7a-ethyldihydro-1H,3H,5H-oxazolo[3,4-c]oxazole

Toxicity to aquatic algae and cyanobacteria

Description of key information

Two algal growth inhibition toxicity tests were conducted with EDHO according the standard test guidelines.   One test was conducted with the freshwater green alga, Pseudokirchneriella subcapitata, and the second test was conducted with the marine diatom, Skeletonema costatum.  
The most recent study, the marine diatom test with Skeletonema costatum, was conducted under GLP with analytical verification of exposure concentrations.  The results were based on measured concentrations at time 0 to accurately reflect the toxicity of the product, including the parent EDHO and any metaboites that formed since renewal was not possible for this test type.   Concentrations of parent EDHO were not quantifiable at the conclusion of the test (96 hours).  The test was conducted under the OPPTS 850.5400 (Public Draft) (1996) guideline and the 96-hour endpoint for the marine diatom is the primary regulatory endpoint (this species is not an OECD 201 species).  The ErC50 based on the growth rate endpoint at 96 hours was 2.09 mg product/L and the NOEC was 1.34 mg product/L.
The second algal test was conducted in 2002 with the freshwater green algal species, Pseudokirchneriella subcapitata, according to the OECD 201 guideline.   This test was also conducted under GLP with analytical verification.  For this test, the results were presented based on time-weighted average (TWA) rather initial concentration.  The 72-hour ErC50 based on the growth rate endpoint was 1.08 mg EDHO/L TWA and the NOEC was 0.513 mg EDHO/L TWA.

Key value for chemical safety assessment

EC50 for freshwater algae:

1.08 mg/L

EC50 for marine water algae:

2.09 mg/L

EC10 or NOEC for freshwater algae:

0.513 mg/L

EC10 or NOEC for marine water algae:

1.34 mg/L

Additional information

Two algal growth inhibition toxicity tests were conducted with EDHO according the standard test guidelines.  One test was conducted with the freshwater green alga, Pseudokirchneriella subcapitata, and the second test was conducted with the marine diatom, Skeletonema costatum.

 

Results from both studies were similar although one (the marine diatom) was based on initial measured concentrations and the second (the freshwater green alga) was based on time-weighted average concentrations. The results from the Pseudokirchneriella subcapitata growth inhibition test determined a 72-hour ErC50 based on the growth rate endpoint of 1.08 mg EDHO/L TWA and a NOEC of 0.513 mg EDHO/L TWA. The results from the Skeletonema costatum growth inhibition test determined a 96-hour ErC50 based on the growth rate endpoint of 2.09 mg product/L and a NOEC of 1.34 mg product/L.

 

Since no renewal is possible with algal growth inhibition tests and the EHDO is susceptible to hydrolysis and degrades fairly rapidly, it is probably more realistic to present the data based on the initial measured concentrations because the toxicity observed will be a function of the EHDO plus the metabolites rather than the parent EDHO alone. Since the green algal test used time-weighted average, if the concentrations translated to initial measured concentrations they would be higher than reported. For the purposes of this assessment and to provide a more conservative approach, the freshwater green alga endpoints will be considered as the most sensitive species and used in the derivation of PNEC values.