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The investigation continues, as Ed Kolodziej, UW, (left) and Jen McIntyre, WSU, coordinate their studies of the mysterious chemical that comes off tires and kills coho salmon in urban streams. Here, the two are seen in Seattle’s Longfellow Creek, where coho often come to die. Largely hidden behind McIntyre is Zhenyu Tian, the researcher on Kolodziej’s team credited with isolating and identifying the tire-related compound, 6PPD-quinone. Photo: Mark Stone, UW

Discovery of tire-related chemical that kills coho salmon sparks widespread response

Scientists, legislators and manufacturers are responding in various ways to the recent groundbreaking discovery of a deadly chemical derived from automobile tires, a chemical that can rapidly kill coho salmon swimming in urban streams.

Researchers are trying to better describe the chemical signature and biological function of the newfound chemical, 6PPD-quinone, along with related compounds. One major goal is to find an alternative chemical that can prevent dangerous cracking in tires without poisoning the environment.

U.S. House Committee on Natural Resources, Subcommittee on Oversight and Investigations, holds a hearing on the toxic chemical associated with tires, July 15, 2021

Tire manufacturers acknowledge that they had no idea that 6PPD-quinone even existed, although the chemical’s parent compound, 6PPD, has been touted as essential to tire safety and used in nearly every tire on the market. Without 6PPD, tires are readily attacked by ozone, and the resultant cracking increases the danger of tire failure, manufactures say.

This ozone attack is what adds oxygen to 6PPD in tires to produce the more toxic compound 6PPD-quinone, according to researchers at the Center for Urban Waters, affiliated with University of Washington-Tacoma. The findings were published in the journal Science in January.

I described the 20-year search for this deadly chemical and the final sprint to the finish in a story for the Encyclopedia of Puget Sound. (The Center for Urban Waters is the parent organization of Puget Sound Institute.)

Now, the search for a replacement chemical has become a high priority, involving state and federal governments, research institutions and tire companies. At the same time, other efforts are focused on methods that might be able to remove the chemical from stormwater that washes off roadways, carrying the toxic chemical into nearby streams.

Washington state

In both Washington and California, state agencies responsible for protecting the environment from damage caused by toxic chemicals are beginning to spearhead investigations into the problems with 6PPD-quinone and looking at potential solutions.

Earlier this year, the Washington Legislature appropriated $718,000 to the Washington Department of Ecology to fund studies and analyses of the 6PPD-quinone problem. Of that, $195,000 will be used “solely for the department to carry out an assessment of potential hazards of 6PPD and other chemicals or chemical classes and breakdown products used as anti-oxidants and/or anti-ozonants in tires…” A report is due to the Legislature by Dec. 1.

Ken Zarker, manager of Ecology’s Pollution Prevention Section, said the assessment will include a first look at nine “chemicals of interest” that may or may not become safer additives for tire manufacturers to use in new tires. The nine chemicals will be assessed for their toxicity by putting them through the established “GreenScreen” program, which rates a chemical’s hazards to human health and the environment.

Washington State Capitol, Olympia

“That will give us a snapshot of the toxicity of the chemicals,” Ken told me, adding that full alternative assessments could come later.

At this time, 6PPD-quinone in automobile tires is not eligible for review under the Safer Products for Washington program, because automobiles are exempt under the law, Zarker said. The Safer Products program is busy with a host of other chemical assessments, as I described in “Our Water Ways” on June 10.

The remaining $523,000 appropriated by the Legislature is to be used by Ecology to work on roadway infrastructure and “best management practices” to study ways to reduce and treat toxic runoff. That work will involve a partnership with frontline researchers from UW Tacoma’s Center for Urban Waters and Washington State University’s Puyallup Research and Extension Center, along with the Washington Department of Transportation. One goal is to develop a standard laboratory method for measuring 6PPD-quinone and related chemicals in various media, such as soil, water and sediments. That report is due to the Legislature by Nov. 1, 2022.

“Now we can begin testing and treatment for it,” said Brandi Lubliner, coordinator of Ecology’s Stormwater Action Monitoring program. “A suitable alternative is the primary long-term goal.”

Future projects could involve assessing where 6PPD-quinone can be found on the landscape, what other sources may exist, and how the chemical moves through the environment under various conditions, Lubliner said. Other questions include how strongly 6PPD-quinone and possible substitutes bind with soils and how well street sweeping can remove these chemicals from roads.

A 6PPD Subgroup has been established within Ecology’s ongoing Stormwater Workgroup, which is focused on the problems of stormwater pollution.

Many environmental organizations in the Northwest are joining the effort to find solutions to the 6PPD-quinone problem, according to Mindy Roberts, Puget Sound Program director for Washington Environmental Council. Because the discovery of 6PPD-quinone was announced just before the start of the last legislative session, supporters for additional research had to work quickly to obtain special funding in the last session, she said.

Even before the discovery of 6PPD-quinone, a wide variety of experts and interest groups were involved with the Washington Department of Transportation in proposing stormwater projects that could reduce pollution coming from highways. Now, officials are preparing to shift those priorities, as needed, in light of the new findings about 6PPD-quinone.

Studies in biochemistry

The ongoing research is a mixture of lab studies and field studies to figure out how these tire-related chemicals and similar compounds are changed in the environment and affect living organisms, said Ed Kolodziej, who is leading the studies at the Center for Urban Waters.

“Most of the work is focused on 6PPD,” he said. “We need to understand the chemistry much better.”

Jen McIntyre, based at the WSU’s Puyallup research center, continues to be largely focused on the biological effects of 6PPD-quinone. She and other researchers suspect that the chemical is causing injury to coho via the circulatory system, based on studies involving toxic stormwater.

“What we know so far about the mechanism of toxicity is that this chemical disrupts the blood-brain barrier,” McIntyre told a congressional panel in July. “We injected a tracer dye into the heart of fish and allowed it to circulate through the blood vessels and then rinsed it out with clean saline water.

“In the control fish in clean water, none of the dye was left behind. But in runoff fish (exposed to the toxic chemical), the dye was found in various organs after the rinse-out and could be seen leaking from the gills during circulation,” she told a subcommittee of the House Committee on Natural Resources.

Jen said the tracer dye was able to get into the brain of fish exposed to the toxic chemical, something that should never happen under normal conditions. “That’s what the blood-brain barrier is for,” she noted during her testimony, which begins at 22:30 in the top video on this page.

The findings about leakage across the blood-brain barrier are outlined in a scientific paper published by McIntyre and her colleagues in February in the Canadian Journal of Fisheries and Aquatic Sciences. (Lead author: Stephanie Blair.)

One of the intriguing, yet unanswered questions, remains why coho are affected to such a severe degree, while chum salmon get by seemingly unscathed in the same waters.

A major question for McIntyre and researchers across the globe is whether 6PPD-quinone causes one or more toxic effects that are more difficult to observe than sudden death. That question of sublethal toxicity stands front and center, not only for salmon but for other fish and a multitude of aquatic organisms.

The uncertainty about toxicity in humans is another great concern, given the potential for exposure from playfields that use recycled tires as a base.

In previous studies, researchers were able to remove what were then unknown toxic compounds by filtering toxic road runoff through ordinary organic compost. While it would be extremely costly to filter all stormwater from roads and freeways, filtration offers at least one alternative for solving the toxic problem. Such a solution might prove useful in selective locations, considering that a replacement chemical could be years away.

California efforts

Meanwhile in California, the state’s Department of Toxic Substances Control has designated tires as a “priority product” under its Safer Consumer Products program. The decision to review 6PPD came with the full support of the U.S. Tire Manufacturers Association, as described in a letter to DTSC (PDF 69 kb).

The rationale for reviewing tire chemicals is outlined in a June “discussion draft” of a “Product-Chemical Profile for Motor Vehicle Tires Containing (6PPD).” The 88-page document (PDF 2.8 mb) covers the chemical properties, potential exposure routes, potential for harm, regulatory background, potential alternatives and other state-of-the-science issues. Public comments on the draft are being accepted until Sept. 17.

California’s three-year work plan for priority products covers a number of products besides tires. The analysis of each product involves screening, public engagement, product-chemical profile and rulemaking. The result can be a requirement that manufacturers conduct an alternatives analysis before the agency decides whether other chemicals, methods or actions should be required to better protect human health and the environment.

At the end of July, the Department of Toxic Substances Control held an online workshop covering various aspects of the problem involving toxic chemicals involving tires. That presentation is shown in the third video on this page with supporting information on the DTSC website.

Tire manufacturers

The U.S. Tire Manufacturers Association has emphasized that member companies are greatly concerned about their impact on the environment, but the association has also raised many questions about the newly discovered transformation product, 6PPD-quinone — which, they stress, is a chemical that was never actually put into tires.

Sarah Amick, senior counsel for the USTMA, has said 6PPD-quinone may not be long-lived in the environment, based on a report that researchers were unable to measure the compound directly in stormwater runoff.

To identify gaps in knowledge and to develop a plan to fill those gaps, the global tire industry formed a task force that includes the USTMA, the European Tyre and Rubber Manufacturers Association, and the World Business Council for Sustainable Development’s Tire Industry Project, Amick said in written testimony (PDF 685 kb) for a subcommittee of the House Committee on Natural Resources.

The Tire Industry Project, which has long been focused on the environmental effects of tires, is conducting its own studies into 6PPD-quinone, Amick said. Scientists involved in the research are expected to publish peer-reviewed studies on the subject sometime next year.

The USTMA is not only supporting California’s in-depth review of 6PPD-quinone, she said, the association suggested that the Department of Toxic Substances Control take up the matter, as described in a letter last December (PDF 69 kb), Amick said.

In her congressional testimony, Amick explained why the industry wanted California to conduct the review: “A review of 6PPD in tires under the Safer Consumer Products Regulations provides a rigorous, transparent, scientific, regulatory framework to analyze whether alternatives exist that will enable tire manufacturers to meet Federal Motor Vehicle Safety Standards as established by the National Highway Traffic Safety Administration.” See also the USTMA factsheet on 6PPD.

In an online workshop put on by the California agency at the end of July (third video), Howard Colvin, an independent technical consultant to the tire manufacturers, emphasized the importance of an array of chemicals used to make tires. He explained that tires are carefully formulated to produce a particular performance — depending on the use of the tire. Today’s formulations also ensure vehicle safety by preventing the breakdown of the rubber through oxidation and ozonation, he said.

“The antioxidants help to keep the tire from breaking down due to temperature and oxygen effects,” Colvin said. “The antiozonants are used to impede the exposure to ozone, which is in our atmosphere and on the surface of the tire.”

The compound 6PPD prevents the breakdown of rubber from both oxygen and ozone under dynamic conditions when the tire is rolling at high speed on the highway, as well as under static conditions when the vehicle is standing still.

“Without the use of this material and materials like this,” he said, “the tire will crack and degrade very rapidly, potentially leading to a catastrophic failure.”

A compound used as an anti-ozonant must be able to diffuse through the rubber and come to the surface of the tire, where it reacts with ozone, he noted. It must diffuse and react fast enough to protect the rubber from ozone but not so fast that it is used up before the tire comes to the end of its useful life. Also, the chemical must be one that that will not cause problems during the manufacturing process, as the rubber is formed into a tire.


U.S. Reps. Marilyn Strickland, D-Tacoma, and Derek Kilmer, D-Gig Harbor, have been seeking federal funding in support of studies dealing with the toxic-tires problem.

Strickland was able to gain support in the House for a $1-million appropriation for research on the tire chemicals within the Interior and Environment “minibus” appropriations bill. It’s called “minibus,” as opposed to “omnibus,” because the overall appropriations legislation rolls together several smaller bills.

As Congress works through these spending packages and possible budget reconciliation, Strickland and Kilmer are calling on top congressional leaders to support $1.5 million for research into 6PPD-quinone within the reconciliation package. A letter — also signed by Reps. Suzan DelBene (D-Medina), Pramila Jayapal (D-Seattle), and Adam Smith (D-Bellevue) — expresses urgency about the need to get answers through research.

“Biologists have observed coho salmon dying from mysterious symptoms in Pacific Northwest urban streams for decades,” the letter (PDF 146 kb) states. “However, it is only in the past year that a team of scientists from the University of Washington’s Center for Urban Waters and Washington State University’s Washington Stormwater Center discovered the cause: a toxic chemical called 6PPD-quinone, created when a commonly used antiozonant in tires interacts with ozone. This chemical runs into local streams when it rains, entering the bloodstream of coho salmon and killing them…

“Including dedicated funding to accelerate research into this toxic chemical in the reconciliation package is an essential part of our coordinated strategy to recover salmon, uphold our treaty and trust obligations to tribal nations, and ensure that our children can enjoy the same extraordinary ecology and biodiversity that makes the Pacific Northwest unique,” the letter concludes.

The breakdown in proposed spending:

  • $900,000 “to sharpen laboratory-based surveillance methods to conduct further research on the occurrence and impacts of novel consumer product chemicals in U.S. coastal waters.”
  • $250,000 to study the impacts of tire chemicals on aquatic species. “Although the tire chemical break-down product 6PPD-quinone has been found to be toxic to coho salmon, we don’t know if this or related chemicals are toxic to other organisms.”
  • $250,000 to develop and refine stormwater treatment technologies for the removal of 6PPD and 6PPD-quinone from stormwater.
  • $125,000 to empower communities, and in particular historically underrepresented communities, with accessible Puget Sound simulation modeling tools.

Comments provided in a related news release:

Rep. Marilyn Strickland: “Salmon are an integral part of our history, culture, economy, and way of life in the Pacific Northwest, especially for our Tribal nations. Our salmon are dying now, and we cannot afford to wait another two decades for the next research breakthrough. We must robustly fund research into 6PPD-quinone today, which is why Rep. Kilmer and I are urging Congress to take action now to save our salmon before it’s too late.”

Rep. Derek Kilmer: “We know that toxic stormwater runoff is one of the biggest threats facing Puget Sound salmon recovery. That’s why Rep. Strickland and I are working to secure federal support for critical research that will help scientists and researchers understand the link between tire debris and the health of our Sound — as well as the species that depend on it. I am grateful for Rep. Strickland’s leadership and partnership as we work to advance this urgent priority.”

Joel Baker, professor and science director at University of Washington Tacoma’s Center for Urban Waters: “There is still so much we don’t know about 6PPD-quinone, the impact of this toxic chemical on Washington’s coho salmon, and how other species are impacted in other geographies. Science must lead the way. We thank Congresswoman Strickland for her advocacy and bringing this issue to Congress.”

John Stark, director of Washington State University’s Washington Stormwater Center: “The funding proposed in this letter would dramatically accelerate research of 6PPD-quinone and its impact on Washington wildlife and waters. We are grateful to Congresswoman Strickland for listening to the science, taking action, and fighting to put this issue on the national radar in Congress. We look forward to working with her on this important research.”

The budget process is more confusing than ever this year, but Strickland promises to keep pushing for research to get answers about 6PPD-quinone. As Katrina Martell, her press secretary, wrote in an email: “As Congress negotiates spending packages and reconciliation, Rep. Strickland will continue to pursue every opportunity to direct funding to research the toxic chemical endangering coho salmon.”

2 Replies to “Discovery of tire-related chemical that kills coho salmon sparks widespread response”

  1. Is anything being done about all the tires we see on our dives in the Puget Sound, there were put all over the place to become reefs, but they are not good for the fish. Seems like it would be a good Idea to remove the actual tires out of the water first and then work on a way to stop new pollution from coming in? Or work on both at the same time?

    I see that in Guam and in Florida there is work to remove all the tire reefs for the same reason. It would be great if we can also do this up here in the Seattle area.

    Thanks for your time Scuba Jess

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