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Downstream of the drain
The route a synthetic fibre takes once it leaves the house, from the wastewater plant to rain falling on a national park.
0 to 100 m
It starts at the kerb. Tyre and road wear is among the largest single sources of microplastic anywhere, and it begins within a few metres of a road.
0.81 kg of tyre wear shed per person per year, global average
Kole and colleagues pulled the scattered tyre wear literature into one per capita estimate. The global average is 0.81 kilograms a year, with national figures running from 0.23 to 4.7. Brake wear adds roughly another 8 percent on top of the car tyre figure.
What changes it. Hours on the road are the part a household sets. Vehicle weight and road surface set the rest.
Sources: Kole 2017
About 90 percent of the larger airborne particles beside a busy road, by volume, came from traffic abrasion
Sommer's team set passive samplers beside busy roads in industrial, farming and urban settings. Single particle analysis of more than 500 particles put traffic abrasion at around 90 percent of the larger airborne particles by volume. Most of those particles were composites of tyre, road and brake material.
What changes it. Driving speed, traffic flow and the mix of vehicles changed the mix of particles from road to road.
Sources: Sommer 2018
2 to 355 fibres landing on every square metre of Paris each day
Dris and colleagues left steel funnels open on an urban and a suburban site in Paris and collected the fallout. Deposition ran between 2 and 355 fibres per square metre per day, and the urban site was consistently higher. About 29 percent of the fibres were synthetic or a mix of synthetic and natural material.
What changes it. The urban site measured higher than the suburban one throughout, which puts density of the surrounding city behind the gap.
Sources: Dris 2016
575 to 1,008 microplastics deposited per square metre of central London each day
Wright's team measured atmospheric deposition on a roof in central London. Every sample held microplastic, at 575 to 1,008 particles per square metre per day. Fibres made up 92 percent of the count and fifteen different petrochemical polymers were identified.
What changes it. Deposition tracked the wind, and the study traced fibrous and non-fibrous particles back to different parts of the city.
Sources: Wright 2020
1.1 to 24.6 particles per litre of urban stormwater runoff
Werbowski and colleagues sampled wet weather runoff from twelve watersheds around San Francisco Bay. Every sample carried anthropogenic particles, between 1.1 and 24.6 per litre, well above the plant effluent draining to the same bay. Fibres and black rubbery fragments, most likely tyre and road wear, were about 85 percent of the total.
What sets the number. A rain garden sampled in the same study caught 96 percent of the debris on average and all of the black rubbery fragments.
Sources: Werbowski 2021
How much of a tyre's wear goes into the air rather than staying on the road. The split between the coarse fraction that settles at the kerb and the fine fraction that travels is modelled, not counted.
1 to 1,000 km
The gutter runs to a treatment works, the works sends its solids to a field, and the field drains to a river. Each step moves what came before somewhere new, and most of it ends up in the sludge.
65 million particles leaving one wastewater plant every day
Murphy's team sampled a secondary plant serving about 650,000 people at every stage of treatment. Influent carried 15.7 particles per litre and final effluent 0.25, a drop of 98.41 percent. At that plant's volume the fraction left over still works out at about 65 million particles a day.
What sets the number. Nothing here is a household setting. What does not leave in the water leaves in the sludge.
Sources: Murphy 2016
1.1, then 3.5 particles per gram of dry soil, after one and after five sludge applications
Corradini and colleagues matched 31 farm fields to ten years of sludge application records. Median counts climbed from 1.1 particles per gram of dry soil after one application to 3.5 after five. The sludge itself held 18 to 41 particles per gram, and 90 percent of those were fibres.
What sets the number. Repetition is the variable here. Counts tracked how many times a field had been dressed, not how long ago.
Sources: Corradini 2019
80, then 1,076 particles per kilogram of soil, after 5 and after 24 years of plastic mulching
Huang's team analysed 384 soil samples across 19 Chinese provinces, then went back to sites that had been mulched for decades. Counts were 80.3 particles per kilogram after five years, 308 after fifteen, and 1,075.6 after twenty four. Infrared analysis matched the particles to the films themselves.
What changes it. How much film a region uses tracked the residue left in its soils, so consumption drives the number rather than time alone.
Sources: Huang 2020
90 percent of 29 Swiss floodplain soils contained microplastic
Scheurer and Bigalke sampled 29 floodplains in Swiss nature reserves, drawing on catchments that cover 53 percent of the country. Nine in ten held microplastic, and concentrations rose with the population of the catchment upstream. Particles also turned up in remote, unsettled high mountain sites.
What sets the number. Small particles dominated the counts, which the authors read as delivery by wind rather than by water alone.
Sources: Scheurer 2018
1.15 to 2.41 million tonnes of plastic carried from rivers to the ocean each year
Lebreton's team modelled river inputs from waste management, population density and hydrology, then calibrated against published measurements. The estimate was 1.15 to 2.41 million tonnes a year, and twenty rivers, mostly in Asia, carried 67 percent of it. Jambeck and colleagues had earlier put land-based input at 4.8 to 12.7 million tonnes from 192 coastal countries in 2010.
What changes it. National waste handling is the biggest term in both models, which is why the same discarded bottle reaches a river in one country and a landfill in another.
Sources: Lebreton 2017 Jambeck 2015
More than 1,000 rivers account for 80 percent of riverine plastic reaching the ocean
Meijer and colleagues rebuilt the river model at higher resolution, adding land use, wind, rainfall and the river network itself. Emissions turned out to be spread across up to a hundred times more rivers than earlier work had suggested. More than a thousand rivers carry 80 percent of the annual total, and small urban rivers are among the most polluting.
What sets the number. This is a model, not a set of measurements. It changes where the plastic comes from, not how much there is.
Sources: Meijer 2021
How much of what lands on a field stays on it. Soil counts are snapshots. The share that blows away, washes off, or works down the profile has never been followed through a full year.
10,000 km, and back
The rivers empty and the particles spread across an ocean. Some of it sinks, some of it is eaten, and some of it blows back over the land.
5.25 trillion plastic particles afloat at the ocean surface, weighing 268,940 tonnes
Eriksen's team ran 680 surface net tows and 891 visual transects across all five subtropical gyres between 2007 and 2013. Their calibrated model put a minimum of 5.25 trillion particles at the surface, weighing 268,940 tonnes. Lebreton's later survey of the North Pacific accumulation zone counted 1.8 trillion pieces spread over 1.6 million square kilometres.
What sets the number. Both figures describe the surface only. Microplastics were 94 percent of the pieces in the Pacific zone but 8 percent of its mass.
Sources: Eriksen 2014 Lebreton 2018
4 orders of magnitude more microplastic per unit volume in deep-sea sediment than in contaminated surface water
Woodall's team sampled deep-sea sediment from the Atlantic, the Mediterranean and the Indian Ocean. Microplastic fibres were up to four orders of magnitude more abundant per unit volume than in contaminated surface water. The authors read the sea floor as the likely resting place for the fraction that surface surveys cannot account for.
What sets the number. This is a sink rather than a source, and what settles there is unlikely to come back to the surface.
Sources: Woodall 2014
0 to 1,674 particles per kilogram of sea salt, against 0 to 148 in rock salt
Kim's team analysed 39 commercial salt brands from six continents, 28 of them sea salts. Sea salt ran from 0 to 1,674 particles per kilogram, rock salt from 0 to 148, and lake salt from 28 to 462. Counts in unrefined sea salt correlated with modelled plastic emissions from the rivers nearby.
What changes it. Where the salt was made mattered more than the grade, and the Asian sea salts were highest in this set.
Sources: Kim 2018
1 in 4 individual fish sold for food carried man-made debris in the gut
Rochman's team bought fish and shellfish from markets in Makassar and in California and opened the digestive tracts. Debris turned up in 28 percent of individual fish in Indonesia and 25 percent in the United States. Everything recovered in Indonesia was plastic, while the American samples were mostly textile fibres.
What changes it. The study looked only in the digestive tract, which is not eaten with a gutted finfish.
Sources: Rochman 2015
0.36 to 0.47 particles per gram of mussel and oyster flesh at the point of eating
Van Cauwenberghe and Janssen measured farmed blue mussels and Pacific oysters at the point people would eat them. Mussels held 0.36 particles per gram of wet flesh and oysters 0.47. For European shellfish eaters at the top of the range that works out at roughly 11,000 particles a year.
What changes it. Bivalves are eaten whole, gut included, which is why their counts read higher per gram than finfish.
Sources: Van Cauwenberghe 2014
Up to 14,400 particles per litre of Arctic snow, and up to 154,000 in European snow
Bergmann's team took snow from ice floes in the Fram Strait and from sites in the Swiss Alps, Bremen and Bavaria. Microplastic showed up in 20 of 21 samples, at 0 to 14,400 particles per litre in the Arctic and 190 to 154,000 in Europe. Allen's team, sampling a remote Pyrenees catchment, logged 249 fragments, 73 films and 44 fibres per square metre a day.
What sets the number. Air mass analysis in the Pyrenees traced particles across up to 95 kilometres, so remote does not mean untouched.
Sources: Bergmann 2019 Allen 2019
2.9 to 9.6 particles per cubic metre of marine air on the French Atlantic coast
Allen's team sampled the air just above the sea on the French Atlantic coast during onshore winds, blowing in from the sea, and offshore winds, blowing out to it. Onshore air averaged 2.9 particles per cubic metre and offshore air 9.6, identified by Raman spectroscopy. Bubble bursting and wave action appear to throw particles back out of the sea along with the sea salt.
What changes it. The global figure the authors extrapolate, 136,000 tonnes a year blowing ashore, rests on a pilot study with a small sample.
Sources: Allen 2020
The size of the return trip. Spray, rain and snow clearly carry particles back over land, but nobody has closed the budget between what leaves a river and what comes home on the wind.