A toxic chemical from car tires is polluting rivers and creeks in Santa Cruz County and the Central Coast, posing a severe threat to endangered coho salmon, steelhead trout, and potentially humans.
Researchers tested 14 creeks, six roadway outfalls, and two estuaries in Central California during a recent storm and found the contaminant, known as 6PPD-quinone, in 98% of the water samples.
The compound forms when tire rubber wears down on roads and washes into waterways via stormwater runoff.
In Santa Cruz County, 6PPD-quinone was discovered in the San Lorenzo and Pajaro Rivers, and Aptos, Arana, Carbonera, Branciforte, Soquel, Trout, Wilder, Waddell, and Scott Creeks.
“This research confirms what we already knew about the presence of 6PPD-quinone in the San Lorenzo River watershed, but it also provides additional information about other local watersheds that was previously unknown,” said Chris Berry, Watershed Compliance Manager for the City of Santa Cruz.
The research, published last month in Environmental Science and Technology Letters, states that more than three-quarters of the storm samples surpassed EPA safety thresholds, and nearly 70% exceeded lethal levels for young coho salmon.
Waterways located near busy highways, such as Branciforte Creek in Santa Cruz County, recorded the highest concentrations.
“The fact that road surface contaminants are affecting our salmon and trout populations is not new. Toxins like copper brake dust and oils in road runoff have been a concern for years,” said Ben Harris, Executive Director of the Monterey Bay Salmon and Trout Project (MBSTP). “But this 6PPD-quinine study is relatively new and it’s concerning because this stuff is extremely toxic in a very low concentration.”
6PPD is a chemical commonly added to products made from plastics and rubber, such as car tires, to increase their life span. When car tires wear down, the 6PPD reacts with ozone in the air to create the compound 6PPD-quinone, which is significantly more toxic than the original chemical.
In 2020, Pacific Northwest researchers published a study that linked it to the decline of Puget Sound coho populations. This landmark work sparked an immediate wave of hundreds of international studies across environmental science, toxicology, and engineering..
Last month’s report on 6PPD-quinine levels in local watersheds is just more bad news for the Central Coast’s coho population, a species that has struggled to survive for decades.
Between the 1940s and the 1960s, Santa Cruz and San Mateo county coho numbers dropped from an estimated 350,000 fish to 10,000. By 1995, scientists counted only 50 to 60 spawning adults, mostly in Waddell and Scott Creeks.
In addition to toxins like 6PPD-quinone, biologists attribute the local coho population’s collapse to habitat destruction, disease outbreaks, temperature extremes and environmental cataclysms like atmospheric rivers and wildfires. Coho landed on the endangered list in 2006.
While research on 6PPD-quinone’s environmental impact continues, state agencies and researchers are working on long-term solutions such as alternative tire materials, more efficient infrastructure, and better stormwater management.
According to the U.S. Tire Manufacturer’s Association, the tire industry is actively screening and testing dozens of potential chemical replacements that protect rubber without forming toxic quinone. They are also exploring physical or structural changes in rubber vulcanization and alternative antioxidant formulas.
In a December 2024 report, Caltrans outlined current early strategies to address 6PPD-quinone. These include stormwater monitoring programs, the management of stormwater runoff along state highways, the installation of water treatment facilities and bioswales filtering systems, and the introduction of permeable or porous road surfaces to minimize runoff.
So is 6PPD-quinone a danger to humans? Research is ongoing.
The National Toxicology Program and the EPA have formed a federal working group to assess potential human health effects, but biomonitoring studies in the Puget Sound have already detected 6PPD-quinone in human urine, blood, and cerebrospinal fluid.
While early laboratory tests on human cells and rodents suggest that high doses can trigger oxidative stress, inflammation, DNA damage, and liver or nervous system changes, researchers have not yet established whether real-world environmental levels are high enough to cause these issues in people.
