Assessment of the long-range environmental transport potential of cyclic volatile methylsiloxanes: a critical review

Long-range environmental transport potential (LRETP) is a key scientific factor underlying recent discussions about whether certain organic chemicals may warrant consideration as persistent organic pollutants (POPs). Here, we review the evidence base for the LRETP of cyclic volatile methyl siloxanes (cVMSs). The majority of cVMSs are released into the air, with smaller fractions entering surface water (via wastewater) and soil (via sewage sludge). Transport distances of cVMSs in water are likely to be low due to high combined loss rates for volatilisation, hydrolysis and net deposition of sediment-associated material. In contrast, the long-range atmospheric transport potential (LRATP) of cVMSs is likely to be high due to very high volatility and moderate reaction rates, compared with advection. However, the potential for deposition from air to surface media (e.g. in remote regions) is much less certain. Multimedia models suggest that cVMSs are likely to have long characteristic travel distances (CTDs) in air (2500-7800 km), although empirical travel distances (ETDs) are shorter (1000-2500 km) and potential for deposition is low. Field evidence for atmospheric deposition is often confounded by local contamination, analytical artefacts and (in the case of biota) uncertainties in diet, exposure locations and migration paths. The overall weight-of-evidence suggests that deposition from air to surface media in remote regions is very low for cVMSs.