Cyclopentasiloxane – technically decamethylcyclopentasiloxane, or D5 – is a type of silicone. This, and two similar chemicals (D4 and D6) are used in many health and beauty products.

Scientists have previously raised concerns about the potential persistence of D4, D5 and D6 in the environment, and the possibility that they may bioaccumulate. Today, there are still a number of questions about exactly how much of a risk these chemicals pose. However, based on the available evidence, the all three have been identified as substances of very high concern in the European Union.

From 31 January 2020, D4 and D5 will only be permitted at concentrations below 0.1% in wash-off products (e.g. those used in the shower) in the European Union. However, the listing of D4, D5 and D6 as substances of very high concern means that their use could be further controlled in the future. This development has prompted a number of new studies.

One of these studies has looked at the contribution of D5 in leave-on products (like moisturiser) to environmental levels, compared to the contribution from wash-off products. The study uses a model to determine the likely contributions. The model aims to account for variability in the way health and beauty products are used.

Three time periods are considered. In 2010, which is assumed to represent peak use of D5 in wash-off products in the European Union, it is estimated that 762 tonnes of the chemical were washed down European drains. Of this, around 28 tonnes are estimated to have come from leave-on products. This is because the D5 is slowly released to the atmosphere after being applied, and there is only a small amount left when the user washes.

By 2016, use of D5 in wash-off products in the European Union had dropped from 1000 tonnes per year to 50 tonnes per year, but use in leave-on products stayed about the same (13,600 tonnes in 2010 to 11,700 tonnes in 2016). In this year, 36.7 tonnes of D5 were washed down the drain from wash-off products, and 24.1 tonnes were washed down the drain from leave-on products.

Based on this information, the study authors note that phasing out D5 from wash-off products altogether will result in a 97% reduction of emissions on 2010 levels. That said, the acceptability of ongoing emissions – however small – is currently not clear.

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