OCTAMETHYLCYCLOTETRASILOXANE (OMCTS), A CASE-STUDY - SUMMARY AND AQUATIC RISK ASSESSMENT

This paper presents an aquatic ecological risk assessment, including a n aquatic effects assessment, an exposure assessment, and a characteri zation of risk to aquatic organisms. In a number of acute and chronic studies with several aquatic species, octamethylcyclotetrasiloxane (OM CTS) toxicity was observed, but only after extended, continuous exposu res of 7 to 14 d; survival was the only biological end point affected. The symptoms of toxicity observed were highly consistent with narcosi s as the mechanism of action. For all aquatic toxicity testing, the lo west NOEC was 4.4 mu g/L for rainbow trout in both 14-d and 90-d tests . The physicochemical characteristics of OMCTS (low solubility, high K -ow, and very high Henry's law constant) indicate that the water conce ntrations should be low and transient, consistent with results of envi ronmental-fate studies that demonstrate that OMCTS moves rapidly out o f water. The primary route of release of OMCTS to the environment is a ssociated with use in personal-care products, which would potentially result in release to air and water via public sewer systems. Environme ntal-fate models predict low concentrations of OMCTS in air and water. Monitoring studies indicate mean measured concentrations of OMCTS in sewage treatment plant effluents of 0.06 to 0.41 mu g/L. Assuming 50% bioavailability of OMCTS and a dilution factor of three, calculated me an concentrations of dissolved OMCTS in surface waters range from 0.01 to 0.068 mu g/L. Comparison of the lowest no-observed-effect concentr ation (NOEC) from all toxicity testing with this predicted range of su rface water concentrations indicates a conservative margin of safety t o 64 to 444 for aquatic organisms. Rapid volatilization and additional dilution in most aquatic environments will increase this margin of sa fety even further. Based on all available evidence, the risk of OMCTS to aquatic ecosystems is characterized as very low.