6PPD – Edmonds Environmental Council

Untangling chemical names and looking back over 200 years of 6PPD history

Link to original PSI article
By Christopher Dunagan

Published July 17, 2026

The tire chemical 6PPD is known worldwide for its deadly effect on salmon, but it also has a complex history. We look at how this history connects to important breakthroughs in modern transportation and medicine and how the unique chemical structure of 6PPD makes it an environmental hazard.

The history of 6PPD-quinone intersects with the history of quinine, derived from the bark of the cinchona tree, and follows an unlikely path from malaria treatment to a deadly tire chemical.

Five and a half years ago, the world was suddenly confronted with the chemical names 6PPD and 6PPD-quinone — a pair of related chemicals that quickly became notorious for their deadly effects on coho and other salmonid species.

Since then, hundreds of scientific papers have evoked these names, as researchers worldwide struggle to determine why 6PPD-quinone (often shortened to 6PPD-Q) can be extremely harmful to some fish while nearly harmless to others.

The history of 6PPD — which logically begins in the early days of the automobile — involves decades of searching for chemicals that would allow car owners to get more mileage out of their tires. Chemicals with “PPD” in their names became central in the effort to protect tires from ozone, a ubiquitous compound that destroys rubber. Now, with the understanding that ozone converts 6PPD to the highly toxic 6PPD-quinone, the search has resumed for a less-poisonous compound that can, hopefully, protect tires as well as 6PPD has done.

As the research advances, the names 6PPD and 6PPD-quinone are becoming familiar to toxicologists and practically anyone concerned about salmon and the environment. What is rarely discussed, however, is what the letters “PPD” stand for, the meaning of the number “6” or how the word “quinone” relates to “quinine,” a traditional medicine used to treat malaria.

The following descriptions examine the history of the chemical, from the origin of its structure to its eventual use as a tire preservative.

The foundation of 6PPD in organic chemistry

The untold story of quinone

It is no accident that quinone, a class of familiar compounds, is just one letter away from quinine, the first effective treatment for malaria and considered by some to be the all-time greatest discovery in herbal medicine. The indigenous people of the Andes of South America used the bark of the cinchona tree for centuries to treat fevers and disease.

By the early 1600s, the medicinal bark was introduced to Europe after doctors found that it could treat malaria. While the bark was certainly an effective treatment, it was caught up in a controversy for a time, according to an article published by the National Institutes of Health. Uncertainty about the treatment may have been caused by impurities in the ground-up bark given to patients.

In 1820, French scientists Pierre-Joseph Pelletier and Joseph-Bienaimé Caventou isolated the active compound found in the cinchona bark. They named it “quinine” from “quina-quina” (meaning bark of barks), the Inca name for the sacred bark of the cinchona tree.

The bark of cinchona trees (left) which grow in tropical forests contains quinine, the compound credited with treating malaria. Scientists Pierre-Joseph Pelletier and Joseph-Bienaimé Caventou isolated this active ingredient in 1820, depicted in Ernest Board’s oil painting “The Discovery of Quinine” (right). Photo: Forest & Kim Starr (CC BY 3.0). Painting: Wellcome Collection (CC BY 4.0).

Because the bark was in high demand, chemists raced to synthesize quinine without relying on trees. It was a challenging endeavor, and the quest played an important role in the development of modern organic chemistry, according to a 2005 historical report by two Argentine authors.

Although the word “quinone” is just one letter away from “quinine,” the chemical relationship is more complicated. “Quinone” is wrapped up in the chemistry of the medicinal bark and predates the name “quinine,” according to scientific literature of the early 1800s.

In 1806, 14 years before the French scientists isolated quinine, Louis Nicolas Vauquelin, another French scientist, identified a natural acid in the quinquina bark, using a slight variation of the Inca name.

“Let us give it the name ‘acide kinique’ from the word ‘quinquina,’ until — when it is better known in its nature and its combinations — a better name may be given to it,” Vauquelin said in the original peer-reviewed manuscript. In English, “acide kinique” became “quinic acid.”

Vauquelin became famous for his work on quinic acid and many other discoveries. He did not know, at the time, that a German pharmacist, Friedrich Christian Hofmann, had isolated the same acid from cinchona bark in 1790 and named it “Chinasäure” (“china acid” — no connection to the country). In German, “China” remains the word for cinchona even today.

About 1838, Russian chemist Alexander Voskresensky oxidized quinic acid and obtained a yellow substance. He named it “Chinoyl,” using the ending “yl” on “Chin” to designate an organic “radical.” According to one theory at the time, a radical was considered a chemical building block that would remain stable through chemical changes.

A few years later, by 1844, German chemist Friedrich Wöhler concluded that the yellow substance was instead a complete, neutral molecule. Consequently, he renamed it “Chinon,” the German word corresponding to the English “quinone.”

Wöhler wrote: “The name ‘Chinoyl’ proposed … for this substance cannot be retained, because the ending “yl” is customarily used to designate an organic radical, which Chinon is not. I therefore give preference to the latter name, ‘Chinon.’”

For at least a few years, quinone retained its identity as an individual compound with its pair of double-bonded oxygens attached to a single ring of benzene. But by the 1860s, as chemists expanded their research into larger-ring structures, chemical dictionaries began to consider the quinones as a “class of compounds” with a specific structural pattern.

Thus, what was formerly “quinone” became “benzoquinone,” because it involved a single ring of benzene. There is also “naphthoquinone” involving the double-ringed naphthalene, “anthraquinone” involving the triple-ringed anthrylene, and so on for other ring configurations.

The list of quinones in everyday products today is quite long. While this class of chemicals tends to raise concerns because of their reactive nature, toxicity is a function of their chemical environment. Many are safely used, and some are even essential components of normal biological function.

For example, Coenzyme Q10, known as CoQ10, is a natural antioxidant found in every cell. It fuels cellular energy and protects cells from oxidative damage. Levels naturally decline in older people, especially those taking statin medications. Studies have shown that CoQ10 supplements can help certain people with specific medical conditions.

Phylloquinone (Vitamin K1), which is found in leafy green vegetables, plays a role in blood clotting. Plastoquinone, found in all green plants, is a key component in photosynthesis.

All sorts of dyes also are built from quinones, especially naphthoquinones and anthraquinones. The natural dye Henna, which provides a distinctive red-orange stain, is a naphthoquinone. Brilliant synthetic dyes Alizarin (vibrant red), Disperse Blue 3 and Solvent Orange 57 are all anthraquinones.

Natural herbal laxatives rely on anthraquinone glycosides. Widely used cancer drugs — also anthraquinone derivates — work by disrupting the DNA of cancer cells. Modern treatments for malaria are less often quinine – which is not a quinone at all — and more often drugs like Atovaquone, a highly modified variation of naphthoquinone.

Quinones are NOT found in pure hydrogen peroxide — a familiar chemical used for first-aid, cleaning with oxygen bleaches, teeth whitening and many other functions. But virtually all manufacturing of hydrogen peroxide involves the use of anthraquinones.

The history of 6PPD as a rubber preservative

The evolution of tire chemicals

Even before the invention of the automobile, a self-made chemist, Charles Goodyear, revolutionized the use of rubber and paved the way to modern tires by using chemical additives. The highly successful antiozonant 6PPD came into prominence in the 1960s — about midway through the history of tire chemistry. Many other additives with names like DPPD, IPPD and CPPD have been tried with varying success.

If told completely, the ongoing story of tire chemistry must begin with the earliest use of rubber at least 3,600 years ago. Ancient civilizations in Central America played games with rubber balls on ballcourts of various sizes, according to archeologists who have unearthed ballgame structures and equipment. The Olmec people of Southern Mexico, who predated the Mayans and Aztecs, harvested rubber from the Panama rubber tree (Castilla elastica) and used it to fashion footwear, craft tools and make rubber bands as well as rubber balls.

Europeans were fascinated with the bouncy rubber balls brought back by explorers and scientists in the early 1500s. But rubber became more of a household name after 1770, when British chemist Joseph Priestley recognized that the dried sap from these special trees could “rub out” graphite pencil marks. He named his invention “rubber,” which caught on and became the name for the material itself.

The use of natural rubber was limited by a critical property related to temperature: It becomes soft and sticky in the heat of summer, yet stiff and brittle in the cold of winter. In 1839, Charles Goodyear, living in Wolburn, Mass., made a breakthrough that would alter the course of industry when he accidentally spilled a mixture of raw rubber and sulfur onto a hot stove. The heating created crosslinks to maintain the strength of rubber at all temperatures, a process that became known as vulcanization.

The wood engraving “An Amazing Revelation” depicts Charles Goodyear discovering vulcanization after accidentally dropping rubber and sulfur onto a hot stove, circa 1839. Illustration: From James D. McCabe, 1871 engraving/Linda Hall Library (public domain)

Goodyear’s discovery launched a vast array of products, from waterproof raincoats to life jackets, along with industrial gaskets, hoses, conveyor belts and cylinder linings for steam engines.

When the first modern car — the 1886 Benz Motorwagen — came on the market, it rolled on solid rubber tires made of vulcanized rubber. The ride was slow and rough. In 1895, the Michelin brothers, André and Édouard, produced the first removable pneumatic tire, transforming automobiles from slow-moving motorized carriages to fast, long-distance vehicles.

In the early 1900s, tire chemists began blending carbon black, a fine soot, into tires to strengthen their resistance to road wear and degradation from ultraviolet light. Tires went from a light grey — the natural color of rubber — to jet black.

In 1906, chemist George Oenslager, based in Akron, Ohio, found he could speed up the slow vulcanization process with the use of “accelerators,” which are catalysts that lower the energy needed for sulfur to break its own bonds and become linked to rubber molecules. Curing time went from hours to minutes.

During the 1920s, chemists began experimenting with complex organic molecules, including chemicals in the family of phenylene diamines (PPDs). These additives helped to capture free radicals and atmospheric ozone that can break the polymer chains.

“The eﬀect of ozone on the degradation of tire-rubber compounds was not fully understood until the 1930s,” according to a report (PDF) by the Interstate Technology and Regulatory Council. “At that time, a typical tire lasted only 10,000 miles or roughly two years.”

During World War II, supplies of natural rubber from Asia became limited, so chemical engineers in the U.S. and Russia invented synthetic rubber produced from crude oil. Their products fulfilled wartime needs for rubber until rubber-tree supplies resumed after the war. During the 1950s, chemists improved the formulas for synthetic rubber, and by the mid-1960s synthetic materials outpaced natural rubber in tires.

After World War II, the U.S. Army maintained an extensive inventory of vehicles, whose tires were experiencing cracking and deterioration. Government chemists conducted a search for compounds to better preserve the rubber. They were aware of earlier antiozone compounds, such as DPPD (diphenyl-PPD, and IPPD (isopropyl-PPD).

While DPPD and IPPD both helped, DPPD reacted too slowly with ozone to fully solve the problem, and IPPD reacted so quickly that it was prematurely used up, according to the ITRC report. The “Goldilocks” chemical, with the right reactivity and ability to migrate to the rubber’s surface layer, turned out to be 6PPD, the “6” standing for a six-carbon unit as part of the PPD structure.

In 1965, the Monsanto Company obtained patent rights to the antiozonant 6PPD, which the company marketed under the trade name Santoflex 13, just one product in a line of Santoflex antioxidants. After the patents expired in the 1980s, other companies began producing and selling 6PPD, sometimes mixed with other compounds. Today, 6PPD is found in nearly every tire on the road. For other PPD derivatives, check out the table in the technical document by California’s Department of Substances Control.

Now that 6PPD has become associated with environmental problems, some of these other PPD compounds have been considered as a replacement. One candidate is CCPD, a compound that has been known to chemists for years and appears to be much less toxic to coho than 6PPD.

While 6PPD is highly effective at blocking the damaging effects of ozone on tires, the chemical reaction with ozone leads to 6PPD-quinone, one of the most toxic chemicals ever produced.

Ground-level ozone is a highly reactive compound produced under energetic conditions, such as when sunlight strikes pollutants in automobile exhaust. Ground-level ozone should not be confused with the atmospheric “ozone layer,” an essential feature of our planet that helps to block damaging ultraviolet rays. On the road, ozone is known to break the chemical bonds of rubber in tires, leading to degradation and shortening a tire’s life. The value of 6PPD is that this chemical is able to capture the ozone before it can attack the rubber. In the process, we get the formation of 6PPD-quinone.

Chemical structure

The structure and meaning of 6PPD

N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine

That is the long and cumbersome name for an amazing, yet dangerous chemical used in tires, abbreviated as 6PPD. It is a complex, multi-functional compound with a complex name to match. It’s no wonder that this formal name is rarely used except at the beginning of scientific articles.

While this complex name may seem intimidating, untangling the nomenclature is not so difficult if you understand that organic chemistry is something like Tinker Toys, Legos or a map of highways connecting cities together. Atoms link to each other according to basic rules of nature — such as carbon atoms each providing four links of attachment to other atoms. The nomenclature thus follows the structure.

Ignoring the energy consumed or delivered in chemical reactions, we can build 6PPD and other closely related chemicals from a structural point of view, starting with benzene, one of the basic compounds in organic chemistry, and then adding onto it. In common terms, C stands for carbon, H for hydrogen, N for nitrogen and O for oxygen.

Figure 1. The chemical structure of 6PPD starts with benzene (A), depicted here two ways. Carbons are assumed to be in the corners the simpler, lower version. To benzene, two amino groups (B) are added, resulting in p-phenylenediamine (C). An additional benzene ring is then added by replacing a hydrogen in one amino group, resulting in N′-phenyl-p-phenylenediamine (D) to which 1,3-dimthylbutyl- (E) is added. The result is 6PPD. Graphic: PSI

Figure 1A shows the location of carbon and hydrogen atoms of benzene and below that a simplified version, in which carbons are assumed to occupy the corners and hydrogen atoms go where needed to complete the picture. Note the four connections for each carbon atom, with double bonds counting as two.

To the benzene ring (drawn as a hexagon for convenience), we need to add two amino groups (—NH₂) (FIgure 1B) on opposite sides (Figure 1C). Amines are interesting, because they are found in many biologic systems. Adding a hydrogen to an amino group gives us ammonia (NH₃). Adding a carbolic acid group (—COOH) and a side chain can lead to one of many amino acids — the building blocks of protein and other biological compounds.

According to chemical nomenclature, if we attach one chemical group to a benzine ring, the basic ring is known as “phenyl.” If we add two groups, the ring becomes “phenylene.” So if we add two amino groups to our benzene ring, it becomes phenylene diamine, “di” meaning two (Figure 1C).

The location of the amino groups also comes into play. When they are opposite each other on the ring, as in 6PPD, they are called “para,” and may be designated with the letter “p.” Now we have the last half of our complex name for 6PPD: “p-phenylenediamine,” or “PPD” This is a recurring structure in the most basic search for alternatives to 6PPD.

Now we will build onto our growing structure, starting with an add-on to one amino group on p-phenylenediamine. This is done by replacing a hydrogen on the amino group with a phenyl group (another benzene ring). The accent mark (“prime”) on the “N” is used simply to tell one nitrogen atom from another. We now have N′-phenyl-p-phenylenediamine (Figure 1D).

Let’s now construct the last piece to be added, the 1,3-dimethylbutyl group (Figure 1E). It starts with a butyl group, which has four carbons, and then adds two methyl groups, each with a single carbon. The methyl groups are added to the first and third carbon atoms in the butyl group, thus giving us 1,3-dimethylbutyl (Figure 1E).

Next, we attach the 1,3-dimethylbutyl group to the nitrogen on the available amino group that is part of N′-phenyl-p-phenylenediamine, completing the 6PPD structure: N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (Figure 2).

Figure 2. The “6” in 6PPD comes from the six carbon atoms in the attached dimethylbutyl group. Graphic: PSI

Where does the name “6PPD” come from? The PPD part is the p-phenylenediamine that we constructed in this exercise. The number 6 comes from a shorthand used years ago by chemists in the rubber industry and eventually adopted as official nomenclature by the International Organization for Standardization. The “6” itself comes from the six carbon atoms that are found in the attached dimethylbutyl group.

Other compounds using numbers for naming include 7PPD and 8PPD, specific compounds that contain seven- and eight-carbon chains, respectively, attached to one of the two amino groups on PPD. Others are 77PD, which have identical 7-carbon chains, one attached to each of the amino groups. Likewise, for 88PD.

Among the earliest antiozone compounds were DPPD, or diphenyl-PPD, and later IPPD, or isopropyl-PPD (Figures 3A and B). “Diphenyl” indicates two phenyl groups, one on each of the amines of PPD. “Isopropyl” indicates a three-carbon propyl group (as in propane) with the attachment point on the middle carbon.

Since 6PPD has come under fire for its environmental problems, other PPD compounds are being considered as a replacement, including CCPD (Figure 3D). Like 77PD and 88PD, the double letters “CC” represent twin groups on opposite ends of our PPD structure. Specifically, CCPD has two cyclohexyl groups, while CPPD (Figure 3C) has one cyclohexyl group and one phenyl group. “Cyclo” means “circle,” so cycohexyl (and cyclohexane) is a six-carbon benzene ring with hydrogen atoms in place of double bonds between the carbons.

Figure 3. Early antiozone compounds used in tire manufacturing include DPPD (A) and IPPD (B) which differ in their phenyl and isopropyl groups. Replacement compounds for 6PPD under consideration include CPPD (C), and CCPD (D), which differ in their phenyl and cyclohexyl groups. Graphic: PSI

What scientists didn’t know until December 2020 was that many of the PPD compounds — most significantly 6PPD — produce a quinone structure while reacting with ozone. The chemical name for 6PPD-quinone is:

2-anilino-5-[(4-methylpentan-2-yl)amino]cyclohexa-2,5-diene-1,4-dione.

You don’t see that name often; most chemists just go with the abbreviation 6PPD-quinone or 6PPD-Q.

To understand the transformation, let’s start with simple benzene and bring in ozone (O3). The result is benzoquinone, as shown in Figure 4:

Figure 4. Ozone oxidizes benzene to phenol, which the diagram shows becomes more reactive, then converts to 1,4-benzoquinone. Graphic: PSI

Notice the two double-bonded oxygens tied to carbons on the benzene ring. From benzene, we can advance to ozone oxidation — or ozonation — of 6PPD (Figure 5):

Figure 5. Ozone oxidizes 6PPD through hydroxylation of the aromatic ring to produce 6PPD-quinone, the toxic compound that kills coho salmon. Graphic: PSI

As we have seen, the nature of this chemical class called quinones can be both toxic and beneficial, going well beyond the positive effects of 6PPD and the dangerous effects of 6PPD-Q. The questions now become: Can one or more safer replacements for 6PPD be found? Will chosen chemicals avoid the formation of quinone, or will they be designed with quinones that do not disrupt biological functions?

About the Author:
Christopher Dunagan is a senior writer at the Puget Sound Institute.

Following the Flow: Mapping the Road to Cleaner Water

Stories from the people protecting and recovering Puget Sound

July 7, 2026
Written by Sara Bruestle, freelance writer

Salmon are icons in Washington: Their long-distance migrations shape the state’s natural environment and robust economy. Strong salmon runs are vital to healthy ecosystems, the fishing and tourism industries, Tribal treaty rights, food security, cultural heritage, and a way of life.

With 14 salmon and steelhead populations in Washington listed under the Endangered Species Act, their recovery is a statewide investment. Washington is investing billions to restore and protect fish habitat and address threats to salmon.

One of the state’s strategies for salmon recovery is improved stormwater management. Salmon need clean streams, rivers, wetlands, estuaries, and oceans as they migrate hundreds or even thousands of miles during their lifecycles. Despite laws and regulations aimed at protecting water, population growth has contributed to increasing pollution in these waterways.

As cities and towns expand, the number of roads, bridges, and parking lots increases, resulting in more pollutants running into surrounding watersheds. Stormwater runoff can harm fish, limiting their survival rate. One pollutant derived from tires is exceptionally deadly to some salmon.

Gorst Creek

The Gorst Creek watershed on the Kitsap Peninsula is one top-ranked location the Washington State Department of Transportation (WSDOT) is prioritizing in efforts to improve water quality in critical fish habitat. It supports runs of Chinook, coho, and chum salmon, as well as steelhead and cutthroat trout.

The Suquamish Tribe releases 1.8 million salmon into the creek each year. As the salmon leave the Tribe’s rearing ponds, they imprint on the unique odors and geomagnetic fields of the Gorst Creek watershed. This ensures that years later the salmon can migrate back to their home stream to spawn.

The watershed is also home to the notorious Gorst bottleneck, where Highway 3 and Highway 16 merge at the head of Sinclair Inlet. The interchange funnels over 85,000 vehicles daily, as it serves as the main land connection between the Kitsap Peninsula and the rest of Puget Sound.

Traffic heavily backs up during Seattle-Bremerton ferry sailings and Puget Sound Naval Shipyard shift changes. Thousands of cars passing through the narrow corridor get stuck in the Gorst gridlock several times a day.

Chemicals, like those from tires, are released on roads and washed into local bodies of water where they harm salmon and other aquatic life. Credit: iiievgeniy

When it rains, pollution from the traffic washes from Gorst Creek into Sinclair Inlet, including 6PPD-quinone (6PPDQ), a toxic chemical that forms when tires degrade and that researchers identified as fatal to coho salmon in 2020.

For these reasons and more, the Suquamish Tribe and Kitsap County have marked the Gorst Creek watershed as a top priority for stormwater mitigation on WSDOT’s Stormwater Retrofit Prioritization Web Map.

“Sinclair Inlet is a pretty heavily impacted body of water because there’s a lot going on right there in Gorst,” says Joe Rutan, Kitsap County’s county roads engineer. “It’s a highly developed area that is in need of change, and we want to make sure the county is involved in that change.”

Prioritization Map

WSDOT launched the interactive web map in 2024 to pinpoint needs for stormwater retrofit projects that mitigate stormwater impacts from the state transportation system. This collaborative planning tool maps and prioritizes locations along 14,561 miles of state highway for future mitigation.

The state legislature earmarked stormwater retrofits that aid in salmon recovery, ecosystem health, reducing pollution, and addressing health disparities, pledging $500 million in a Move Ahead Washington transportation funding package.

WSDOT surveyed Tribes, towns, cities, counties, and conservation groups, asking them to rank their top priorities for stormwater upgrades. Since the map’s launch, many locations have been top ranked by more than one respondent.

WSDOT’s interactive Stormwater Retrofit Prioritization Web Map shows high priority segments of highway across the state.

“We would like to capitalize on this high level of interest and leverage our investments by accomplishing more than WSDOT could do on its own through partnerships,” says Tatiana Dreisbach, WSDOT’s stormwater retrofit outreach and innovation lead.

The map incorporates perspectives and expertise from across the state to help WSDOT update a transportation system that was primarily built before the Clean Water Act. Through partnerships, those surveyed can steer project plans toward solutions that provide the greatest return on investment – perhaps removing fish barriers or upgrading streambank protection at the same time.

“The WSDOT Stormwater Retrofit Program is working to create and streamline partnership pathways to collectively do more with limited resources, to strategically develop and redevelop the built environment in smart ways,” Dreisbach says.

WSDOT turns to the map when compiling an annual list of standalone stormwater retrofit projects for Move Ahead Washington funding.

Of the 14,561 miles of highway prioritized on the map, 63% are ranked a high priority due to pollution, 30% are ranked a high priority for the salmon population, and 3% are ranked high for existing health disparities. Twelve percent of the highways, or 1,680 miles, score high in all three areas of concern. This includes the Gorst Creek watershed.

Toxic Stormwater

Decades ago, coho salmon traveling upstream through urban waterways to spawn started showing alarming symptoms: They were disoriented, rolling onto their sides, gasping for air, and dying before they could lay their eggs. No one understood why.

For years, scientists tested the water and ruled out typical culprits like heavy metals, disease, high water temperatures, and low oxygen levels. The actual killer was an invisible byproduct of tires.

In 2020, Washington State University and University of Washington researchers linked 6PPDQ to these mass die-offs of coho salmon around Puget Sound over the last 25 years.

Water monitoring data helps Kitsap County meet regulatory requirements and make informed decisions about future water quality improvement projects. Photo courtesy of Kitsap County.

Tire manufacturers use a preservative called 6PPD to prevent rubber from cracking. As tires roll, they shed rubber dust onto roads. When the dust reacts to ozone in the air, the 6PPD transforms into 6PPDQ.

As 6PPDQ is recognized to be one of the most toxic pollutants to fish, there is a widespread urgency to address stormwater impacts. Trace levels – as low as 41 to 95 parts per trillion – are lethal to coho salmon and harmful to other sensitive salmonids, such as steelhead and cutthroat trout. As little as half a drop in an Olympic-sized swimming pool can be lethal. Researchers found that exposed coho salmon experienced mortality rates of 40% to 100%. The fish can die within hours.

In response, the state legislature authorized $500 million over 16 years, starting in 2023, for WSDOT stormwater retrofits as part of its Move Ahead Washington funding package. An additional $15 million was added in 2023 for the legislature’s request, which emphasizes green solutions such as bioswales.

Moving Ahead

WSDOT’s Move Ahead Washington pilot project is on the other side of Puget Sound, 68 miles from Gorst by highway. The Ship Canal Bridge Stormwater Treatment Facility in Seattle will filter polluted runoff from I-5 before it flows into Lake Union.

The project’s scope could treat stormwater from up to 80 acres across multiple jurisdictions, making it WSDOT’s largest standalone stormwater retrofit project to date. It also represents the busiest stretch of highway in the state: Nearly 240,000 vehicles travel across the bridge daily.

“The Ship Canal Bridge has the highest volume of traffic, so it’s some of the dirtiest water coming from the state transportation infrastructure,” Dreisbach says. “This project will be a huge win for water quality and salmon enhancement.”

WSDOT is in the pre-design phase for the project, which includes data collection and community engagement for the planning and development of three design alternatives. Treatment facility construction is expected to begin in 2028.

Over Move Ahead Washington’s 16 years, the state legislature has ambitiously committed to fund $515 million for stormwater retrofit projects. However, recent budget constraints have limited Washington’s ability to provide a steady stream of funding.

WSDOT faces some challenges with the current funding schedule. No funding is allotted between 2027 and 2031, while over $300 million is budgeted between 2035 and 2039. With cash flow issues, the department will be pressed for time and resources to deliver results.

“WSDOT cannot implement a steady stream of stormwater retrofit projects due to funding gaps and pulses, limiting WSDOT’s ability to address urgent stormwater impacts from 6PPDQ and other pollutants in the near term,” Dreisbach says, adding that the hope is that the legislature will optimize the package’s funding schedule.

Kitsap County staff collect information on water quality, stream conditions, and aquatic habitat to identify potential issues, track trends over time, and assess whether stormwater management efforts are effectively protecting streams. Photo courtesy of Kitsap County.

Tribal Interests

Since 2024, 15 federally recognized Tribes have marked 50 locations on WSDOT’s Stormwater Retrofit Prioritization Web Map as top priorities for stormwater mitigation, most of them along state highways encompassing Puget Sound: Ground Zero for 6PPDQ. Tribal rankings are elevated to the top of the list for project scoping. Not only does WSDOT value Tribal rights and interests, but Tribes’ priorities such as water quality and salmon recovery merge with the department’s.

The Suquamish Tribe marked the Gorst Creek watershed as a top priority for stormwater mitigation because it is a 6PPDQ hotspot and the location of salmon restoration work. In addition to its Gorst Creek Rearing Facility, the Tribe operates two Chinook rearing ponds and yearling raceways in the creek. All together, the Tribe’s annual hatchery runs add up to an estimated 2.1 million salmon. While 1.8 million Chinook salmon are released into the creek, another 300,000 coho are transferred to the nearby Agate Passage net pens.

“For more than 40 years, the Suquamish Tribe has operated the Gorst facility to protect and enhance salmon populations,” says Charlie Kratzer, Suquamish Tribe’s hydrologist. “Through a network of hatcheries, rearing facilities and marine net pens the Tribe supports ceremonial, subsistence, and commercial harvests while contributing to the long-term recovery of natural salmon stocks.”

The Suquamish Tribe and Kitsap County hope that the Gorst Creek watershed will make WSDOT’s project shortlist. Kratzer and Rutan agree that the WSDOT Stormwater Retrofit Prioritization Web Map is a useful tool that will help meet the state’s priorities for stormwater management improvements. They’re both interested in partnering on a stormwater retrofit project that will filter contaminants like 6PPDQ from polluted runoff before it reaches streams, helping improve salmon survival.

“How our county roads interact with the state roads and the water is important,” Rutan says. “We need to address that in an environmentally sensitive, caring way.”

Other Ways Washington is Addressing Stormwater Impacts

Washington leads the nation in addressing 6PPD and its toxic derivative, 6PPDQ, following its discovery in 2020. Funding stormwater retrofits that treat the salmon-killing chemical is just one of many strategies in addressing the crisis. In addition:

Tribes petitioned the Environmental Protection Agency (EPA) to establish risk-management rules to restrict or eliminate 6PPD use in tires. The EPA granted the petition in 2023.
In 2024, Washington became the first state to pass legislation regulating 6PPD in tires. It requires manufacturers to report consumer products that contain 6PPD.
The Washington State Department of Ecology (Ecology) is collaborating with manufacturers to find alternate chemical preservatives that could replace 6PPD in tires, and evaluate if those chemicals are actually safer.
Cities are implementing specialized bioretention systems such as rain gardens and engineered soils to filter out 6PPDQ in stormwater runoff, preventing the chemical from washing into waterways. Ecology is also testing new stormwater treatment technologies.
Ecology now oversees statewide water testing in critical streams, wetlands, and estuaries to monitor the presence of 6PPDQ. The data is used to track 6PPDQ hotspots to guide stormwater management and targeted water-quality cleanup projects.
In 2024, Washington was also the first state to establish a numeric water-quality limit for 6PPDQ to regulate contamination. The legal limit is 12 parts per trillion.
During the 2026 state legislative session, the House introduced a bill requiring manufacturers to phase out 6PPD in tires sold in Washington. Although the bill didn’t pass, legislators are likely to propose similar bills in the future.

The Washington State Department of Transportation is implementing Box of Rain, a new stormwater treatment technology, to help prevent stormwater pollutants from reaching salmon-rearing streams. Photo courtesty of WSDOT.

More WSDOT Stormwater Management Partnerships

WSDOT is also working to improve water quality throughout Washington with programs like Box of Rain, Adopt-a-Downspout, and the Roadside Vegetation & Beautification Permit.

Box of Rain is a new stormwater treatment technology developed in partnership with The Nature Conservancy and Stewardship Partners. Like a rain garden in a box, the bioretention system removes pollutants from a bridge’s downspouts before they can enter nearby waterways.

Adopt-a-Downspout is a Stewardship Partners model that asks community volunteers to maintain local Boxes of Rain. For example, the Northwest Fly Anglers have adopted the boxes installed at the Ship Canal Bridge. More volunteer opportunities are in the works.

The new Roadside Vegetation & Beautification Permit expands the scope of an existing permit for WSDOT right-of-way improvements to do more than plant trees and shrubs. It now includes stormwater mitigation to reduce pollutants and control flows. For example, the Duwamish Valley Industrial Greening group applied for the permit to create a stormwater interpretive center in the Georgetown neighborhood of Seattle. The project plans to use the Box of Rain technology to reduce stormwater pollution from an on-ramp to I-5, provide community access to a beautified right-of-way, and install interpretive boards explaining the value of stormwater mitigation.

For more information, view the WSDOT Stormwater Retrofit Prioritization Web Map or visit WSDOT’s Stormwater Retrofit Move Ahead Washington toolbox.

Tire companies race to replace deadly tire chemical

By Christopher Dunagan
Salish Sea Currents Magazine

Published June 05, 2026

The chemical known as 6PPD protects the rubber in tires from degrading by preventing the formation of cracks in the tread and sidewalls. But when 6PPD meets ozone, it transforms into 6PPDQ, a chemical deadly to some species of salmon and trout. Photo: Griffinstorm/Wikimedia (CC BY-SA 4.0)

A chemical found in tires is leading to the deaths of untold numbers of coho salmon in Puget Sound every year. Chemical companies are searching for an alternative but say the unique chemistry and function of 6PPD are major challenges. This article is part one of a three-part series.

In Seattle’s Longfellow Creek, coho salmon are still dying in large numbers before they get a chance to spawn, according to observers. But now a long-term solution to the problem may be inching closer.

Just over five years ago, scientists identified a deadly chemical associated with automobile tires that has been blamed on the untimely deaths of thousands of coho and other vulnerable salmonids. Such losses have been observed not only in Longfellow Creek but in urban streams throughout the Salish Sea. Based on known and suspected toxic levels, salmon may be dying in populated areas from California to Alaska, while numerous trout and char species may be experiencing problems in waterways throughout the world.

Coho salmon in Longfellow Creek are dying before they have a chance to spawn. Photo: AdobeStock

Researchers have been exploring potential alternatives to the tire chemical, known as 6PPD, which blocks destructive ozone before it can attack and destroy the rubber in tires. In the process, 6PPD transforms into 6PPD-quinone (6PPD-Q), one of the most toxic agents known to science.

For nearly 40 years, observers have been watching returning coho become disoriented, keel over and die. Over time, they narrowed the list of suspects — from something in stormwater to automobiles, to tires and finally to a single specific chemical. The effects of 6PPD-Q still reverberate through the aquatic food web, where losses to coho and other populations are immeasurable, experts say.

Although the chemistry of 6PPD has been well known to tire engineers, nobody ever took a serious look at the transformation product, 6PPD-Q, until focused studies examined what was killing coho in Puget Sound. Using advanced techniques, University of Washington researchers with our affiliate the Center for Urban Waters — along with scientists from Washington State University and the National Oceanic and Atmospheric Administration — were able to isolate 6PPD-Q from more than 2,000 compounds found in stormwater.

Now, getting 6PPD out of tires and eventually out of streams has become an all-out goal of numerous tire manufactures, government and nongovernment scientists, as well as agencies responsible for the health of our ecosystems. Outside pressure also is increasing from individuals and organizations concerned about the future of Puget Sound and other waterways.

Alternatives under study

At least a dozen companies and groups associated with the tire industry are working on ways to remove 6PPD from tires. But tire chemistry is complicated, involving 10 to 15 different constituents working together. No simple solutions have emerged.

Possible replacements under review include chemicals closely related to 6PPD along with newly invented compounds. Some ideas would necessitate significant changes to the rubber-making process. Other ideas are focused on innovative tire designs, a few made with little or no rubber.

Despite the intense effort, finding a replacement for 6PPD is widely recognized as a monumental challenge. That’s because the chemical is exceedingly good at its job — which is to extend the life of tires. For the better part of a century, chemists have been perfecting ways to keep tires from falling apart. Today’s designs cover a wide range of tires with 6PPD as a universal constituent, making tires last longer while increasing safety for drivers and passengers.

“The safety, performance and sustainability” of tires remain the “uncompromising priority” of the U.S. Tire Manufacturer’s Association, said Stephanie Schlea, the organization’s vice president for environment, health, safety and sustainability.

USTMA is leading a consortium of 36 tire-related companies searching for alternatives to 6PPD. As of last August, the consortium was studying 24 potential alternatives, but USTMA officials say that number may have changed as studies advance.

“The chemicals present in tires today all perform specific and integrated functions, and tire composition cannot responsibly be modified without great care, including extensive and rigorous testing,” said Schlea in a letter (pdf) to the Washington Department of Ecology. “Any alternative identified must continue to ensure compliance with Federal Motor Vehicle Safety Standards and other consumer, vehicle and tire manufacturer requirements.”

Rubber formulations vary depending on the type and weight of a vehicle along with different traction specifications for weather and road conditions. While many rubber additives are used in various parts of a tire, 6PPD has become universally accepted as the leading chemical for rubber preservation.

Players in the race for 6PPD alternative

Tire manufacturing facility equipment holding tires

6PPD in action

Here’s how it works: Natural rubber is highly vulnerable to an attack by ground-level ozone, often produced from pollution (including automobile emissions) in the presence of sunlight. The 6PPD on a tire’s surface reacts rapidly with the ozone before it can attack the rubber, which would otherwise crack and eventually break apart.

As 6PPD reacts with ozone, the chemical is used up and must be replaced by 6PPD that migrates to the tire’s surface from the underlying rubber. If 6PPD migrates too slowly, it won’t be there in time to protect the rubber from ozone. If it migrates too quickly, it can dissipate and be used up prematurely. Some experts call 6PPD the “Goldilocks” of tire preservatives for its optimal rate of migration and rapid reaction with ozone.

Any replacement compound must maintain a defensive posture at the surface of the tire, but that’s not the only benefit derived from 6PPD. For example, heat and mechanical stress can create highly reactive free radicals within the rubber polymer chains, which provide structure to the tire. The presence of oxygen encourages chain reactions, which break apart the rubber at the molecular level. The presence of 6PPD quells the chain reactions by “scavenging” free radicals. These antioxidant properties — separate from ozone blockage — mean that 6PPD does double duty in maintaining a tire’s integrity.

In the sidewalls of tires, 6PPD also helps to maintain a flexibility essential to a tire’s performance. Engineers also tout 6PPD’s ability to mix easily into natural rubber and man-made polymers that make up the bulk of a tire. The chemical also performs well during a heating process, called vulcanization, which results in chemical bonding with sulfur, adding strength yet flexibility to the final product.

The multiple attributes of 6PPD make it difficult to find a single chemical to replace it, said Erick Sharp, CEO and founder of ACE Laboratories in Northeast Ohio, a firm involved in the development and testing of potential alternatives to 6PPD.

“We have tried a lot of things, and I don’t think you are going to have a one-for-one replacement for 6PPD,” Sharp said. “The final solution is likely to be a multi-faceted system replacement.”

For example, if a replacement chemical inhibits the vulcanization process — unlike 6PPD — then another chemical might be added to maintain the needed “cure” rate for a specific tire, Sharp said. Virtually all tire manufacturing involves chemicals, called accelerants, that can help establish a cure rate conducive to the desired chemical bonding among raw materials. If the cure rate is altered significantly by new chemicals, then a manufacturing plant might need to be restructured for additional processing, he said.

Of course, any new rubber formulations must be tested for toxicity. An initial test in Sharp’s lab involves exposing new rubber candidates to high levels of ozone and identifying the transformation products released into water. Toxicity, based on published data, is a major factor in deciding whether further study of a particular material is warranted.

Although the challenge is considerable, one or more viable alternatives to 6PPD may not be far away, Sharp said. “With what we have seen, I think all the tools are in the toolbox to get there.”

Urgency for fish populations

Longfellow Creek, which drains a 2,685-acre area in West Seattle, has become Exhibit A for the killing effects of 6PPD-Q on coho salmon. The once-natural stream runs through the heart of the immense Duwamish Industrial Area. Around 1939, it is believed that a localized coho population went extinct, probably because of pollution and fish-passage problems — including a 3,300-foot culvert that carries the stream under parking lots and industrial facilities before releasing the water into the West Duwamish Waterway.

Approximate location of Longfellow Creek which drains a 2,685-acre area in West Seattle. The stream’s pathway includes a 3,300-foot culvert that conveys water under parking lots and industrial facilities before releasing it into the Duwamish River. Map: PSI using Google Earth imagery.

During the 1990s, community groups and volunteers began to restore natural streamside vegetation in an area just upstream of where the stream dives into the long culvert. Around the same time, rocks were added just below that culvert to raise the downstream channel, allowing fish an easier entryway to the culvert when they return from the ocean. In addition, “skylights” were installed by cutting into the top of the culvert to increase light and encourage salmon to swim through the otherwise dark and uninviting pipe.

By the end of the 1990s, coho were beginning to return to Longfellow Creek, a likely result of stream and fish-passage improvements. The first to arrive apparently were strays from hatchery operations. For the first time in years, fair numbers of coho were seen in the vegetated section of Longfellow Creek upstream of the long culvert, as well as areas farther upstream.

But all the work on stream restoration became a somewhat hollow victory, because of the unexpected and gruesome deaths among the returning coho. Many fish that survived the long and tortuous trip from the ocean were dying on the doorstep of their home stream before they could complete their life cycle with reproduction for a new generation.

In some years, the rate of so-called pre-spawn mortality for coho in Longfellow Creek has been estimated as high as 90 percent. Last fall, among the 130 coho that were found dead in the creek, 55 percent had died before spawning, according to observers with Puget Soundkeeper, an environmental group that has been monitoring coho returns since 2015. Another 20 percent were believed to have spawned, but spawning success or failure could not be determined for 24 percent of the dead fish, according to reports.

Another 366 coho were spotted alive in the stream, but that includes an unknown number of fish that may have been counted more than once.

Scientific investigations have found that the highest concentrations of 6PPD-Q can be measured in streams whose watersheds contain a multitude of heavily traveled roads. In Puget Sound, such areas are located in Seattle, Tacoma and Everett, as well as heavily developed areas throughout the region, according to computer models. Concentrations of 6PPD-Q have been found to spike during and shortly after rainstorms, when accumulated toxics are washed off the roads.

Sean Dixon, executive director of Puget Soundkeeper, said he appreciates the research being done to find an alternative to 6PPD, but the urgency doesn’t seem to match the hazard. People need to understand that this tire chemical is one of the most toxic chemicals ever produced by humans, he said, arguing that it has been getting into waters throughout the world and causing serious ecological effects — some known but some yet to be revealed..

“From the perspective of the fish, a ban on 6PPD needs to be in place now,” Dixon said. “You can have the world’s best rainbow trout stream, but if a county road or bridge is nearby, you will have tire particles going down into the stream, and it won’t be the same.”

Dixon noted that it has been more than five years since 6PPD-Q was discovered and 40 years since dying fish were first reported in urban streams. Still, representatives for the tire industry say a solution could be years in the future — and that’s hard to accept, he added.

“It blows my mind in this day and age that we can’t move faster when the whole world recognizes the problem,” Dixon said. “I think we could (move faster) if society decided that we need to.”

Unfortunately, he continued, 6PPD is just one of many problems that salmon encounter in Puget Sound today. Others are warm waters caused by loss of streamside vegetation, fish-passage problems caused by poor road building, and a variety of dangerous chemicals from multiple human sources.

Dixon pointed out that “green infrastructure,” in which natural materials are used to filter stormwater, have been effective at reducing pollution, including 6PPD-Q. Because of the cost of retrofitting stormwater systems in urban areas, many jurisdictions have been slow to make renovations, he noted.

“There are solutions,” Dixon said. “We just need to align our management of the environment with the solutions that we know exist.”

Beyond coho salmon

The search for alternative chemicals that could replace 6PPD is closely connected to toxicological studies examining the chemical’s effects on a multitude of species, including humans. Studies have found that coho salmon are extremely sensitive to 6PPD-Q, succumbing to death at lower concentrations than any other species examined so far — although data remains sparse for many animals.

Recent studies have shown that young coho, as well as older fish, are highly susceptible to 6PPD-Q. If exposed, they can be killed in urban streams during their early rapid-growth period, when coho typically spend their first year of life in freshwater.

Recent information shows that coastal cutthroat trout are nearly as sensitive to 6PPD-Q as coho salmon. Photo: Lukeward6/iNaturalist (CC BY-NC)

Dying coho alerted experts from the beginning about a serious pollution problem in urban streams, and coho remain the most sensitive to 6PPD-Q of any species studied. But new information is continually coming forth. Last year, researchers with the U.S. Geological Survey in Seattle reported that coastal cutthroat trout are nearly as susceptible to the chemical as coho. Other studies have shown that lake trout, brook trout and whitespotted char are somewhat less sensitive. Rainbow trout come next on the sensitivity scale, while Chinook, sockeye and pink salmon are far less sensitive, being able to survive under extreme environmental conditions. Chum salmon seem to be the least susceptible of all.

These reports of sensitivity are based on experiments that measure the minimum concentration of 6PPD-Q that can kill half the fish in a sample when exposed for a specified amount of time. Researchers are just beginning to understand what may happen when fish are exposed at lower, sublethal concentrations. For example, ongoing studies have shown that the chemical can impair muscle function and reduce metabolism, with reports of diminished swimming ability under some conditions.

How 6PPDQ affects fish at the cellular level is not yet well understood, but several studies have revealed that it can cause leaky blood vessels, notably in the brain and gills. Metabolic disruption and immune system activation have been found in some experiments, with potential but uncertain links to vascular dysfunction. Interestingly, some studies suggest that differences in 6PPD-Q sensitivity among species may be related to how quickly different fish are able to detoxify the chemical in their livers.

As for effects on humans, 6PPD-Q has been found in multiple human tissues, and studies on laboratory animals suggest that the chemical may be toxic to people, according to the Washington State Department of Health. More research is needed to determine what human health conditions may be caused by 6PPD-Q and at what levels. Researchers are investigating metabolic stress, reproductive deficiency, developmental impairment, DNA damage and potential effects on various organs.

On a broader scale, 6PPD-Q may disrupt entire ecosystems by affecting organisms at the base of the food web. Limited research so far shows that growth, reproduction and metabolism may be impaired in freshwater invertebrates — including microalgae, water fleas and snails.

First look at alternatives

A first rapid assessment of alternatives to 6PPD was published by the Washington Department of Ecology in November 2021, less than a year after the discovery that 6PPD-Q was killing coho. But the report considered only the toxicity of 6PPD compared to 10 early replacement candidates. It did not evaluate potential transformation products that could result from real-life conditions, such as exposure to ozone.

Six of the chemicals mentioned in the report are closely related to 6PPD, and some could potentially produce quinone forms, the authors acknowledge. If so, those transformation products might be more toxic than the alternatives themselves — such as when ozone transforms 6PPD into the far-more-toxic 6PPD-quinone. With little or no information available about potential transformation products and their effects on coho salmon, the report concluded that more research was needed.

The other four chemicals mentioned in the report appear to be less hazardous than 6PPD-Q, but limited information is available on their transformation products. Three of the four may not sufficiently protect rubber from ozone attack, according to chemical profiles, reinforcing the idea that a simple replacement may not be feasible.

By July 2024, a consortium of tire companies had finalized what is called a “Stage 1” analysis required under California’s Safer Consumer Products law. The analysis selected seven possible alternatives for further study from an original list of 60 candidates given consideration. A month later, during an annual update, the consortium reported in writing that 24 unidentified chemicals were still being reviewed.

Of the seven in the Stage 1 report, four are close relatives of 6PPD. All of those showed promising potential for ozone protection. While their quinone forms had various toxicities, they all appeared to be less hazardous than 6PPD-Q, according to the report. Whether their toxicities are within an acceptable range is a question yet to be answered.

The remaining three are octyl gallate, a food preservative with antioxidant properties; Irganox, an antioxidant compound often used in plastics; and specialized graphene, a substance in which carbon atoms interlock together to form super-thin sheets. Graphene proponents say the material can increase the strength of rubber and possibly reduce, but not eliminate, the need for anti-degradant chemicals. All these are undergoing further review. (See alternatives analysis reports.)

Final recommendations from the consortium are due on Aug. 19 under California regulations. (Tires came under limited state authority in October 2023, when the state’s Department of Toxic Substances Control declared tires with 6PPD as a “priority product.”) Officials with the U.S Tire Manufacturing Association have expressed confidence that one or more viable alternatives to 6PPD will be reported as part of the upcoming “Stage 2” report.

Transformation products resulting from the ozonation of chemicals in the PPD family remain a concern for those seeking alternatives to 6PPD. But the risks became a little clearer recently in a study (pdf) conducted by the U.S. Geological Survey. Instead of using live fish, the study measured the effects of various transformation products on immortalized cell lines from coho salmon, Chinook salmon and rainbow trout.

Increasingly used in toxics studies, immortalized cell lines result from normal cells that are altered in a lab to keep them dividing indefinitely. These laboratory cells respond to chemical exposure like normal cells in many ways. The advantage is that results can be achieved more quickly and consistently without exposing live animals.

Justin Greer, a USGS toxicologist at the Western Fisheries Research Center in Seattle, said studies of tire additives were turned upside down by the discovery of 6PPD-quinone, which is not a tire additive at all. Rather the chemical is the result of the reaction of 6PPD with ozone, a chemical that can destroy a tire under normal driving conditions.

To find an alternative to 6PPD, chemists must understand not only the toxicity of the tire additive but also the toxicity of the transformation products generated through chemical reactions on the road and in the environment, Greer said.

“Specifically,” he added, “tire chemicals are meant to react under normal conditions — so we know that the transformation products are important.”

The USGS study examined the effect of ozonation products — include quinone forms — of five compounds closely related to 6PPD, including some still under consideration as potential replacements for the tire additive.

As expected, 6PPD-Q was highly toxic to coho salmon, as revealed by a disruption in metabolic activity in the laboratory-grown cells. Two related chemicals, 7PPD-Q and IPPD-Q showed metabolic declines in coho cells but at higher concentrations than for 6PPD-Q. Metabolic activity for the Chinook salmon and rainbow trout cells were not significantly affected by the quinone forms of either 7PPD or IPPD.

Expanding the experiment, the researchers looked at the full mixture of transformation products, many unidentified, following ozone treatment of the base PPD chemicals.

“We observed that ozonated mixtures of 6PPD, 7PPD and IPPD were typically more toxic that the purified quinones,” states a report on the project. Those findings suggest that the PPD-quinones are not the only transformation products harmful to fish; others not only exist but can add to the toxicity of the quinones alone.

For the Chinook cell line, no effects were seen when exposed to pure 7PPD-quinone, a single transformation product. Yet multiple transformation products resulting from ozonation of 7PPD produced a measurable reduction in metabolic activity. Likewise, for rainbow trout, the ozonated mixtures produced greater toxic effects than pure quinones not only for 7PPD but also 6PPD and IPPD.

While 6PPD may have a variety of transformation products, it appears that 6PPD-Q, the quinone form, is by far the most potent to coho salmon. The effects of other chemicals formed during ozonation are not even comparable.

For three other relatives of 6PPD, namely DPPD, 77PD and CCPD, no effects were seen in the lab-grown cells of coho, chinook or rainbow trout when exposed to their quinone forms. The researchers note, however, that immortalized coho cells are less sensitive than living coho cells, so the question of sensitivity among all the test cells warrants further examination.

While these experiments clearly demonstrate the importance of studying the full range of transformation products, this study did not try to identify the actual chemicals in the mix. Consequently, the authors could not entirely rule out the possibility that unknown contaminants had influenced the results.

About the Author

Christopher Dunagan is a senior writer at the Puget Sound Institute.

6PPD Update – January 2026

Communications and Outreach

Ecology’s current work to address 6PPD and 6PPD-quinone (6PPDQ) is highlighted below. This update includes current agency actions. Contact us at 6PPD@ecy.wa.gov.

Momentum Builds for Solutions to Tire Wear Pollution: Last month, Ecology hosted a virtual 6PPD State of the Science Forum, connecting researchers, policy practitioners, and partners from across the globe to share knowledge and accelerate progress.

Report reveals how urban runoff continues to threaten coho salmon health

January 27, 2026
Puget Soundkeeper

Link to My Edmonds News article

Pre-spawn mortality – *Photo courtesy Puget Soundkeeper*

Since 2015, Puget Soundkeeper has monitored coho salmon health in Seattle’s Longfellow Creek, producing data on the impacts of urban pollution on salmon survival. For years, Longfellow Creek has been the epicenter of research studying the effects of 6PPD-quinone — a toxic tire chemical lethal to coho salmon.

According to a Puget Soundkeeper news release, exposure to 6PPD-quinone causes a condition called Urban Runoff Mortality Syndrome (URMS), a disease characterized by symptoms such as disorientation and gasping for air, often killing coho within 24 hours. Furthermore, this chemical has been strongly linked to Pre-Spawn Mortality (PSM), where adult salmon die before successfully reproducing, the news release said.

Puget Soundkeeper’s annual Pre-Spawn Mortality Survey analyzes the spawning success of coho salmon in Longfellow Creek to better understand the impacts of 6PPD-quinone. From October through December, trained volunteers count and dissect returning coho salmon, particularly assessing the presence of eggs and milt to determine whether spawning was successful.

Results from the 2025 study show that 55.5% of coho salmon died before spawning. This aligns with data collected since 2015, which shows that 49-90% of returning salmon die before reproducing each year.

“This level of pre-spawn mortality is devastating,” said Ewan Henderson, clean water program specialist at Puget Soundkeeper. “The dramatic reduction in successful spawning opportunities is a huge blow to our ecosystems, particularly for our resident orcas who already face major challenges as their food sources dwindle.”

According to the press release, Green Stormwater Infrastructure (GSI) projects, such as rain gardens or bioswales, can effectively remove toxins from runoff and prevent harmful chemicals from entering local waters. In addition to GSI projects, a bill (HB 2421) aimed at removing 6PPD from tires was introduced to the Washington State House and Senate Environment Committees last week.

Puget Soundkeeper will be recruiting the next cohort of salmon surveyors in August. More information will be made available on their website.

To learn more about the 2025 Salmon Survey report results, visit pugetsoundkeeper.org/volunteer/salmon-surveys.

Reader view: Dead coho in Shellabarger Creek — was stormwater runoff to blame?

By Greg Ferguson and Jane O’Dell
December 6, 2025

*A dead coho salmon. (Photo courtesy Students Saving Salmon)*

Link to My Edmonds News article

On Oct. 16, a group of Edmonds-Woodway High School students with the Students Saving Salmon Club were performing routine water quality testing in Shellabarger Creek when they saw something disturbing: a young coho smolt swimming weakly on its side. On the shore nearby was another dead coho smolt.

New Test Measures Toxin from Tires in Marine Life, Helping Understand Risk to Marine Species and Ecosystems

NOAA Fisheries
September 17, 2025

NOAA scientists develop method to track toxic chemical in finfish, shellfish, and marine mammals.

Scientists discover ‘potential breakthrough’ in protecting salmon from urban killer

March 28, 2025 at 6:00 am

By Amanda Zhou
Seattle Times staff reporter

For decades, toxic tire dust has choked coho salmon before they can spawn in their natal streams. Now, King County scientists say they have made a “potential breakthrough” in how to save them.

Western Washington University student researchers split a sample of treated stormwater into smaller amounts for analysis. (Courtesy of Curtis Hinman)

Link to Seattle Time article

Why are so many Coho salmon dying? The answer might be in your tires

By Camila Domonoske, Jessica Yung, Rebecca Ramirez
November 20, 2024

For decades, Coho salmon were turning up dead in urban streams the Pacific Northwest. The salmon would stop swimming straight, and then die before they had a chance to spawn. Researchers worried that unless they figured out the cause, the species would eventually go extinct.

From tires to nail polish, we’re studying a new round of consumer products with toxic chemicals

Draft report open for formal public comment

by Cathy Hamilton-Wissmer
November 1, 2024

In May 2024, our Safer Products for Washington team identified a new set of toxic chemicals in everyday consumer products. That’s important because it gives us an opportunity to reduce exposure to toxic chemicals and prevent their release into the environment. We want your feedback on the products we’ve identified as significant sources of these toxic chemicals.

Tire anti-degradant (6PPD) and 6PPD-quinone (6PPDQ)

Department of Ecology

6PPD is a chemical that prevents automotive tires from degrading (i.e., breaking down) and helps them last longer. When 6PPD is exposed to air, it reacts with ozone to create 6PPD-quinone (also known as 6PPDQ). 6PPDQ is lethal to coho salmon and can contaminate water systems.

We work with Tribal and local governments, state and federal agencies, academic institutions, and industrial organizations to reduce the pollution and sources of 6PPDQ released from tires in Washington.

Article continues…

Link to DOE article