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pacific salmon

Car tire chemicals are killing salmon and steelhead

The chemical 6PPD, added to tires to prevent degradation, is causing a “complete breakdown of the blood brain barrier” in fish, a new study found.

Since the early 2000s, Barb French observed an unexplainable phenomenon among coho salmon in the Pacific Northwest’s Puget Sound.


When the fish returned to their natal streams to spawn, a point in their life cycle when they are typically in excellent health, they behaved strangely.

“They’d swim into the banks of the creeks,” French, a researcher at the National Oceanic and Atmospheric Administration, told EHN. “They were very disoriented, even swimming sideways.” The fish lost their sense of direction, gaping their mouths at the water’s surface and splaying their fins. Within a few hours, they would die.

Last year, a group of Washington researchers pinpointed the cause of these mass fish kills: 6PPD, a chemical added to tires to prevent them from breaking down. When 6PPD, which has been used in tires since the 1970s according to the U.S. Tire Manufacturers Association, reacts with ozone in the atmosphere, it creates 6PPD-quinone (6PPD-q), a compound that leaches into urban stormwater and watersheds. This derivative chemical has proven difficult to identify and study and is even harder to regulate given that the chemicals in tires are proprietary and not disclosed by tire manufacturers.

However, new research led by NOAA scientists found 6PPD-q harms other species in the salmonid family besides coho, including steelhead trout.

The team investigated how different salmonids react to undiluted, 6PPD-q-loaded, urban stormwater, which is rarely treated before it reaches larger bodies of water. Nearly all the coho salmon died right away after being exposed to the polluted water, and up to 42% of the steelhead trout were killed too, though their deaths were delayed by one to two days.

French and her colleagues tried removing the steelhead from the contaminated water and placing them in clean water, but once they had been exposed to the chemical, they still died.

While the research focused on Pacific fish, the implications extend to other salmonids across the U.S., and perhaps other aquatic life.Even if data on humans is still scarce, some states are taking notice of the potential impacts and beginning to regulate the chemical and source safer alternatives.

Much of this research is nascent and ongoing because scientists only identified 6PPD and 6PPD-q in 2021, though scientists observed the effects for decades before knowing what was causing the mass fish kills. “You can’t measure things in the stormwater if you don’t know what they are,” said French.

“A complete breakdown of the blood brain barrier”

tire chemicals

6PPD has been used in tires since the 1970s, according to the U.S. Tire Manufacturers Association.

Credit: Corey/flickr

6PPD-q still puzzles scientists, but Washington State University researcher Stephanie Blair has made headway in understanding why the fish die. Blair observed that when coho salmon are exposed to the chemical, they swim to the water’s surface where the concentration of dissolved oxygen is highest – which, she figured, was an extreme response to oxygen deprivation, even though oxygen was abundant throughout the water.

Blair noticed that as the fish developed symptoms, their blood thickened. Blair used a fluorescent tracer to visualize the circulatory system of the fish and found that plasma was leaking out of blood vessels and filling brain tissue. “That’s a complete breakdown of the blood brain barrier that will shut down neuronal function and very quickly cause death,” Blair told EHN.

Many conservationists hope that at a lower dilution, 6PPD-q might not be as hazardous to salmonids. But even at low concentrations, the chemical poses a high risk.

“We’ve found quite high concentrations of 6PPD-quinone in our waterways,” Ezra Miller, a scientist at the San Francisco Estuary Institute, told EHN. In the San Francisco Bay and nearby watersheds, many stormwater samples taken by the San Francisco Estuary Institute have exceeded the species LC50, a standard measure of toxicity indicating the concentration of a chemical that is expected to kill half of the exposed population. A sensitive population will have a low LC50, while a population that can tolerate a higher concentration has a higher LC50.

The LC50 threshold for steelhead is higher than for coho, but Miller predicts the Bay Area 6PPD-q levels are high enough to harm steelhead, and the case is likely the same for any U.S. waterway near an urban area. Even if the concentration of 6PPD-q does not kill steelhead, it might produce sublethal effects that limit their ability to reproduce and evade predators or prevent their cardiovascular system from developing properly.

Researchers are only beginning to investigate the effects of 6PPD and 6PPD-q on humans and other organisms. But the ubiquity of 6PPD-q in our waterways means it could potentially be passed up the food chain to humans. Salmon and steelhead are canaries in the coalmine, explained Blair; as many other organisms, including humans, have blood brain barriers that 6PPD-q has the potential to disrupt.

There are thousands of chemicals in urban stormwater, many from tires. French worries not only about these “parent chemicals,” but also how they decompose and interact with each other and the environment.

States take action 

Though 6PPD is present in tires across the country, Washington and California are leading the nation in their efforts to limit its concentrations in the environment. In October 2022, the Washington State Department of Ecology released a new report on stormwater management, aimed at reducing concentrations of 6PPD and 6PPD-q in runoff through source control (such as street sweeping and roadside cleanup) and flow control (such as soil filtration columns that filter out 6PPD-q). Meanwhile, the California Department of Toxic Substances Control is taking steps to classify 6PPD as a “priority product,” which would force companies to disclose their use of 6PPD and conduct analyses to find safer alternative chemicals.

“It will be a long time before we can do product replacement or green chemistry,” Nat Scholz, ecotoxicology program manager at NOAA and senior author on French’s study, told EHN. Not only because it takes a long time to find a nontoxic alternative, but also because tire engineering has to account for human safety, Scholz added.

So for now, scientists and engineers are treating the stormwater itself. Green infrastructure solutions like soil filtration columns catch chemicals in urban stormwater before they enter watersheds. But it will take an enormous amount of green infrastructure to ensure water quality for a fish population so sensitive to 6PPD-q as coho salmon is.

6PPD-q has been polluting and killing salmon and steelhead for decades, long before we even knew the compound existed, Scholz told EHN. Since then, the scientific tools to recognize its pollution, such as molecular biology to understand how chemicals affect organisms and analytical chemistry to break apart complex mixtures, have advanced. And the complexity and scale of the problem have grown with urbanization. But our approach to solutions has not kept pace. “Most of our worldview around water quality and salmon habitats hasn't really changed since the 1970s,” said Scholz. Fish will keep dying until we adapt.

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