6PPD, or N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, is a chemical compound added to vehicle tires. It primarily functions as a preservative, extending tire life and safety. While serving an industrial purpose, its presence has become an environmental concern. This compound and its transformation products are widely investigated due to their environmental distribution.
The Role of 6PPD in Tires
6PPD is added to tire rubber as an antioxidant and antiozonant. Atmospheric oxygen and ground-level ozone degrade rubber, causing brittleness and cracking. This degradation compromises tire integrity and safety. By reacting with these elements, 6PPD forms a protective film on the tire surface, preventing deterioration and enhancing durability.
As tires wear down from friction with roadways, they shed tiny fragments into the environment. These microscopic fragments, known as tire wear particles, contain 6PPD. The 6PPD within these particles can migrate to the rubber surface, a process called “blooming,” making it available to react with ozone. This release is a natural consequence of tire use.
Transformation into a Toxin
Once released, 6PPD reacts with ground-level ozone. This process, called ozonation, converts 6PPD into 6PPD-quinone (6PPD-q). 6PPD-quinone, not the original 6PPD, is the primary environmental contaminant of concern. This transformation was first recognized in 2020, despite 6PPD’s use in tires since the mid-1970s.
Studies show that during ozonation, up to 81% of 6PPD mass can react within hours, yielding 6PPD-quinone at 1% to 19% for pure 6PPD and 0.95% within tire wear particles. A considerable portion of 6PPD intended to protect tires converts into this different chemical. The resulting 6PPD-quinone is no longer effective as an antiozonant, transforming into a more harmful substance.
Environmental and Wildlife Impacts
The environmental impact of 6PPD-quinone became widely known through its link to “urban runoff mortality syndrome” in coho salmon (Oncorhynchus kisutch) in the Pacific Northwest. For over two decades, biologists observed mass die-offs of adult coho salmon in urban streams after rain events, before spawning. Researchers in Washington State identified 6PPD-quinone in 2020 as the probable cause.
Stormwater runoff from roads washes tire wear particles containing 6PPD-quinone into streams and rivers. Concentrations in stormwater are lethal to coho salmon after hours of exposure. Affected fish exhibit symptoms including increased surface swimming, lethargy, disorientation, and loss of equilibrium and buoyancy, often leading to death within hours.
Beyond coho salmon, research indicates 6PPD-quinone is acutely lethal to other fish species, including brook trout (Salvelinus fontinalis) and rainbow trout/steelhead (Oncorhynchus mykiss), at low concentrations. Lethal concentrations (LC50) are 0.095 µg/L for coho salmon, 0.59 µg/L for brook trout, and 1.0 µg/L for rainbow trout. Scientists are investigating its effects on a broader range of aquatic organisms, as its widespread presence suggests pervasive contamination.
Potential Human Health Implications
The widespread presence of 6PPD-quinone raises questions about its potential effects on human health. Studies have detected both 6PPD and 6PPD-quinone in human urine, indicating exposure. A biomonitoring study in South China found these compounds in urine samples from adults, children, and pregnant women, with detection frequencies from 60% to 100%. Pregnant women showed higher median concentrations of 6PPD-quinone (2.91 ng/mL) compared to adults (0.40 ng/mL) and children (0.076 ng/mL).
Research into 6PPD-quinone’s specific health effects on humans is preliminary. Some studies suggest potential links to adverse health effects like hepatotoxicity (liver damage), enterotoxicity, pulmonary toxicity, neurotoxicity, reproductive toxicity, and cardiotoxicity in animal models and human cells. However, the full scope of risks at environmentally relevant exposure levels is not yet understood. Researchers continue to investigate human exposure pathways and long-term health outcomes.
Regulatory and Industry Responses
In response to 6PPD-quinone’s toxicity findings, regulatory bodies and the tire industry have begun action. The U.S. Environmental Protection Agency (EPA) developed an agency-wide action plan for 6PPD and 6PPD-quinone, coordinating research and funding with federal partners, tribes, states, and industry. This follows an August 2023 petition from Native American tribes, urging the EPA to regulate or prohibit 6PPD use in tires under the Toxic Substances Control Act (TSCA).
At the state level, Washington became the first state to pass legislation addressing 6PPD in motor vehicle tires in March 2024. This bill aims to reduce 6PPD sources to protect aquatic life, especially salmon, and identified tires containing 6PPD as a priority product within Washington’s Safer Products Program. California also identified tires containing 6PPD as a priority product in October 2023 through its Department of Toxic Substances Control.
The tire industry, including the U.S. Tire Manufacturers Association (USTMA), is engaged in discussions and undertaking research to find 6PPD alternatives. Manufacturers submitted preliminary analyses to California in March 2024, with final reports expected by mid-2026. No direct “drop-in” replacement has yet met all federal safety and performance requirements without forming a toxic byproduct. Research into compounds like 7PPD and 77PD is ongoing, and alternative rubber formulations are being explored.