PORTLAND, July 7 — The rubber dust that scatters across every highway, every city street, every driveway in America is now drawing the attention of scientists far beyond the watersheds where salmon have been dying. On a stretch of asphalt outside this Pacific Northwest city, the evidence is invisible to the naked eye: microscopic fragments of tire tread, ground down by millions of passing vehicles, carrying a chemical cocktail that researchers are only beginning to understand.
A 2017 study from the International Union for Conservation of Nature estimated that tire wear accounts for up to 28 percent of primary microplastics released into the environment worldwide. In the United States, these particles are a major component of road dust and runoff.
They are not merely fragments of rubber. They carry additives — zinc, other compounds, and a chemical called 6PPD that is used in virtually every tire on the market to prevent the rubber from degrading in ozone. The discovery in 2020 that a transformation product of 6PPD — 6PPD-Q — is acutely toxic to salmon marked a turning point.
Suddenly, the tire dust that had been dismissed as a nuisance became a suspected killer in Pacific Northwest streams. Now, new research has expanded those concerns to human health, with studies linking tire chemicals to Alzheimer’s disease.
The implications are vast. Urban areas concentrate tire pollution along road corridors, where wind and water carry particles into soil, waterways, and the air residents breathe. By nightfall on any given day in any American city, the evidence is settling into the environment, unnoticed.
The chemistry of the road
The chemical 6PPD is added to tire rubber to prevent ozone from cracking the material. It works. Without it, tires would fail rapidly in sunlight and smog.
But as tires wear down, the particles that slough off contain both 6PPD and its transformation products, including the highly toxic 6PPD-Q. These particles are light enough to be transported by wind.
They enter the air in urban areas, settle into roadside soil, and wash into storm drains that feed rivers and estuaries. Regulatory agencies are beginning to take notice. The U.S. Environmental Protection Agency is assessing the risks posed by 6PPD and its transformation products, a process that could lead to restrictions or requirements for alternative formulations.
The EPA’s scrutiny follows the salmon toxicity findings and now the Alzheimer’s research, which has broadened the stakes from ecosystem health to human health. On the ground, the problem is concentrated and persistent.
Tire pollution does not degrade quickly. It accumulates. Residents along major highways describe a fine black dust that coats windowsills and porches.
That dust is tire wear. It contains zinc, used in tire manufacturing, and the 6PPD family of chemicals. In Portland, where salmon runs have been a cultural and economic touchstone, the connection between the road and the river is tangible.
The same rain that washes tire particles into the Willamette River carries them to the Columbia, and eventually to the Pacific. The salmon that return to spawn encounter 6PPD-Q at concentrations that can kill coho salmon within hours of exposure.
From fish to humans
The leap from salmon to Alzheimer’s is not as strange as it might seem. Both involve the same fundamental mechanism: the toxicity of 6PPD-Q to biological systems. In salmon, the chemical causes a specific syndrome — rapid mortality after exposure to stormwater runoff.
In human brain tissue, researchers have found evidence that tire particles can enter the body and that the chemicals they carry may contribute to neurodegenerative processes. The new Alzheimer’s research, published in recent months, has put tire chemicals on the radar of public health officials in a way that even the salmon die-offs did not.
The scale of the problem is enormous. Every vehicle on the road sheds tire particles. Unlike exhaust emissions, which have been reduced by catalytic converters and cleaner fuels, tire wear is a function of friction and weight.
Heavier vehicles — SUVs, trucks, electric vehicles with large batteries — generate more tire particles. The shift toward heavier vehicles in the American fleet may be increasing the load of tire microplastics entering the environment even as tailpipe emissions decline.
The chemicals in tire particles do not stay put. They leach into water. They volatilize into air.
They accumulate in soil. In urban areas, where road density is highest, the concentration of tire particles in dust and runoff can be measured in parts per million. The discovery of 6PPD-Q’s toxicity to salmon in 2020 triggered a wave of research into the environmental fate of tire chemicals.
That wave is now reaching human health, and the early findings are troubling.
What regulators face
The EPA’s risk assessment of 6PPD is still underway. The agency must weigh the benefits of the chemical — preventing tire degradation and extending tire life — against the environmental and human health costs. The chemical is used in virtually all tires globally.
Replacing it would require reformulating an industry standard that has been in place for decades. The alternatives are not yet proven at scale.
On the ground, the problem is immediate. Residents near highways breathe tire particles. Children play in playgrounds built near roads.
Urban stormwater, untreated, carries tire chemicals into rivers and lakes. The salmon in the Pacific Northwest are a sentinel species — their die-offs are a warning.
The new Alzheimer’s research suggests that warning extends to humans.
