The Controversy Surrounding 6-PPD and 6-PPD-quinone_1

Introduction

The “6PPD State of the Science Virtual Forum” Day 1, hosted by the Washington Department of Ecology, was held. In this session, researchers from the United States and internationally presented and shared the latest findings on water pollution and ecological impacts caused by 6PPD and 6PPD-quinone.

6PPD is an antioxidant used in tires and is released into the environment together with wear particles generated during driving. When it reacts with ozone in the atmosphere, it transforms into 6PPD-quinone, a substance that has recently been shown to exhibit strong toxicity to fish, particularly salmonid species.

This forum is positioned as a venue to organize the scientific understanding of this issue and to share directions for future countermeasures.

1. Background of the Discovery

In 2020, in the Pacific Northwest, the cause of mass mortality of returning spawning Coho salmon was identified as 6PPD-quinone. This discovery became a turning point clearly demonstrating the relationship between urban runoff and tire wear particles.

Since then, research has rapidly expanded, and multifaceted investigations are progressing on toxicity mechanisms, environmental dynamics, treatment technologies, and policy responses.

2. Behavior in Urban Watersheds

Several presentations consistently highlighted the following characteristics.

・Concentration rises sharply immediately after rainfall begins
・Peaks concentrate within the first 24 hours
・Acute lethality at ppt levels
・Correlation with urbanization rate, road density, and traffic volume

Watershed monitoring has confirmed threshold exceedances in many urban rivers.
Simultaneous detection with other road-derived metals such as copper and zinc has also been reported.

3. Ecological Impacts

Effects on Adult Fish

Exposure to urban runoff has been reported to cause pre-spawning mortality rates reaching 60–90%.

Effects on Juvenile Fish

Experiments using actual river water have shown cases where exposure resulted in about 80% mortality. Chronic exposure has also confirmed developmental delay and morphological changes.

Interaction with Water Temperature

An increase in water temperature may raise uptake rates by about 50%. Combined stress with climate change is a concern.

4. Research on Toxicity Mechanisms

Research on metabolites of 6PPD-quinone has shown that toxicity differs significantly depending on positional isomers.
Only specific structures exhibit high toxicity, suggesting binding with selective molecular targets.

This molecular-level understanding will become fundamental information for future alternative design.

5. Progress in Treatment Technologies

Field demonstrations using improved bioretention soils reported that 6PPD-quinone could be reduced to below the detection limit.

The enhancement of existing urban stormwater management technologies is attracting attention as an implementable countermeasure.

6. Policy Trends

・Regulatory petitions by U.S. tribal governments
・Reassessment in Canada
・Expansion of research support at the state level

The connection between science and policy is entering an accelerated phase.

Overall Summary

The 6PPD-quinone issue is not simply a matter of chemical regulation.
It is positioned within a complex structure involving urban transportation, material design, drainage infrastructure, and climate change.

Forum Day 1 showed that
scientific understanding of the phenomenon and the search for solutions are progressing simultaneously.

Future focal points include

・Quantification of exposure conditions
・Evaluation of long-term impacts
・Design of alternative additives
・Implementable runoff control technologies

Advancing cross-sector collaboration across these areas.

6PPD exists within tire materials.
However, its impacts extend across entire watersheds.

The intersection of materials science and urban environmental management is being reconsidered.