When the Rubber Hits the Road... It Doesn't Disappear

Air and microplastic pollution from car tires is significant and environmentally pervasive – and as yet unregulated

by Gavin MacRae

When we think of air pollution, smokestacks and tailpipes probably come to mind. Plastic in the ocean? That’s synonymous with discarded packaging and single-use items.

But another pernicious, far less recognized source of air and plastic pollution is emitted during the normal wear and tear of car tires. Unlike car exhaust or plastic packaging, there are no regulations to restrain the pollution, and little attention paid by the public or policymakers.

During everyday driving, tire and road wear particles (TRWP)1 – tiny, cigar-shaped bits of rubber tread laced with asphalt – tear free as the tires scrub against the road. It’s how tires wear out.

Together with similarly fine particles emitted from brake pad wear, these non-exhaust emissions are now believed to make up around 60% of harmful airborne particulates under 2.5 microns (PM 2.5) and 73% of particulates under 10 microns (PM 10) from road transport.2 (Asphalt itself also emits hazardous pollutants, but that’s another story)3

Research hasfound that microplastics make up 11% of the flow of plastic to the oceans, and tire particles make up 78% of those microplastics by mass.

PM 2.5, the smallest particles, are particularly dangerous. The pollution is linked to a range of cardiovascular, respiratory, and developmental health conditions, and an increased risk of premature death. Recent research also suggests air pollution worsens symptoms of COVID-19, especially in congested urban areas.4

According to a 2020 report by the Organization for Economic Cooperation and Development, a 37-country intergovernmental body focused on world trade, non-exhaust sources will grow to dominate particulate emissions from road traffic as early as 2035, as tailpipe emissions dwindle.5

As tires wear, it’s been calculated that 10-30% of the tread rubber ends up in the environment.6 Worldwide, that works out to an estimated 6.1 million ton mountain of tire dust every year.7

And the fragments aren’t just tiny bits of rubber trees, but microplastics. The rubber in modern car tires is a roughly 60/40 mix of synthetic and natural, with sulphur, zinc, carbon black, aromatic oils, and other materials added for wear resistance and better rolling characteristics.8

Airborne

So where does that mountain of tire particulate end up?

Lumping in particles from brake wear, some 550,000 tonnes are carried by wind every year to the far corners of the earth, according to research published in Nature Communications in 2020.9 That includes over 200,000 tonnes that enter the oceans, and 86,000 tonnes that land in regions of snow and ice, where, like black carbon, the particles’ dark colour absorbs sunlight and speeds melting.10 The smallest particles can remain airborne for weeks, and previous research has confirmed microplastic contamination in snow from the Arctic to the Alps.11

Scientists cracked a mystery last year when they discovered that a tire rubber additive is the cause of mass die-offs of spawning coho salmon that have plagued Puget Sound for decades.

Waterborne

Particles that aren’t carried by wind can be washed off roadways by runoff, to enter creeks and rivers, and eventually the ocean.

Research by The Pew Charitable Trusts, universities, and other NGOs found that microplastics make up 11% of the flow of plastic to the oceans, and tire particles make up 78% of those microplastics by mass. Plastic pellets trail behind at 18% of ocean microplastics; textiles and grooming products contribute four per cent.

The particles are not inert. Scientists in Washington state cracked a mystery last year when they discovered that a tire rubber additive is the cause of mass die-offs of spawning coho salmon that have plagued Puget Sound for decades.12 13 The additive, 6PPD, (N-(1,3-Dimethylbutyl)-N´-phenyl-1,4-benzenediamine) is an antioxidant found in tires all over the world. When the chemical reacts with ozone in the atmosphere it changes to become toxic to salmon. Some urban streams in Washington have seen 40-90% of returning coho struck down within hours of returning to spawn.

Inventories of microplastics in other coastal areas show tire particles are pervasive. One study found nearly half of the seven trillion microplastic particles estimated to flush into San Francisco Bay with stormwater annually are likely tire rubber.14 On the Atlantic coast, research has found tire fragments appear to be the large majority of microplastics in the water and sediments of Charleston Harbour, Southern Carolina.15

Microplastics have been shown to stunt the reproductive capacity and growth of fish, cause genetic damage to shellfish, and fatally clog the guts and gills of shrimp.16 17 18

The health effects of microplastics in human food are so far unclear, but we may well find out soon. Research from Netherlands notes that with microplastic uptake documented in hundreds of marine species, “human exposure to microsized and nanosized tyre particles via the consumption of aquatic food species seems apparent.”19

Solutions?

The problem of tire and road wear particles is expected to get worse as consumers’ yen for larger, heavier SUVs and trucks continues. Heavier vehicle weights translate to more tire and brake wear.20

And unlike exhaust emissions, electric vehicles cannot help. Despite regenerative breaking that saves brake wear, the increased weight of EV batteries and torque of electric motors causes more tire wear than comparable internal combustion engine vehicles.21

Solutions have been proposed, such as mandating low-wear tire formulations, low-wear road surfaces, and lower speed limits. Roadside “bioretention” rain gardens have been shown to filter 90% of waterborne microplastics, and other contaminants, from stormwater before it reaches waterways.22

As well, a prototype car-mounted device to capture tire dust using an electrostatic charge has been invented by The Tyre Collective, a startup from a team of students at the Imperial College London and Royal College of Art in the UK.23

Perhaps most important, the same changes that will do heavy lifting to decarbonize road transport – walkable cities, bicycles, buses, and trains – could nearly halve microplastic tire pollution by 2040, The Pew Charitable Trusts says.24

As yet, no tire-specific measures have been implemented, and tire and road wear particles remain largely under the radar. According to emissions testing firm Emissions Analytics, “The challenge to the industry and regulators is an almost complete black hole of consumer information undone by frankly out-of-date regulations still preoccupied with exhaust emissions.”25


 

  1. Tire & Road Wear Particles | U.S. Tire Manufacturers Association
  2. Non-Exhaust Emissions from Road Traffic | Air Quality Expert Group Report
  3. Yale study finds dangerous emissions from asphalt long after it’s laid
  4. Executive Summary | Non-exhaust Particulate Emissions from Road Transport : An Ignored Environmental Policy Challenge | OECD iLibrary
  5. Executive Summary | Non-exhaust Particulate Emissions from Road Transport : An Ignored Environmental Policy Challenge | OECD iLibrary
  6. Non-exhaust traffic related emissions – Brake and tyre wear PM
  7. Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment
  8. An unknow object: the tire -Materials | Michelin The tire digest
  9. Atmospheric transport is a major pathway of microplastics to remote regions | Nature Communications
  10. As black carbon accelerates Arctic melt, UN ponders ‘ineffective’ regulation of a key culprit | Watershed Sentinel
  11. Microplastics ‘significantly contaminating the air’, scientists warn | Pollution | The Guardian
  12. Tire dust killing coho salmon returning to Puget Sound, new research shows | The Seattle Times
  13. Pollution from car tires is killing off salmon on US west coast, study finds | Fish | The Guardian
  14. Final Draft 9.25.table 4.1 test 3 (1) -Microplastic Levels in SF Bay -Final Report.pdf
  15. Microplastic in two South Carolina Estuaries: Occurrence, distribution, and composition | ScienceDirect
  16. Research on the Influence of Microplastics on Marine Life
  17. Size-and shape-dependent effects of microplastic particles on adult daggerblade grass shrimp (Palaemonetes pugio) | PubMed
  18. Tires: An Emerging Threat to Our Waterways, Our Seafood, and Ourselves? | NRDC
  19. Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment
  20. Tyres Not Tailpipe | Emissions Analytics
  21. Tyres Not Tailpipe | Emissions Analytics
  22. Multiyear Water Quality Performance and Mass Accumulation of PCBs, Mercury, Methylmercury, Copper, and Microplastics in a Bioretention Rain Garden | Journal of Sustainable Water in the Built Environment | Vol 5, No 4
  23. The Tyre Collective
  24. Breaking the Plastic Wave: Top Findings for Preventing Plastic Pollution | The Pew Charitable Trust
  25. Tyres Not Tailpipe| Emissions Analytics

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