Loose fibers from functional clothing, discarded PET bottles, or abandoned packaging: When such plastic materials enter the environment, they often persist there for a long time. UV radiation, precipitation, and ozone cause them to become brittle – and over time, mechanical forces break them down into increasingly smaller particles. Swirled up by the wind, they are carried further through the atmosphere. Until now, it was unclear how much microplastic ends up back in Swiss soil and water bodies through this process.
Researchers at Empa, together with colleagues from Eawag and Agroscope, have now systematically documented this for the first time. According to the study, which was supported by the Federal Office for the Environment (FOEN), approximately 219 tons of microplastics settle in Switzerland each year in areas below 2,000 meters in elevation. “Sensitive” areas are particularly relevant here: The researchers estimate that about 78 tons settle on agricultural land and about 10 tons settle directly into water bodies each year.

Microplastics can also be found in rural areas
Measurements were taken over the course of an entire year at five locations: Zurich, Dübendorf, Magadino, Payerne, and on Mount Chaumont in the Jura Mountains. Each month, the researchers collected samples of wet and dry deposition – that is, particles captured and washed out of the air by rain and snow, and those that settle due to gravity. The researchers analyzed microplastic particles with diameters ranging from 20 to 215 micrometers. At the urban site in Zurich, they found an average deposition of 881 particles per square meter per day. At the other sites, the values – ranging from 249 to 331 particles – were significantly lower and relatively similar.
It comes as little surprise that Zurich has more than twice the level of contamination. Dense population, traffic, and plastic products in outdoor spaces all contribute to this. What was more unexpected for the team led by Empa researcher Christoph Hüglin, however, was how similar the values were outside big cities. “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.” The levels from Dübendorf, Magadino, Payerne, and Chaumont are comparable. This suggests that microplastics in the atmosphere are not just an urban phenomenon but also reach less densely populated regions.

Concrete figures for a hard-to-quantify environmental problem
Previous studies had already shown that plastic particles are transported through the atmosphere. According to Hüglin, however, many results are difficult to compare because they use different methods and size ranges. “We wanted to collect data for Switzerland and apply an analytical method that delivers the most reliable results possible.” To do this, the team used the joint particle laboratory of Eawag, Empa, and Agroscope. There, the collected samples were prepared, applied to a filter, and then analyzed using imaging infrared microspectroscopy. Instead of examining only individual suspicious particles, the researchers developed a method to automatically scan randomly selected filter areas point by point. This made it possible to determine which particles were actually made of plastic and what type of plastic they were.
The researchers most frequently found polyethylene terephthalate (PET: 31 percent), polyethylene (PE: 26 percent), and polypropylene (PP: 21 percent). These plastic particles have various sources: Polyethylene and polypropylene are among the most widely used plastics and are found, for example, in packaging, films, containers, and everyday products. The high proportion of PET is particularly striking. This plastic is commonly found in beverage bottles, but is also present in polyester fibers for textiles. According to the researchers, textile fibers could therefore also contribute significantly to microplastic pollution in the atmosphere. However, the study cannot determine the specific products from which the individual particles originate. Emissions from traffic must also be clearly distinguished: While tire wear is generally classified as microplastics, it was analyzed in a separate, ongoing study for analytical reasons.

Back to the Earth's surface, wet or dry
Microplastics return from the atmosphere to the Earth’s surface in two ways: The particles are either washed out by rain or snow, or they settle during dry weather. “Precipitation is a very efficient process for cleansing the atmosphere,” explains Hüglin. In Switzerland, however, dry deposition accounts for the larger share – about 60 percent of the mass. Both pathways contribute to particles reaching the ground, plants, or bodies of water directly. For Swiss water bodies, the researchers estimate direct atmospheric input at around 10 tons per year. By comparison, a parallel Eawag study on microplastics from Swiss wastewater treatment plants estimates the input via treated wastewater at only about five tons annually.
However, when measured against the total mass of all airborne particles – including desert dust, soot and combustion residues, and biological particles – microplastics account for only a very small proportion: between 0.02 and 0.12 percent. Over large areas, however, the constant deposition leads to considerable quantities that do not degrade at all or degrade only very slowly. Much remains unknown about the environmental and health impacts of microplastics. What is clear, however, is that plastics are widespread and that the finest plastic particles do not simply dissolve in the air, on land, or in water. “Plastic is a very good material for many applications,” says Hüglin. “But in accordance with the precautionary principle, we should avoid littering, use plastics only where necessary, and close material cycles as much as possible.”