The Environment Agency’s environmental risk assessment for octamethylcyclotetrasiloxane (D4) is based on the methods outlined in the European Union (EU) Technical Guidance Document (TGD) for the risk assessment of new and existing chemicals. The persistence, bioaccumulative, and toxic (PBT) status is assessed, and a ‘quantitative’ risk assessment made by comparison of exposure with effects. Persistent, bioaccumulative, and toxic status assessment The persistence of D4 in sediment means it potentially meets the criteria for a PBT substance. It is unlikely to meet the persistent organic pollutants (POPs) screening criteria in long-range transport. Laboratory studies indicate that D4 is not readily biodegradable in aquatic systems. However, it is difficult to interpret some of the results because of the rapid loss of D4 through volatilisation, but the final products of the hydrolysis of D4 are not thought to have PBT properties. Based on the rapid hydrolysis, D4 is not persistent or very persistent in surface water. However, the persistence in sediment is not as clear. Although D4 is highly volatile and degrades relatively rapidly in surface water by hydrolysis, it has a high log octanol–water partition coefficient (Kow) value and so it is also expected to adsorb onto sediment. Very limited data are currently available as to the persistence of D4 adsorbed onto sediment, but they suggest that the half-life may approach 120 days at room temperature. Thus, D4 could potentially meet the criteria for persistence in sediment. Although volatilisation from water is likely to be the dominant removal mechanism of D4 from aquatic systems, it is detected in some sediments. However, the high rate of volatilisation is likely to limit the amount of D4 that ultimately reaches the sediment compartment. The bioconcentration factor (BCF) for D4 in fish is 12,400 l/kg, and so it meets the very bioaccumulative criterion. D4 has a long-term fish no-observed-effect concentration (NOEC) 4.4 μg/l, a fish NOEC of 4.4 μg/l from a 14-day prolonged acute toxicity study, and a long-term NOEC of 7.9 μg/l from a Daphnia magna reproduction study. In addition, it is classified as a category 3 reprotoxicant. Therefore D4 meets the toxic criterion. The overall conclusions of the PBT assessment are that D4 potentially meets the criteria for a PBT substance when the persistence in sediment is considered, but this is based on a poorly reported preliminary study. However, even though the amount of D4 that reaches the sediment compartment is likely to be limited, further experimental and modelling work to evaluate its persistence in sediment is in progress. Thus the PBT assessment should be revisited once the results are available.