In 2020, scientists at the University of Washington solved a decades-old ecological mystery: why coho salmon were dying en masse in Pacific Northwest urban streams just before spawning, apparently healthy one moment and dead within hours of arriving in fresh water. The culprit was a single molecule shed by vehicle tires — present at parts-per-trillion concentrations, lethal to coho, and nearly invisible to conventional water monitoring. That molecule was 6PPD-quinone, and its discovery reframed how researchers think about tire rubber as a source of water contamination reaching far beyond the streams where the salmon died.
What Is 6PPD-Quinone?
6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine) is an antiozonant added to virtually all modern vehicle tires. Its job is straightforward: it diffuses to the tire surface and reacts preferentially with atmospheric ozone before ozone can attack the rubber polymer chains, preventing the surface cracking that shortens tire life. Without 6PPD, tires would degrade rapidly in urban air. With it, tires last long enough to shed an estimated 1 kilogram of rubber per car per year as tire wear particles.
When 6PPD at the tire surface encounters ground-level ozone, the reaction that protects the rubber produces a byproduct: 6PPD-quinone. This transformation product is chemically distinct from its parent compound and far more biologically active. It is shed from tires continuously — in wear particles that fall to road surfaces, from tire-dust plumes, and through direct abrasion on wet pavement — and washes into stormwater with the first significant rainfall.
The 2020 paper by Tian et al. in Science identified 6PPD-quinone as the cause of what researchers had called “urban runoff mortality syndrome” in coho salmon — documented since the 1990s in Puget Sound-area streams, where pre-spawn mortality in urban tributaries reached 40 to 90 percent of returning adults in some years. The team screened tire rubber extracts against a stormwater chemical library and traced the kill signal to 6PPD-quinone, which is lethal to coho at concentrations as low as 0.8 micrograms per liter, with some median lethal concentration estimates in the parts-per-trillion range. The species sensitivity is striking: chum and sockeye salmon are far less sensitive at environmentally relevant concentrations, while coho die rapidly.
6PPD-quinone is now understood to be a near-ubiquitous product of modern road surfaces wherever tires contact ozone-containing air — which is to say, essentially everywhere vehicles operate.
How It Gets Into Drinking Water
The pathway from tire to tap follows the stormwater system.
Every millimeter of rain on a paved road mobilizes tire wear particles and associated transformation products, including 6PPD-quinone, and carries them into storm drains. In most US cities, storm drains discharge directly to receiving waters — rivers, reservoirs, and coastal estuaries — with no treatment. Unlike PFAS, which enters drinking water through point sources such as industrial discharges or military base runoff, 6PPD-quinone is a nonpoint-source contaminant distributed across every road surface and parking lot in every city that has stormwater-to-surface-water connections.
Drinking water intakes on rivers and reservoirs that receive urban stormwater are the primary exposure point for the drinking water system. The contaminant has been detected in surface water samples globally, in road runoff, in urban streams, and in household dust. A 2024 study of human urine found detectable 6PPD-quinone in samples from exposed populations, confirming that the compound reaches people through some route — whether drinking water, dust inhalation, food, or a combination is not yet fully characterized.
Groundwater risk is lower because soil and aquifer media filter tire wear particles, though dissolved transformation products are not fully retained. Utilities drawing from surface water in urbanized watersheds face the highest potential for 6PPD-quinone in source water.
Like microplastics — another tire-derived contaminant category — 6PPD-quinone travels the stormwater pathway from road to receiving water. The two share a source but have different chemical fates: tire wear particles carry both, but 6PPD-quinone is a dissolved transformation product rather than a microparticle.
Health Effects (Known and Unknown)
The human health data for 6PPD-quinone is limited. The compound became a subject of serious toxicological research only after the 2020 Science publication, and the human epidemiology does not yet exist. What is known comes from aquatic toxicology, early rodent studies, and mechanistic inference.
In fish, the evidence is unambiguous at ecotoxicological concentrations. Coho salmon exposed to environmentally measured concentrations of 6PPD-quinone in stormwater develop a characteristic syndrome: loss of equilibrium, disorientation, and rapid death within hours. The mechanism appears to involve disruption of cardiovascular function and gill physiology, though the precise molecular target is still under investigation. The narrow species sensitivity — coho highly vulnerable, chum and sockeye substantially less so — suggests a receptor or metabolic pathway that varies across salmonid species.
In mammals, the picture is at an earlier stage. Rodent studies have reported liver toxicity at elevated doses, including elevated liver enzymes and histopathological changes in hepatic tissue. Developmental effects have also been observed in some animal models. The compound has been flagged as a suspected endocrine disruptor based on structural analogy with other phenylenediamine-class chemicals, though confirmatory mechanistic studies are limited.
In humans, no epidemiological study has established a dose-response relationship. The 2024 detection of 6PPD-quinone in human urine confirms exposure via some pathway, but urine detection is a biomarker of exposure, not an established measure of harm. Long-term human risk from chronic low-level exposure through drinking water is genuinely unknown.
The honest framing: 6PPD-quinone is demonstrably lethal to a keystone species at concentrations found in urban stormwater, is detectable in human bodies, and shows preliminary toxicological signals in mammals — but the human health risk characterization needed for regulatory action does not yet exist. The situation parallels early PFAS science in the 2000s, when ecological signals and animal data preceded epidemiological evidence by years.
EPA Regulation and Limits
6PPD-quinone is not regulated under the Safe Drinking Water Act. There is no Maximum Contaminant Level (MCL), no MCL Goal (MCLG), and no monitoring requirement for public water systems. The contaminant does not appear on the current Unregulated Contaminant Monitoring Rule (UCMR 5 or the proposed UCMR 6), meaning utilities are not collecting the systematic national data that would be the precondition for regulation.
Regulatory movement is happening, but through a different statute. In August 2023, the EPA granted a citizen petition filed under the Toxic Substances Control Act (TSCA) Section 21 by three West Coast tribal nations — the Puyallup Tribe, the Port Gamble S’Klallam Tribe, and the Yurok Tribe — seeking federal action on 6PPD. The tribes, whose fishing rights and food security are directly tied to coho salmon populations, argued that 6PPD-quinone poses an unreasonable risk to both ecosystems and human communities. EPA’s acceptance of the petition committed the agency to initiate a TSCA Section 6 risk evaluation — the same formal risk assessment process used for other high-priority chemicals.
In 2024, EPA issued an information request to the tire industry seeking data on 6PPD production volumes, uses, and potential alternatives. California, which has moved more aggressively on emerging contaminants than the federal government, announced it would list 6PPD-quinone as a Priority Product under its Safer Consumer Products program — a designation that triggers a mandatory alternatives analysis by manufacturers.
| Standard / Action | Status | Notes |
|---|---|---|
| US EPA MCL (drinking water) | None | No SDWA action initiated |
| US EPA UCMR monitoring | Not listed | Absent from UCMR 5 and proposed UCMR 6 |
| TSCA Section 6 risk evaluation | Initiated 2023 | Triggered by tribal citizen petition; timeline not finalized |
| EPA tire industry information request | Issued 2024 | Seeks production and hazard data |
| California Safer Consumer Products | Listed 2024 | Priority Product designation; alternatives analysis required |
| Washington State | Monitoring ongoing | State agencies tracking coho mortality; no product ban yet |
| Canada | Under assessment | Environment and Climate Change Canada evaluating 6PPD |
No jurisdiction has set a drinking water limit for 6PPD-quinone as of May 2026. Regulation, if it comes, is more likely to target the tire manufacturing process under TSCA or equivalent chemical control statutes than to set a drinking water MCL — because the most effective intervention is reducing how much 6PPD-quinone is produced and shed, not treating it after it reaches water.
How Widespread Is It?
Measured data is sparse because monitoring is not required, but the available studies suggest 6PPD-quinone is as geographically widespread as the stormwater it travels in.
The compound has been detected in urban stormwater runoff across the United States, Canada, and multiple European countries. Surface water detections have been reported in streams and rivers receiving urban drainage in the Pacific Northwest, California, and elsewhere. A global household dust study detected 6PPD-quinone in samples from multiple countries, consistent with indoor deposition of road dust carried in on shoes and through ventilation. The 2024 human urine study found detectable 6PPD-quinone in a meaningful fraction of samples from populations in urbanized areas.
Concentration variability is high. Stormwater during the first flush of a rain event carries far higher 6PPD-quinone loads than baseflow conditions. Urban areas with heavy traffic and high impervious surface coverage generate more concentrated stormwater than suburban or rural areas. Because no US utility is currently required to test for 6PPD-quinone, the true prevalence in finished drinking water is unknown at a national scale.
How WaterVerge Tracks 6PPD-Quinone
6PPD-quinone is not currently in the EPA’s Safe Drinking Water Information System (SDWIS) — there is no enforceable standard to violate and no federal monitoring requirement that would generate utility-level data. It is also absent from UCMR 5, the most recent national monitoring cycle for unregulated contaminants.
This creates the same data gap that applies to microplastics: the absence of data reflects the absence of a monitoring requirement, not the absence of the contaminant. WaterVerge will incorporate monitoring data onto city pages as soon as EPA, state agencies, or research programs publish results at the public water system level. The TSCA Section 6 risk evaluation may eventually generate national-scale monitoring data as part of the risk characterization process.
The most relevant proxy currently available is a utility’s source water type: surface water utilities drawing from heavily urbanized catchments face the highest plausible 6PPD-quinone exposure. Check your city’s profile to see your utility’s source water designation and any other emerging contaminant detections on record.
How to Remove 6PPD-Quinone
Research on drinking water treatment effectiveness for 6PPD-quinone is at an early stage, but several treatment categories have been evaluated and the results are directionally consistent with what is known about the compound’s chemistry.
| Treatment Method | Removal Effectiveness | Notes |
|---|---|---|
| Activated carbon (GAC/PAC) | Moderate to good | Promising in lab studies; contact time and carbon type matter |
| Reverse osmosis | Effective | Membrane exclusion; removes dissolved organics broadly |
| Coagulation / sand filtration | Unreliable | Conventional treatment; limited removal of dissolved fraction |
| Bioretention / green infrastructure | Effective at source | Practical stormwater mitigation before reaching intakes |
| Oxidation (ozone/UV) | Under investigation | May transform to other products; not yet characterized |
Conventional drinking water treatment — the coagulation, sedimentation, and sand filtration sequence used by most large utilities — does not reliably remove 6PPD-quinone. These processes are designed primarily to remove turbidity, pathogens, and particles; the dissolved chemical fraction passes through.
Activated carbon is the most promising conventional treatment technology. Granular activated carbon (GAC) contactors and powdered activated carbon (PAC) both show adsorptive affinity for 6PPD-quinone in laboratory studies. Removal rates depend on carbon type, dosage, contact time, and source water matrix. Utilities that already deploy activated carbon for taste, odor, or PFAS control may achieve some 6PPD-quinone removal as a co-benefit.
Reverse osmosis removes 6PPD-quinone effectively by physical membrane exclusion — RO membranes reject dissolved organic compounds across a broad size range, and 6PPD-quinone’s molecular weight (382 g/mol) is well within the rejection range for standard RO membranes. For household use, a certified under-sink RO system provides the most comprehensive protection across emerging contaminants including 6PPD-quinone, PFAS, and microplastics. See our guide to best reverse osmosis systems for tested models and certifications.
Bioretention and green stormwater infrastructure — rain gardens, bioswales, constructed wetlands — are the most effective intervention before 6PPD-quinone reaches a drinking water intake. Studies have shown meaningful attenuation in bioretention soil media, and Pacific Northwest municipalities have begun investing in green infrastructure partly in response to the coho mortality findings. This is an upstream solution, not a household option, but it is where the most impactful mitigation is likely to occur.
For households that want to act given current uncertainty, an under-sink reverse osmosis system is the most defensible choice. It addresses 6PPD-quinone alongside a broad spectrum of emerging contaminants for which monitoring data is similarly incomplete. If you are unsure what is in your tap water, the first step is testing: see our guide to how to test your tap water for options that go beyond standard utility reports.
Check Your City
6PPD-quinone sits at the frontier of what drinking water monitoring tracks. No utility is currently required to test for it, no national concentration dataset exists, and the regulatory process that might eventually set a limit is measured in years, not months. What is clear is that the contaminant is widely distributed wherever road runoff reaches surface water, that it is lethal to ecologically important species at very low concentrations, and that early human biomonitoring confirms exposure.
Search your city on WaterVerge to see your utility’s source water type, any emerging contaminant data on record, and the full contaminant profile available from federal monitoring. If your utility draws from a river or reservoir in an urbanized watershed and you want to reduce your exposure to tire-derived contaminants and other emerging chemicals before regulators catch up, a reverse osmosis system at the kitchen tap is the most comprehensive step available to households today.
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