Empa measures 219 tonnes of microplastic falling on Switzerland below 2,000 metres each year

The total covers one size class. The analysis ran on particles between 20 and 215 micrometres across ^[7], so finer material is not counted, and neither is tire wear, which the team pulled into a separate study still under way for analytical reasons ^[19]. Both exclusions can only push the true mass up ^[27].

Farmland takes about 36 percent of the national total, water bodies about 5 percent, and the remaining 131 tonnes a year settles on everything else below 2,000 metres ^[21][20].

What was more unexpected for the team was how similar the values were outside the big cities ^[10]. The Zurich site ran a factor of 2.7 to 3.5 above the other four ^[22], and those four, one of them a ridge in the Jura Mountains, sat within about a third of one another ^[5][28]. "The closer the location is to densely populated areas, the higher the levels we would have expected. But we don't see such a difference," said Christoph Hüglin, the Empa researcher who led the work ^[11]. The published account credits dense population, traffic and plastic products in outdoor spaces for the urban excess ^[25].

That narrow spread is also what makes a national tonnage defensible from five stations. Had the non-urban sites disagreed among themselves, an area-weighted total would be an extrapolation across an unmeasured gradient; instead the gap to bridge is between one city and everywhere else ^[8].

Monthly samples of wet and dry deposition went to the joint particle laboratory of Eawag, Empa and Agroscope, onto filters, and under imaging infrared microspectroscopy ^[6][14]. Instead of picking out particles that looked like plastic, the team scanned randomly selected filter areas point by point ^[15]. Selecting suspicious particles yields identifications; scanning a random area yields a rate, because the denominator is the area scanned and not the analyst's judgement. "We wanted to collect data for Switzerland and apply an analytical method that delivers the most reliable results possible," Hüglin said ^[12]. According to Hüglin, many earlier results are hard to compare because they use different methods and size ranges ^[13].

The three commonest polymers together make up 78 percent of the particles ^[23]. PET led at 31 percent ^[16]. PET is bottle plastic and it is also polyester fibre, so the researchers suggest textile fibres could contribute significantly to atmospheric microplastic ^[17]. The study cannot determine which products the individual particles came from ^[18].