Scientists have found levels of microplastic pollution on beaches around Scapa Flow in Orkney are similar to those in industrialised areas like the Forth and Clyde.
Researchers at Heriot-Watt University in Edinburgh took more than 100 sediment samples from 13 beaches on Hoy and the Orkney mainland.
Some plastic is believed to come from clothes which contain polymer fibres.
These are not caught by washing machine filters or at treatment works.
The micro-particles are thought to have been blown up the east coast.
Although more work is needed, researchers say plastic is carried on complex tidal flows through the Pentland Firth, with Scapa Flow acting like a giant sieve capturing the particles.
Dr Mark Hartl, from Heriot-Watt, said: "The fact that a relatively remote island has similar microplastics levels to some of the UK's most industrialised waterways was unexpected, and points to the ubiquitous nature of microplastics in our water systems.
"We need a baseline for all of the UK's waters, so that we can assess the impact of government policies that aim to reduce marine pollution, such as the microbead ban.
"At present, we only have a patchwork of data from studies in Scotland and comparable North Sea locations."
Heriot-Watt is increasing its focus on the marine environment as part of its Year of the Sea initiative.
Samples for the study were taken by Jenni Kakkonen, a marine biologist at Orkney Islands Council.
She said: "It's important we keep an eye on the seas around us through research of this kind.
"The sites in Scapa Flow are part of Orkney Islands Council's sandy shore monitoring programme and during our annual surveys it was easy to collect the 3cm sand core samples for the microplastics project.
"It came as a surprise to me as well that there was no significant difference between average particle and fibre concentrations found in the Scapa Flow, Clyde and Firth of Forth."
A range of government policies have been announced in recent months to reduce plastic in our seas, including a deposit-return scheme for drinks containers and more recently a ban on plastic-stemmed cotton buds in Scotland.
The plight of marine creatures killed or injured by plastic was highlighted in 2017 by the BBC's Planet Earth II series.