From dying salmon to human urine: tracking 6PPD-quinone

Tyre wear is the part of the microplastics story that has nothing to do with packaging. As tyres abrade on the road they shed rubber particles and the chemicals in them, and one of those chemicals — 6PPD-quinone — turned out to be startlingly toxic to fish. It is now being detected in human urine, and the research on what it does to people is at the very beginning. This is a companion to our earlier report on 6PPD-quinone and the brain; here the focus is the exposure pathway, from road dust to the body.

The salmon that solved a mystery

For decades, coho salmon returning to spawn in urban streams of the US Pacific Northwest died in large numbers after rainstorms, and no one knew why. A 2020 study identified the cause: 6PPD-quinone, a transformation product formed when 6PPD — a globally used antioxidant that keeps tyre rubber from cracking — reacts with ozone in the environment [s1]. The chemical proved acutely lethal to coho at a median concentration of 0.8 micrograms per litre, and surveys of roadway runoff and affected creeks found it at concentrations from under 0.3 to 19 micrograms per litre — routinely above the lethal threshold [s1].

The mechanism is pure road pollution. Rain washes tyre-wear particles and their breakdown products off pavement into storm drains and streams, delivering a pulse of 6PPD-quinone with the first heavy rainfall. The salmon deaths were the visible signal of a contaminant that coats every road surface.

Now measured in people

The exposure does not stop at streams. Road dust is inhaled and ingested, and tyre-wear particles are one of the largest sources of microplastics in the environment by mass. The chemical forms in the open air, where 6PPD in tyre rubber meets ground-level ozone, so it is generated continuously wherever traffic runs, not confined to a spill or a factory [s1]. A 2026 case-control study measured 6PPD-quinone and related quinone compounds in human urine and found them nearly universal: detection frequencies for the target compounds ranged from about 70% to 90% across participants [s2]. Among the group, 6PPD-quinone was one of the most abundant, at mean urinary levels around 1.37 micrograms per gram of creatinine in healthy controls [s2]. Whatever else is unresolved, human exposure is now documented and common.

The health question, wide open

That 2026 study went a step further and looked for an association with colorectal cancer, reporting that people in the highest quartile of urinary 6PPD-quinone had higher odds of the disease than those in the lowest (odds ratio 1.79; 95% confidence interval, 1.26 to 2.32) [s2]. Urinary 6PPD-quinone ran higher in cancer cases than controls — a mean of 1.75 versus 1.37 micrograms per gram of creatinine — and a mixture analysis of all the quinone compounds together suggested a positive joint association with the odds of colorectal cancer, with 6PPD-quinone contributing most strongly [s2]. This is where caution has to be loud. It is a case-control study — it compares people who already have the cancer with those who do not and looks back at their exposure — which cannot establish that the chemical caused anything. People with and without colorectal cancer may differ in many ways that also track with where they live and how much road pollution they encounter. The authors themselves frame it as a signal calling for "further prospective studies," not a conclusion [s2].

So the human health evidence is a single associational study reporting a correlation, against a backdrop of strong evidence that the chemical is toxic to fish and that people are widely exposed [s1][s2]. Those are three separate facts, and only the first two are solid. The near-universal detection in urine is the durable finding here — it establishes internal exposure across a general population, which is the necessary groundwork for any future study that could test whether the exposure matters [s2].

Why it belongs in the plastics conversation

6PPD-quinone widens the frame. Most microplastics coverage is about what people eat and drink; tyre wear is about what everyone breathes and walks through, generated continuously by traffic and concentrated near busy roads. It also illustrates the pattern the whole field keeps repeating: a clear environmental harm — dead salmon — established first, ubiquitous human exposure documented next, and the actual human health effect still unmeasured [s1][s2].

The productive response is the regulatory and engineering one already under way: finding tyre formulations that do not rely on 6PPD, and managing road runoff before it reaches waterways. For an individual, the exposure is largely a function of proximity to traffic, which is not a matter of consumer choice. What 6PPD-quinone does inside the human body is the question the next decade of research has to answer.

This article is informational and is not medical advice.

Sources

• A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon — Science, 2020-12-03
• Associations Between Human Exposure to Substituted p-Phenylenediamine-Derived Quinones and Colorectal Cancer Risk — Toxics, 2026-08-10