How Tyres Pollute the Ocean

Ask most people to name the biggest source of plastic in the sea and they will say bottles, bags or straws. The honest answer is probably the four black rings holding up the car in their driveway, and almost nobody thinks of a tyre as a plastic product at all.

A modern tyre is a composite of natural and synthetic rubber, carbon black, silica, steel, textile and a long list of chemical additives. Synthetic rubber is a polymer. When a tyre wears down over 50,000 kilometres, that lost mass has not evaporated. It has been shed onto the road surface a few micrograms at a time, and a meaningful share of it ends up in water. Globally, tyres are estimated to generate around six million tonnes of wear particles a year, of which roughly 200,000 tonnes are thought to reach the ocean. One frequently cited estimate from the Pew Charitable Trusts attributes 78 percent of ocean microplastics to synthetic tyre rubber, a figure at the high end of the published range but indicative of the scale involved.

What makes tyre pollution distinctive is not only the volume. It is that the particles carry chemistry with them, and one of those chemicals turned out to be among the most acutely toxic substances ever found in surface water.

 

Your Tyres Slowly Disappear

 

Tyre wear is continuous, invisible and proportional to how and what you drive. An average car in the United States sheds roughly five pounds of tyre material a year, around 2.3 kilograms. A European car, typically lighter and driven less, sheds about half that.

The particles span an enormous size range. At the coarse end they are visible crumbs of rubber, often bonded with road dust and bitumen into what researchers call tyre and road wear particles. At the fine end they are ultrafine, below 100 nanometres, and present in extraordinary numbers: Emissions Analytics has estimated that a car's four tyres release on the order of a trillion ultrafine particles per kilometre driven. Particles at that scale behave less like litter and more like air pollution, which is why tyres and brakes now emit more particulate matter than exhaust pipes do in places where tailpipe emissions have been heavily controlled.

Roughly half of what a tyre sheds stays on the road surface, according to work by Dr Samantha Ladewig's team at the University of Auckland. The other half enters the environment. That split is the crux of the whole problem, because the half that stays on the road is recoverable in principle and the half that leaves is not.

 

Rain Carries the Pollution Further

 

The transport mechanism is unglamorous. Particles accumulate on road surfaces during dry weather, rain mobilises them, and stormwater drains carry them into streams, rivers and estuaries. Some settle in roadside soil and sediment along the way. Others travel the full distance to the coast.

Urban stormwater is therefore the main delivery system, and it is a poorly filtered one. In San Francisco, stormwater is estimated to carry around seven trillion microplastic particles into the Bay each year, with roughly half suspected to be tyre-derived. The Auckland study sampled bays adjacent to roads and motorways, including Coxs Bay and Saint Marys Bay, and found tyre particles in every single location tested, in both roadside and coastal sediment. Ladewig's team estimated around 5,000 tonnes entering the New Zealand environment annually.

Two features of this pathway make it hard to interrupt. The first is particle size: fragments run from one or two millimetres down to the nanoscale, and very few existing stormwater treatment devices capture anything at the fine end. The second is that the first flush of rain after a dry spell carries a disproportionate load, so the pollution arrives in concentrated pulses rather than a steady trickle, which is exactly when aquatic organisms are least able to avoid it.

There is also a slower pathway that is much less understood. Rubber fragments sitting in soil release their chemical additives gradually over months or years. How long that takes, and how much eventually reaches water, is one of the larger knowledge gaps in the field.

 

The Chemicals Matter Too

 

For about two decades, biologists in the Pacific Northwest could not explain why coho salmon returning to spawn in urban streams were dying within hours of rainfall. The condition was named urban runoff mortality syndrome. In some streams, the majority of returning adults died before spawning.

A 2020 paper identified the cause. Tyres contain 6PPD, an antiozonant added to stop the rubber cracking. When 6PPD on the road surface reacts with ground-level ozone, it transforms into 6PPD-quinone, a compound that is acutely lethal to coho salmon at concentrations in the sub-microgram per litre range. That is a vanishingly small amount, and it is routinely exceeded in urban creeks after rain.

The toxicity is also strikingly selective. Of eleven fish species tested, four have proved acutely sensitive, with coho salmon the most extreme case. Closely related species, including chum salmon, tolerate concentrations that kill coho outright, and the mechanism behind that difference is still being worked out. The researcher Cassandra Johannessen has described 6PPD-quinone as one of the most toxic substances known, and that is not hyperbole in the context of the species that are affected.

Human exposure is an open question rather than an established harm. 6PPD-quinone has been detected in the urine of children, adults and pregnant women in southern China, which establishes that people are exposed. What that exposure does has not been studied.

 

Why 6PPD Is Difficult to Remove

 

The natural reaction is to ask why the industry does not simply take it out, and the answer is the most important piece of context in the whole story.

6PPD is not a gratuitous additive. Rubber exposed to atmospheric ozone cracks, and a cracked tyre fails. 6PPD migrates continuously to the rubber surface where it sacrificially reacts with ozone, which is precisely why it also sits on the outer surface available to be shed and transformed. The property that makes it protective is the property that makes it an environmental problem. Removing it without a functional replacement would trade an ecological hazard for a road safety one, and no regulator has been willing to do that.

The search for a replacement is therefore a genuine chemistry problem rather than a question of willingness. California's Department of Toxic Substances Control listed motor vehicle tyres containing 6PPD as a Priority Product under its Safer Consumer Products regulations, effective 1 October 2023, which obliges manufacturers selling into California either to remove the chemical, remove the product, or complete a formal alternatives analysis. The department's own list of candidate substitutes, which it stresses it has not vetted for safety or performance, includes Durazone-37, CCPD, graphene and various lignins. The United States Environmental Protection Agency opened a regulatory investigation into 6PPD and 6PPD-quinone under the Toxic Substances Control Act in late 2024, Washington State has published an action plan and alternatives assessment, a coalition of tribes has petitioned the EPA for a ban, and Earthjustice has filed notice of intent to sue tyre manufacturers under the Endangered Species Act.

Reporting in Chemical and Engineering News in August 2026 found that credible candidate antiozonant molecules are now emerging, but that development, testing and qualification will still take years. A tyre compound change has to survive durability, wet grip, rolling resistance, heat and ageing validation before it reaches a road, and the replacement must not turn out to have a transformation product of its own.

 

What the Marine Evidence Actually Shows

 

The carousel's fourth slide is careful to say that marine impacts remain under investigation, and that caution is correct and worth defending.

Almost all of the definitive 6PPD-quinone toxicity work concerns freshwater salmonids in urban creeks, which is where the exposure is most concentrated and where the effect was first observed. The marine picture is thinner. Tyre particles have been found in the gills and stomachs of marine animals, and laboratory studies report neurotoxicity, reduced growth and behavioural abnormalities, but the dose-response relationships that exist for coho salmon in a stream do not yet exist for most marine species in open water. Dilution in a coastal environment is far greater than in a creek, which argues for lower acute risk, while persistence in sediment and uptake by filter and deposit feeders argues for chronic exposure that nobody has fully characterised. Ladewig's observation that selective feeders are ingesting these particles, and that the chemicals attached to them could alter how ecosystems function, is a hypothesis supported by good circumstantial evidence rather than a demonstrated outcome.

Stating this plainly is not a softening of the case. It is the difference between a claim that will hold up and one that will not. The land-based evidence is strong enough on its own: a chemical that reliably kills a keystone fish species in the streams it spawns in is a sufficient reason to act, whether or not the marine toxicology ever matches it.

 

The Electric Vehicle Complication

 

Electrification solves the tailpipe and makes the tyre problem worse. Battery vehicles are heavier than equivalent combustion cars and deliver instant torque, and both increase tyre wear, with estimates putting the increase at around 20 percent. The Auckland research noted that the tyres shedding most were on electric vehicles and utes.

This does not undermine the case for electrification, which remains overwhelming on carbon and air quality grounds. It does mean that non-exhaust emissions become a larger share of the remaining problem as the fleet turns over, and that an environmental strategy built entirely around replacing the powertrain will leave this pollution stream growing. Regulators have begun to recognise this: Europe's Euro 7 standard extends emissions regulation to tyre and brake wear for the first time, which is the first serious attempt to put a limit on something that has never been measured in a type approval test.

 

Ocean Protection Starts on Roads

 

The carousel's closing slide lists the four levers, and the useful observation is that they do not require the same amount of time.

Lower-wear tyres are a design problem that interacts awkwardly with others, because the compounds that wear slowly often perform worse on wet grip or rolling resistance. Standardised abrasion testing and labelling, which Euro 7 introduces, is the precondition for competing on it at all.

Better stormwater filtration is the fastest and most proven intervention, and it is startlingly effective. Rain gardens in San Francisco trapped 96 percent of street litter and 100 percent of black rubbery fragments. In Vancouver, bioretention systems prevented more than 90 percent of 6PPD-quinone from reaching salmon streams. Capture at source is also being attempted mechanically: The Tyre Collective's electrostatic devices mounted at the wheel claim to collect up to 60 percent of tyre and brake particles, with the captured material reusable in new tyres.

Safer chemical formulations are the slowest lever, for the reasons set out above, and the one where regulatory pressure is doing the most work.

Less unnecessary driving is the only lever that addresses volume rather than concentration, and it is the one nobody wants to hear. Every other measure reduces emissions per kilometre. Only this one reduces kilometres.

The honest conclusion is that tyre pollution has been invisible for exactly as long as it was unmeasured, and it is now being measured. Of the four levers, the stormwater one is available immediately, cheap by infrastructure standards, and delivers results that are already documented. The ocean protection that matters most here happens in a roadside planting bed several hundred kilometres from the sea.

 

Note: This article reflects the state of research and regulation as of October 2026, drawing on sources including Yale Environment 360's reporting on tyre pollution, University of Auckland research on tyre particles in coastal sediment, California's Department of Toxic Substances Control Safer Consumer Products listing, US EPA proceedings under the Toxic Substances Control Act, Washington State Department of Ecology's 6PPD action plan, and Chemical and Engineering News reporting on alternatives. Estimates of tyre-derived microplastic volumes vary widely between studies and methodologies, and the share of ocean microplastics attributed to tyres should be treated as an estimate rather than a measured quantity.