Keeping Lethal Tire Chemical 6PPD Out of Salmon Streams

Every time it rains in California, water running off our roads carries tiny particles of worn tire rubber into storm drains, creeks, and rivers. In 2020, researchers at the University of Washington and Washington State University traced decades of mysterious coho salmon die-offs in Puget Sound streams to one chemical in that runoff: 6PPD-quinone. It forms when 6PPD, an additive that keeps tires from cracking, reacts with ozone in the air. 6PPD-quinone is among the most toxic chemicals ever tested on aquatic life. Coho salmon are the most sensitive, often dying within hours of exposure, and studies show it also harms steelhead, cutthroat trout, and Chinook salmon.

A recent study sampled 22 sites around the San Francisco Bay and Central Coast during a single November 2025 storm. The chemical showed up in 98% of samples, and 69% of storm samples exceeded the concentration that kills half of juvenile coho in 24 hours. California's coho salmon are already at a small fraction of their historical numbers, amd this chemical could be a major factor preventing recovery.

CalTrout is tackling the 6PPD problem alongside our partners in the California Salmon and Steelhead Coalition, and with California Coastkeeper Alliance and a statewide 6PPD workgroup of Waterkeepers, Tribes, and conservation groups. We supported AB 756, which would have directed Caltrans to study biofiltration to keep 6PPD out of waterways. In 2026 we joined partners in asking the Legislature for $1 million so the State Water Resources Control Board can support Tribes and NGOs sampling for 6PPD-quinone on the North Coast, Central Coast, and Central Valley. The Coalition is also monitoring for the chemical in coastal streams, and we're briefing legislators and agency staff so decision makers understand what's at stake.

We see three steps to solving this. First, measure the problem: fund monitoring so the state knows where 6PPD-quinone is reaching salmon and steelhead. Second, fix the source: California's Department of Toxic Substances Control has listed tires containing 6PPD as a Priority Product and requires manufacturers to evaluate safer alternatives, and we'll keep pressing for a replacement that works for vehicles and fish. Third, treat the runoff: millions of tires with 6PPD will stay on the road for years, so stormwater solutions like biofiltration, rain gardens, and street sweeping are needed where busy roads drain into salmon streams.