Tires & Brakes Are Significant Sources of Microplastics

July 19th, 2020 by Carolyn Fortuna in Clean Technica

Those minuscule particles of plastics produced by car tires and brakes can’t do much damage, right? They’re so, well, tiny, and nearly imperceptible — yet devastating. Did you know that road microplastics produce more than 200,000 tons of minute plastic particles that blow into the oceans every year?

road microplastics

Image retrieved from CalRecycle

Research titled “Atmospheric Transport is a Major Pathway of Microplastics to Remote Regions” was published in the July, 2020 issue of Nature Communications. The study incorporated 2 methods to estimate the amount of fine particles shed by tires and brakes, which included well-established atmospheric circulation models to assess how the particles are blown around the globe.

Tire wear particles (TWPs) and brake wear particles (BWPs) were the focus of the research team’s global simulations of atmospheric transport of microplastic particles. TWPs and BWPs are produced via mechanical abrasion and corrosion, and both can be quantified relatively well. Such microplastic particles of mean size 0.5–9.5 µm can remain airborne for long periods of time.

“Roads are a very significant source of microplastics to remote areas, including the oceans,” Andreas Stohl, lead researcher from the Norwegian Institute for Air Research, told The Guardian. He said an average tire loses 4 kg during its lifetime. “It’s such a huge amount of plastic compared to, say, clothes,” the fibers of which are commonly found in rivers, Stohl said. “You will not lose kilograms of plastic from your clothing,” unlike vehicle tires and brakes.

Why are Tires & Brakes Such Significant Sources of Microplastics?

An important source of plastics is road traffic emissions. Distinguished vehicle types for road transport include motorcycles, cars, light duty vehicles, buses, and heavy-duty vehicles. The estimates of distance driven for this study were derived using data on fuel use in road transport and supported by national data on vehicle numbers and assumptions of per-vehicle mileage traveled.

Tires consist of a mix of elastomers such as rubber (natural and synthetic), carbon black, steel cord, fibers, and other organic and inorganic components used to improve their stability. Most car braking systems consist of a disc or drum with either a pair of shoes or pads mounted in calipers. Brake linings consist of binders, fibers, fillers, frictional additives or lubricants, and abrasives.

To obtain global emissions of road microplastics, 2 different approaches were used in the study.

  1.  The researchers assumed a constant ratio of TWP emissions to CO2 emissions from the road transport sector using CO2 emissions from the Coupled Model Intercomparison Project inventory. The detailed information of TWP emissions was derived from Northern Europe.
  2. They also relied on the GAINS (Greenhouse gas–Air pollution Interactions and Synergies) model, which is an integrated assessment model where emissions of air pollutants and Kyoto gases are estimated for nearly 200 regions globally. The model considers key economic activities, environmental regulation policies, and regionally specific emission factors. Emissions of non-exhaust PM in GAINS include TWPs, BWPs, and road abrasion, and the calculation is based on region-specific data and estimates of distance-driven (km vehicle type−1 year−1) and vehicle-type-specific emission rates.

Image retrieved from Walker County


The cumulative study data indicated that emissions of road microplastics are concentrated in the eastern US, northern Europe, and large urbanized areas of eastern China, Middle East, and Latin America. This makes sense, as they are the areas where vehicle densities are highest.

The researchers defined the transport efficiency as the ratio between the mass of microplastics deposited in a remote area and the total mass of microplastics emitted globally. In areas surrounded by road microplastic emissions sources, coarse particles were more efficiently deposited than fine particles.

For example, in the Alps, the Mediterranean, Baltic Sea, and South China Sea, transport efficiencies of coarse particles were up to twice of those of the fine particles. Short exposure to coarse particles has been highlighted as a major cause of respiratory diseases (e.g. asthma), especially considering that such regions are heavily populated.

The opposite was the case in remote areas that are far from emission sources, such as the Arctic and Greenland, where deposition of fine particles is greater than that of coarse ones.

Here are the specs:

  • About 15% of the fine road microplastics were transported to the Atlantic Ocean, whereas coarse particles were less efficiently deposited there (TWPs 10%, BWPs 11%).
  • Due to the smaller size of the fine road microplastics, their transport efficiency to the Pacific Ocean was even more strongly enhanced relative to coarse deposition.
  • The South China Sea receives ~2% of airborne road microplastics, at maximum, a large amount considering its relatively small surface. This is due to the fact that Southeastern Asian emissions of microplastics tend to travel towards the South China Sea and the Western Pacific before they turn to the north, all the way to the Arctic.
  • The calculated transport of coarse road microplastics shows a relatively high efficiency over Greenland (TWPs 1.7%, BWPs 2.3%) and over the Arctic Ocean (TWPs 6.8%, BWPs 4.3%) and much smaller to the Southern Ocean (TWPs 1.4%, BWPs 0.5%).
  • Negligible transport to Antarctica was simulated.

All primary sources (TWP and BWP emission data) are publicly available:

Final Thoughts

Reducing microplastic pollution in the greater society is a huge hurdle, and so, too, is the more specific goal to eliminate road microplastics pollution from vehicles.

Stohl, the lead researcher, noted, “The manufacturers will have to respond somehow, if this really becomes a matter of concern.”

Until such a time as more communities become concerned about microplastics pollution, Stohl recommended that people should reduce the use of plastics they can do without and ensure the rest is recycled.

road microplastics

Image retrieved from US EPA