Over a five-month period in 2017-2018, an average of 365 tiny bits of plastic settled every day on each square metre of an uninhabited, high-altitude area in the Pyrenees straddling France and Spain, the journal Nature Geoscience has reported.
"It is astounding and worrying that so many particles were found in the Pyrenees field site," said lead author Steve Allen, a doctoral student at the University of Strathclyde in Scotland.
The study focused on micro-plastics mostly between 10 and 150 micro-metres across, including fragments, fibres and sheet-like pieces of film.
By comparison, a human hair is, on average, about 70 micro-metres in width.
"We would never have anticipated that this study would reveal such high levels of micro-plastic deposits," added co-author Gael Le Roux, a researcher at EcoLab in Toulouse, in southwestern France.
Plastic litter has emerged in the last few years as a major environmental problem.
Up to 12 million tonnes of plastics are thought to enter the world's oceans every year, and millions more clog inland waterways and landfills.
