solutions remained stable between renewals. The toxicological endpoints were evaluated using nominal test item concentrations and the actual concentrations (based on the time-weighted arithmetic mean of the exposure concentrations).

The NOEC (21 d) was calculated to be 62.5 µg/L (nominal) equivalent to 28.1 µg/L (measured) based on reproduction. Statistically significant effects for mortality were observed at 62.5 and 125 µg/L, therefor the NOEC (21 d) for mortality was calculated to be 31.3 µg/L (nominal) equivalent to 16.3 µg/L (measured).

The results of 1,3- and 1,4-DIPN above indicate that the toxicity is above the threshold value of the T-criterion. Both results do not result in fulfilling the T-criterion. Since both isomers were identified by ECHA as the most critical it is concluded that 2,6-DIPN is of no higher toxicity to aquatic invertebrates.

For hazard and risk-assessment the NOEC (21 d) of 0.0118 mg/L was used, which was determined for the long-term toxicity of the isomeric mixture to Daphnids.

Toxicity to aquatic algae and cyanobacteria

For assessment of the toxicity to aquatic algae and cyanobacteria, one valid study is available for DIPN (Vogel/WaBoLU, 1999). In a freshwater algal growth inhibition test according to DIN 38412 part 9, no growth inhibition effect was observed up to a DIPN test concentration close to its maximum water solubility (ca. 0.15 to 0.19 mg/L). The NOEC (72 h) was ca 0.15 mg/L (measured).

Toxicity to microorganisms

For assessment of the toxicity to microorganisms, three studies are available for DIPN. Due to limited reporting (secondary literature or short communication in tabular form), the reliability is 4. As results are very similar, they are used in a weight of evidence approach.

In two tests (UBA/KBwS 1999a, Yoshioka 1985), freshwater single species have been used (Pseudomonas putida, Tetrahymena pyriformis) in a static test. EC10 (16 h) and EC0 (24 h) after 24 h were > 0.16 mg/L and >0.15 mg/L based on growth inhibition, respectively.

The third test used Vibrio fischeri in marine water measuring luminescence inhibition. The EC50 (15 min) was > 0.1 mg/L.

The substance did not exhibit inhibitory effects on the microorganism tested within its range of water solubility (0.1 to 0.2 mg/L).