---
title: Federal Scientists Will Test Two Finalist Molecules to Replace the Tire Chemical Killing Salmon
description: Flexsys narrowed its 6PPD replacement to two molecules and sent them to USGS scientists for toxicity testing. A safe substitute is still years from tires.
author: Darie Nani (Editor-in-Chief)
updated: 2026-08-27T17:20:53.560Z
canonical: https://www.sovereignmagazine.com/article/federal-scientists-will-test-two-finalist-molecules-to-replace-the-tire-chemical-killing-salmon
image: https://cdn.nanimediahouse.com/tire-runoff-salmon-stream-215612.webp
categories: Green Tech
content_type: Analysis
region: United States
publication: Sovereign Magazine
schema_type: Article
---

Flexsys said on August 27 that it had narrowed its search for a replacement for 6PPD, the rubber chemical whose runoff byproduct kills Pacific salmon, to two final candidate molecules. Both were chosen from outside the PPD chemical family, so that neither breaks down into the toxic quinone blamed for the die-offs. The company says both molecules cleared its safety and performance benchmarks in internal and external testing. In the same announcement, Flexsys signed a Cooperative Research and Development Agreement with the U.S. Geological Survey, sending the two candidates to federal scientists at the agency's Western Fisheries Research Center for aquatic toxicity testing.

The agreement is a federal cost-share arrangement: the USGS toxicologists run the tests, and Flexsys funds the work. That makes it an outside check by government fisheries scientists, not a review the company runs on itself, but it is neither a blind third-party study nor regulatory approval. What it does is move two named replacement candidates into independent testing, one concrete step in a search that has run for years with no molecule yet ready for the market.

## The Chemical That Turns Lethal After It Rains

6PPD is an antioxidant added to nearly every tire made, where it shields the rubber from ozone, oxygen and heat and keeps it from cracking. The trouble starts once the tire wears. When tire dust washes onto roads and reacts with ozone, 6PPD forms a second compound, 6PPD-quinone. In 2021, researchers led by Zhenyu Tian reported in the journal Science that 6PPD-quinone was the cause of "urban runoff mortality syndrome," the sudden death of coho salmon returning to spawn in Pacific Northwest urban streams after rain. Tian and colleagues found it kills coho at concentrations on the order of one microgram per liter, within hours.

## Tires Cannot Lose 6PPD Without a Replacement

Pulling 6PPD out of tires is not an option on its own. The chemical is safety-critical, and without an antiozonant, tires crack and fail, according to the tire industry's own analysis. That is the bind the whole replacement effort is built around: a substitute has to match what 6PPD does before anyone can stop using it. A swap also solves only part of the pollution problem. Tire-wear particles are the largest single source of microplastics entering waterways, and changing one chemical does nothing about the particles themselves.

## Regulators and Tribes Have Pushed for Years

The pressure to move has been building since the Science finding. California's Department of Toxic Substances Control designated tires containing 6PPD a Priority Product under its Safer Consumer Products program, effective October 1, 2023, forcing an alternatives analysis. Washington State followed with its own priority-product listing in 2024, and lawmakers there have proposed banning 6PPD in tires by 2035.

Three Pacific Northwest tribes, the Puyallup, Yurok and Port Gamble S'Klallam, [petitioned the Environmental Protection Agency in August 2023](https://www.epa.gov/newsreleases/epa-grants-tribal-petition-protect-salmon-lethal-chemical) to regulate 6PPD under the Toxic Substances Control Act. Salmon sit at the center of these tribes' treaty fishing rights, culture and subsistence. The EPA granted the petition in November 2023 and issued an advance notice of proposed rulemaking a year later. That same month in 2023, two fishing groups, the Institute for Fisheries Resources and the Pacific Coast Federation of Fishermen's Associations, sued 13 tire manufacturers under the Endangered Species Act over 6PPD-quinone's harm to protected coho and steelhead.

## Flexsys Is One Runner in a Wider Race

Flexsys is a major producer of 6PPD and other rubber chemicals, though not the only one; Lanxess and Sennics are large makers as well. The hunt for a replacement is an industry-wide effort, coordinated in part through a U.S. Tire Manufacturers Association consortium running its own alternatives analysis. Narrowing to two candidates and handing them to USGS is Flexsys staking out a position in that race, not the industry crossing a finish line.

## A Substitute Is Still Years From the Road

Even after a replacement molecule is selected, the industry's own analysis says it needs a minimum of about four more years of limited-scale field testing, plus tire-qualification cycles, before it can go into wide use. A winner also has to be affordable at global scale, where 6PPD is roughly a one billion dollar a year market, and it has to clear regulatory and vehicle-safety review. Flexsys has said its first plant for a replacement would come around 2030 or 2031, which puts replacement tires reaching the road broadly in the mid-2030s or later.

In the years between now and then, 6PPD tires stay legal, the compound keeps forming every time it rains, and the coho keep dying in the streams the tribes have fished for generations. The announcement moves a safe substitute one verifiable step closer, but not one that helps the salmon dying now.

## FAQ

**Q: What is 6PPD?**
It is a chemical antioxidant, technically an antiozonant, added to almost all tires. It protects the rubber from ozone, oxygen and heat, keeping tires from cracking and failing. The tire industry considers it safety-critical, which is why it cannot be removed without a working replacement.

**Q: What is 6PPD-quinone?**
It is the compound that forms when 6PPD in tire-wear dust reacts with ozone on roads. Research published in Science in 2021 identified it as the cause of large coho salmon die-offs in urban Pacific Northwest streams after rain, lethal at concentrations around one microgram per liter within hours.

**Q: Are the tires on the road today dangerous to people?**
The research here concerns aquatic life. Scientists identified 6PPD-quinone as lethal to coho salmon at very low concentrations; these sources do not establish effects on human health, and this article does not claim any. The documented harm is to fish, especially coho and steelhead.

**Q: When will safer tires actually be available?**
Not soon. After a replacement is chosen, the industry's own analysis calls for at least about four more years of limited field testing plus tire-qualification cycles before wide use. Flexsys projects its first replacement plant around 2030 to 2031, so replacement tires reaching the market broadly is a mid-2030s prospect at the earliest.

**Q: Would replacing 6PPD end tire pollution?**
No. A 6PPD swap targets one toxic chemical and its salmon-killing byproduct. Tire-wear particles remain the largest single source of microplastics entering waterways, and that problem would continue regardless of which antioxidant tires use.