Publication

The Science of 6PPD and 6PPD-Quinone

September 18, 2024

Executive Summary

On May 3, WSAS organized and spearheaded a convening of relevant interested parties in Seattle, WA, to discuss the science and environmental impact of 6PPD and its transformation product 6PPDQ. This gathering included scientists, state officials, and interested parties who engaged in a detailed examination of these chemicals, their effects on the environment, and potential mitigation strategies. The meeting also aimed to identify data gaps and outline actionable next steps.

The focus of the discussion centered on understanding the properties and environmental impact of 6PPD and 6PPDQ. 6PPD, used in tires to prevent degradation, converts to the toxic 6PPDQ upon reacting with ozone. This chemical contaminant has been linked to high mortality rates in coho salmon, highlighting a critical environmental issue. This conversation explored current research, known effects, and significant gaps in knowledge related to 6PPDQ toxicity and environmental persistence.

The meeting highlighted the highly toxic effects of 6PPDQ on coho salmon, with rapid mortality occurring even at very low concentrations. While the primary environmental source of 6PPDQ is understood to be vehicle tire wear, significant knowledge gaps remain, including the mechanisms of toxicity, impacts on other species, and potential human health effects. Discussions also focused on potential mitigation strategies such as rain gardens, porous pavements, and chemical filtration, noting the challenges and uncertainties associated with each approach. Identifying effective replacements for 6PPD in tires and further research into both mitigation and alternatives were emphasized as crucial next steps.

 

Key Takeaways

Introduction to 6PPD and 6PPDQ

  • 6PPD (6-para-phenylenediamine) is used in tires as an antiozonant and anti-degradant, protecting rubber from oxidation.
  • 6PPDQ (6PPD-quinone) is a toxic byproduct formed when 6PPD reacts with ozone or oxygen. It leaches into the environment from tire wear particles (TWP) and accumulates in near-road waters.

Current Research and Findings

  • Coho Salmon Mortality: 6PPDQ is highly toxic to coho salmon, causing rapid mortality at very low concentrations. The toxicity mechanism is still not fully understood, but it appears to involve severe neurological damage and disruption of the blood-brain barrier.
  • Environmental Presence: 6PPDQ degrades relatively quickly but is continually replenished from tire wear particles. The environmental impact is influenced by factors such as road traffic, rainfall, and stormwater systems.
  • Species Sensitivity: Coho salmon and some other salmonids are particularly sensitive to 6PPDQ, while other species and ecosystems have not been as extensively studied.

Knowledge Gaps

  • Toxicity Mechanism: Further research is needed to understand the exact mechanisms of 6PPDQ’s toxicity, including its effects on different species and potential vascular permeability issues.
  • Impact on Other Species: The effects on other aquatic species and broader ecological impacts are not well understood.
  • Human Health: Potential human health effects of 6PPDQ exposure are not fully known, though preliminary data suggest it may cross the blood-brain barrier.
  • Alternative Chemicals: Identifying and assessing safe and effective alternatives to 6PPD in tires is crucial but remains ongoing.

Mitigation Strategies

  • Rain Gardens: Effective at removing 6PPDQ but require significant space and long-term efficacy studies.
  • Porous Pavements: Potentially effective with the use of carbon fiber additives, but practical implementation and costs are still uncertain.
  • Chemical Filtration: TAML catalysts show promise in degrading 6PPDQ but need further investigation regarding their practical application and environmental impact.

Next Steps

  • Mitigation Research: Focus on researching and developing effective short- and medium-term mitigation methods for 6PPDQ.
  • Pilot Programs: Initiate pilot projects to test and refine mitigation strategies and tire replacements.
  • Collaboration: Continued collaboration among researchers, government agencies, industry, and other stakeholders is crucial for addressing the issue effectively.

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