Assessing the Aquatic Risks of the Cyclic Volatile Methyl Siloxane D4

Assessing the ecological risks of the widely used cyclic volatile methyl siloxane D4 (octamethylcyclotetrasiloxane, CAS Registry Number 556-67-2) to aquatic systems is difficult because of its high volatility and low water solubility, but the potential for long distance atmospheric transport and persistence in the sediments has placed D4 under intense regulatory scrutiny. This paper explores the difficulties inherent in determining the toxicity of D4 to aquatic species and reveals the increased sensitivity of aquatic species tested within artificially closed systems compared to that of similar tests conducted in open systems that allow natural volatilization to occur. The concepts of narcosis mode of action and chemical activity explain the apparent lack of toxicity of D4 to aquatic species under environmentally realistic conditions. Discharge levels for the past 30 years during which D4 has been in use have produced field-measured concentrations that pose negligible risk to aquatic organisms.