Expanding the Science of 6PPD-Quinone: A New Highly Sensitive Species
New results from WFRC reveal that coastal cutthroat trout are highly sensitive to 6PPD-quinone, underscoring the need to find alternative chemicals.
Millions of cars drive along streets and highways every day. But while moving people where they need to go, the vehicle’s tires also leave something behind: a toxic chemical known as 6PPD-quinone.
Rainfall mobilizes roadway contaminants that flow from storm drains into aquatic habitats.
In 2020, a team led by scientists from the University of Washington – Tacoma and Washington State University in Puyallup identified 6PPD-quinone as the chemical responsible for the long-observed die-offs of coho salmon returning to spawn in urban Puget Sound streams. The deaths, seen since the early 2000s, were linked to stormwater runoff containing tire wear particles.
The chemical 6PPD is in all vehicle tires, incorporated to protect the rubber from quickly breaking down, extending tire life and increasing vehicle safety. 6PPD takes the brunt of the corrosive impact ozone has on rubber. When exposed to ozone, it transforms to 6PPD-quinone. This compound sheds from tires and washes into streams during rainstorms.
Concentrations up to 2.5 micrograms per liter (1,000,000th of a gram) have been measured in these waters. While this doesn’t seem like much, research has shown that coho salmon can die at levels as low as a few tens of nanograms per liter (1 billionth of a gram)—an amount equivalent to just a few drops in an Olympic-sized swimming pool. Based on its toxicity in such small quantities, 6PPDQ has been characterized as the second most toxic chemical for aquatic animal life. It has also been detected in human urine, raising concerns for human health.
While other fish species have shown some sensitivity to 6PPD-quinone, none have been found to as sensitive to 6PPD-quinone as coho salmon—until now.
At the U.S. Geological Survey, researchers have helped lead the way in studying 6PPD-quinone and its impacts on fish. Now, scientists at the Western Fisheries Research Center (WFRC) recently published a study demonstrating a second species of fish – coastal cutthroat trout (CCT) - experiences lethal effects upon exposure to 6PPD-quinone at low, environmentally relevant concentrations similar to that of coho.
Coastal Cutthroat Trout Show High Sensitivity to 6PPD-quinone
CCT share similar habitats and life cycles with coho salmon. Adult coastal cutthroat trout (Oncorhynchus clarkii clarkii) lay their eggs in shallow, low-lying waters across the Pacific Northwest. After they hatch, CCT juveniles spend 2-4 years in freshwater creeks before migrating to sea, starting the lifecycle all over again.
To test whether CCT are sensitive to 6PPD-quinone, scientists exposed fish at four different life stages to a range of concentrations and monitored them over a 24-hour period. The results showed a clear pattern: like coho salmon, CCT began to die at concentrations in the nanogram-per-liter range. At all four life stages tested, the lethal concentration of 6PPD-quinone fell within ranges seen in urban streams. High sensitivity in young life stages is particularly concerning for species like CCT that may spend several years in freshwater before migrating to the ocean, potentially increasing the likelihood they will be exposed to the chemical.
Sub-lethal Effects of 6PPD-quinone
While this study showed that 6PPD-quinone can be lethal to CCT at concentrations already present in urban waterways, researchers also wanted to know what happens at lower doses. Sublethal effects are important because even small impairments in behavior or physiology can impact survival and long-term population health.
To assess these effects, scientists focused on two essential behaviors: swimming performance and immune function. Swimming is critical for feeding, migration, and avoiding predators, while immune health helps fish resist infection. By comparing fish exposed and unexposed to 6PPD-quinone, the researchers could see whether even low levels of 6PPD-quinone impaired these functions.
The study found that even at sublethal levels, exposure to 6PPD-quinone reduced the swimming ability of the CCT. This could have significant consequences for CCTs ability to forage, migrate, and avoid predators. However, there was no noticeable impact on viral infection rates in the exposed fish, suggesting that 6PPD-quinone may not interfere with immune responses in CCT. The mixed results of this work highlight the complexity of studying pollutants like 6PPD-quinone.
Video of juvenile coho salmon showing typical signs of urban runoff mortality syndrome in response to 6PPD-quinone.
WFRC is Leading the Way Toward a Safer Alternative
The results of this study and similar work assessing impacts to fish and other species provide critical information that guides efforts to set limits on 6PPD-quinone in watersheds, evaluate technologies to remove the contaminant before it reaches rivers and streams, and to explore chemical alternatives to 6PPD less toxic to fish.
The State of California requires tire manufactures to be actively seeking alternatives to continue to sell tires there, and Washington State recently passed regulation to phase 6PPD out of tires. Through collaborations with the US Tire Manufacturing Association, the US Department of Agriculture, and tire chemical producers including Flexsys, our researchers are leveraging our unique expertise to help advance the search for a viable substitute.
The USGS Western Fisheries Research Center is the only laboratory in the world with the capacity to test a wide range of 6PPD alternatives using high-throughput methods. Through a novel in vitro technique developed at the lab, scientists can test multiple chemical candidates simultaneously in fish cells we grow. After screening the chemicals in vitro, promising alternatives are then tested with live fish to ensure their safety. This two-step approach enables more efficient testing, reduces the number of live fish used for testing, and accelerates the discovery of safer options.
The USGS Kansas Water Science Center, Columbia Environmental Research Center and numerous other USGS Science Centers are also involved in 6PPD efforts, working to assess the chemical’s toxicity, fate and transport in the environment, and on stormwater remediation efforts to remove 6PPD and other chemicals in route to rivers and streams. Visit the USGS 6PPD overview page for more information about USGS broader efforts to understand and address this chemical of concern.