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March/April 2024

Beyond PFAS: What’s up with emerging contaminant N-(1,3-Dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD)?

Sean Thomas Dixon and Kelsey C Furman


  • 6PPD-quinone is one of the most toxic chemicals to aquatic organisms known to science. 
  • Managing 6PPD/q crosses a host of legal regimes, from chemical safety and stormwater control to solid waste, recycling, and endangered species. Like PFAS, 6PPD/q demands cross-agency and nationwide planning.
  • Many initiatives have been launched in recent months to coordinate 6PPD/q planning and response including an EPA workgroup and testing method, stormwater permits, a TSCA petition, and ESA lawsuits. 
Beyond PFAS: What’s up with emerging contaminant N-(1,3-Dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD)?
Ray Kachatorian via Getty Images

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In December 2020, a research team led by scientists at the University of Washington-Tacoma and Washington State University published details about a new chemical—6PPD-quinone, a transformation product of 6PPD formed through interaction with ozone—that causes extreme mortality in several salmonids, including coho salmon. For over a decade leading up to the discovery, the cause of Urban Runoff Mortality Syndrome (URMS), or the stormwater-driven death of salmon prior to spawning, had been known to be connected to cars, tires, and roads, but the underlying chemical remained a mystery. As detailed in a Trends article in 2022, this discovery led to a host of legal conversations and research programs across the nation that then largely focused on California and the Pacific Northwest.

Recently published research about 6PPD-quinone has determined that it is one of the most toxic chemicals to aquatic organisms known to science. In comments made to the Environmental Protection Agency (EPA), the attorneys general of five states (Connecticut, Oregon, Rhode Island, Vermont, and Washington) noted that while coho are the most susceptible to 6PPD-quinone, other species, including rainbow trout (also known as steelhead), are impacted as well, at concentrations found in stormwater. They cited detections of 6PPD-quinone in waterways around Los Angeles (4.1 to 6.1 μg/L), San Francisco (1.0 to 3.5 μg/L), and Seattle (0.3 to 3.2 μg/L) that led to mortality in affected species.

Species          LC50 (μg/L) Test duration (h)
Coho salmon (Oncorhynchus kisutch) 0.04, 24 0.08, 25 0.095 2 24
White-spotted char (Salvelinus leucomaenis pluvius) 0.51 26 24
Brook trout (Salvelinus fontinalis) 0.59 3 24
Rainbow trout/steelhead (Oncorhynchus mykiss) 0.64, 29 1.0,3 2.26 5 96
Chinook salmon (Oncorhynchus tshawytscha) 67.3 24, 82.1 25 24

Reported 6PPD-quinone LC50 concentrations of salmonids, from the Interstate Technology and Regulatory Council (ITRC) Focus Sheet on 6PPD and 6PPD-quinone.

Managing 6PPD and 6PPD-quinone (grouped herein as “6PPD/q”) crosses a host of legal regimes, from chemical safety and stormwater control to solid waste, recycling, and endangered species. Much as the PFAS family of chemistries demanded cross-agency and nationwide planning, quite a few initiatives have launched in recent months to coordinate 6PPD/q planning and response. At the EPA, which has funded URMS research for decades, a work group involving members of research, permitting, headquarters, regional, and chemical safety teams was launched in 2022; recent work by this team includes increased funding for green infrastructure retrofits (a proven pathway for protecting salmon from 6PPD/q), a closer look at tire wear particles as microplastics, and more.

In California and Washington, 6PPD has already been inserted into each state’s safer chemicals program. Under the California Safer Consumer Products regulations, manufacturers of tires for sale containing 6PPD needed to submit Priority Product Notifications for those products by November 2023, and begin work toward alternatives by 2024. Washington launched an Action Plan process in January 2024, bringing together Tribes, local governments, 6PPD and tire manufacturers, community organizations, and state and federal agencies to workshop approaches to stormwater management, 6PPD alternative assessments, and chemical management. Representatives from these states are co-leading an Interstate Technology and Regulatory Council (ITRC) Team on 6PPD and tire anti-degradants that expects to produce a full guidance document by spring 2024.

Clean Water Act issues on the horizon

Two key permits for facilities in Washington State, and a recently released testing methodology, may provide an indication of where stormwater management may be headed in jurisdictions with 6PPD/q–affected fish.

First, in early 2023, the EPA issued a draft stormwater permit for Joint Base Lewis-McChord (JBLM), located in Pierce County, Washington, along and around the Nisqually River and nearby watersheds. This base has stormwater draining to creeks and waterways that are listed as Essential Fish Habitat for, among other fish, coho salmon. As such, the draft permit proposes requiring, “for any retrofit projects initiated during this permit term,” the selection of projects “that have been shown to effectively reduce the pollutants . . . 6PPD-quinone in stormwater discharge as detailed in Ecology’s June 2022 Stormwater Treatment of Tire Contaminants Best Management Practices (BMP) Effectiveness Report.” See JBLM MS4 Fact Sheet, at 27. Washington Department of Ecology, Stormwater Treatment of Tire Contaminants Best Management Practices Effectiveness, 2022. As of this writing, the agency has yet to finalize this permit.

Second, in August 2023, the Washington Department of Ecology (Ecology) released for public comment draft stormwater (MS4) general permits for Phase I and II entities across the state. Of note, the draft permits do not actually mention 6PPD or 6PPD-quinone, but in two places includes tire wear particles as one of many elements to consider in the creation of stormwater management plans. In one section of the Draft Phase I permit, Ecology proposed requiring, for one subwatershed within the permittee’s jurisdiction, “a description of the stormwater facility retrofits needed . . . , including the BMP types and preferred locations and projects to address transportation related runoff, such as tire wear pollutants.” The other reference concerns street sweeping (a new requirement for some permittees), where the agency requires consideration of “[a]reas with significant tire wear, e.g. roundabouts, high traffic intersections, municipal-operated parking lots,” among several other factors, during the development of sweeping plans.

Ecology’s fact sheet speaks more to 6PPD/q, including an acknowledgment that “[d]ue to 6PPD-quinone’s harmful impacts on salmon and fish-bearing watersheds, this stormwater issue also is directly linked to Tribal Treaty Rights.” See Draft MS4 Permit Fact Sheet, at 12. The fact sheet also included a summary of the source control, flow control, and runoff treatment best management practices effective at addressing 6PPD-quinone, and notes that the agency “expects several proposed updates to the permit requirements will provide benefits to address road and tire wear sources [of 6PPD-quinone],” such as first time requirements for street sweeping, incentives for retrofits in high pollution generating transportation areas, and incentives for watershed collaboration at cross-jurisdiction 6PPD/q hotspots. See Draft MS4 Permit Fact Sheet, at 29–30, and 44.

Ecology expects to respond to comments and finalize these permits in July 2024.

Finally, on January 30, 2024, EPA announced a 6PPD-quinone testing method (EPA Method 1634) that will enable government agencies, Tribes, and communities to assess stormwater systems and surface waters. With the method’s recent approval, the number of laboratories able to run test samples should grow, providing nationwide uniformity in environmental enforcement efforts.

“The faster we can identify where problems exist, the faster we can correct them.” ––Casey Sixkiller, EPA Region 10 Administrator, EPA press release announcing 6PPD-quinone testing method for stormwater and surface waters, January 30, 2024

TSCA and ESA: Petitions, approvals, and lawsuits

On August 1, 2023, the Yurok, Port Gamble S’Klallam, and Puyallup Tribes petitioned the EPA under section 21 of the Toxic Substances Control Act (TSCA), 15 U.S.C. § 2620, to “establish regulations prohibiting the manufacturing, processing, use, and distribution of 6PPD in and for tires,” with such regulation “to take effect as soon as practicable, in order to eliminate the unreasonable risk 6PPD in tires presents to the environment.” See TSCA Petition at 1. The petitioning Tribes rely upon salmon and other aquatic species experiencing devastating impacts from the ubiquitous nature of 6PPD/q. The petition argues that the “extraordinary toxic effects” of 6PPD/q are “precisely what TSCA was designed to address” and asserts that 6PPD/q’s toxicity and ubiquity present a level of risk to aquatic organisms that EPA has previously characterized as “unreasonable.” Petition at 5, 9. Five attorneys general wrote to EPA supporting the petition (AG Letter to EPA TSCA Petition Docket, at 5):

“By unavoidably generating and releasing 6PPD-q, an incredibly toxic compound and the most important causal agent for the runoff-related mortality that threatens fish populations  . . . continued use of 6PPD creates an unreasonable risk to salmon and salmon-dependent aspects of the environment.” Attorneys General of Connecticut, Oregon, Rhode Island, Vermont, and Washington.

On November 2, 2023, EPA granted the petition and announced its plan 1. to take action to address 6PPD and 6PPD-quinone in an advance notice of proposed rulemaking under TSCA section 6 (with no projected timeline for rulemaking) and 2. to propose a rule under section 8(d) of TSCA (planned to be finalized by 2025) requiring manufacturers (including importers) of 6PPD to report certain lists and copies of unpublished health and safety studies to EPA.

In approving the petition, EPA specifically cited non-tire sources of 6PPD/q as potential pathways of contamination warranting further research. Further work along these lines of review, which most certainly will dovetail with Washington and California state action planning processes and tire waste management reviews, may drive the agency to take more broadly applicable actions under TSCA.

Shortly after EPA granted the TSCA petition, on November 8, 2023, a conservation group and fishing trade organization (fishing groups) sued 13 of the largest U.S. tire manufacturers over the use of 6PPD in rubber tires. The fishing groups sued in the Northern District of California under section 9 of the Endangered Species Act (ESA), 16 U.S.C. § 1540(g), which prohibits “take” of endangered species. See 16 U.S.C. § 1538(a)(1)(B); see also 16 U.S.C. § 1532(19). The fishing groups claim that the tire manufacturers’ use of 6PPD generates and contributes to contamination of waters near roads with 6PPD-q, imperiling the recovery of 24 populations of coho and Chinook salmon and steelhead trout––three of the species most affected by 6PPD-quinone, and each of which is protected under the ESA. Complaint Exhibit A, at 31.

This ESA action was launched only a few months after a notice of intent to sue letter was sent by the Center for Biological Diversity to the California Department of Transportation, the U.S. Department of Transportation, the Oregon Division of the Federal Highway Administration, the National Marine Fisheries Service, and the Secretary of Commerce for alleged “actions and inactions related to maintenance and management of roads affecting imperiled coho and chinook salmon and their critical habitat in California and Oregon.” The controversy is pending. 

With ESA actions in the queue, state action plans for chemical management under development in California and Washington, key stormwater permits under final review at both state and federal agencies, and the nation running full speed into this election year, the next 18 months of 6PPD and 6PPD-quinone management will likely be quite active. Additionally, there is an emerging field of research on 6PPD/q human health impacts, including tracing of exposure pathways––such as inhalation of tire wear particles in roadside dust, drinking water contamination, or skin contact and incidental ingestion of crumb rubber used for recreational facilities or landscaping.

Looming large over these conversations—for salmon—is the quite simple problem that stems from the simple solution to 6PPD-quinone: green infrastructure. Given the ubiquitousness of 6PPD, the fact that treating stormwater (including with bioinfiltration) can mitigate stormwater’s toxicity, and the challenges in finding alternatives for tire anti-degradants, how will our legal systems respond? To save salmon—and the communities and ecosystems that depend on salmon—significant investments in green infrastructure projects throughout watersheds, on public and private property, for both new projects and existing systems, are clearly needed. But will they be required, and if so, within which legal framework?