Understanding the Impacts of Tire Anti-Degradants 6PPD and 6PPD-Quinone on Airports (2026)
Chapter: 2 What Are Tire Anti-Degradants and What Is the Concern? Background on Tire Anti-Degradants
2 WHAT ARE TIRE ANTI-DEGRADANTS AND WHAT IS THE CONCERN? BACKGROUND ON TIRE ANTI-DEGRADANTS
2.1 What Are Tire Anti-Degradants 6PPD and 6PPD-q?
6PPD belongs to the chemical class para-phenylenediamines (PPDs), which are broadly used as antioxidants or antiozonants in rubber and other products (Gradient 2024). 6PPD acts as an effective stabilizer and anti-degradant by reacting with ozone and other oxygen compounds present in the air to reduce the degradation of tire rubber caused by oxidation. The addition of 6PPD prevents hardening of rubber compounds in the tire and thus improves tire wear life and allows for consistent traction over the lifetime of the tire (ITRC, 2024). 6PPD is the most used antiozonant in vehicle tires.
To extend the service life and slow degradation caused by exposure to oxygen and ozone in the atmosphere, a combination of paraffin and microcrystalline waxes, antioxidants, and antiozonants are added to tire formulations (Rogers, 2016). Para-phenylenediamines such as 6PPD are common antiozonants used in vehicle tires, including aircraft tires (ITRC, 2024) (Rogers, 2016) (Rogers, 2021). Antiozonants work by forming a protective, sacrificial layer that reacts with atmospheric ozone, minimizing the amount of ozone available to attack the rubber (ITRC, 2024). 6PPD is designed to slowly migrate from the interior of the tire to the surface, a process called “blooming,” ensuring that a fresh layer of protection is constantly available at the surface as the tire wears (California Department of Toxic Substances Control, California Environmental Protection Agency, & Safer Consumer Products, 2022). Tire wear produces particles that contain 6PPD, which are known as Tire and Road Wear Particles (TRWP). These particles are released onto paved and unpaved surfaces and ultimately enter airport waste streams and the environment as described further in this First Look.
Effective anti-degradants like 6PPD are critical to maintaining tire durability, which contributes to road and motorist safety. 6PPD is consumed as it reacts with ozone and oxygen, producing 38 transformation products, including 6PPD-q. Over time, as it reacts with ozone and oxygen, the amount of 6PPD in the tire decreases, increasing characteristics of aging in tire rubber. This eventually causes the rubber in the tire to harden, crack, and lose structural integrity.
Before the development of antioxidants and antiozonants like 6PPD and other PPD compounds in the 1960s, rubber products experienced rapid deterioration and failure. Typical tires at that time only lasted about 2 years, and it wasn’t until the 1930s that the role of ozone in tire rubber degradation was understood (ITRC, 2024). Waxes were initially used as a static protectant against ozone, but wax was not effective for rubber products that required flexibility and movement, such as vehicle tires (ITRC, 2024). The U.S. government funded research for methods of preventing tire rubber degradation in response to the failure of tires and other rubber parts on military vehicles after World War II (ITRC, 2024). It was due to this research that chemically modified PPDs were developed to protect rubber compounds against ozone.
Among the first PPDs to be developed and utilized in rubber products were phenyl-p-phenylenediamine (IPPD) and diaryl-p-phenylene diamine (DAPD); however, DAPD was found to react minimally with ozone and IPPD was found to react too quickly with ozone, leading to premature depletion (ITRC, 2024). For an antiozonant to be effective, it must react with ozone at the appropriate speed to provide long-lasting protection. Ultimately, the three PPDs that become commercially viable were 6PPD, 7PPD, and 8PPD.
6PPD was used in tire manufacturing as early as the mid-1960s and into the early 1970s (ITRC, 2024). By 1975, 6PPD accounted for 60% of the antiozonants used in tires (EPA 1975) (ITRC, 2024). 6PPD is currently
the primary anti-degradant used in all manufactured tires, although 6PPD may be combined with other anti-degradants (USTMA 2024), (ITRC, 2024).
2.2 Why are 6PPD and 6PPD-q a concern?
Tire wear caused by the friction between a road surface and a vehicle tire results in the production of fine rubber particulates, or TRWP, which contain both 6PPD and its byproduct 6PPD-q. TRWP, 6PPD, and 6PPD-q may accumulate in areas of high traffic, such as roadways, airports, highways and freeways. As TRWP travel through the environment via wind, water, and soil, they act as a source of 6PPD and other chemical contamination. The fate and transport of 6PPD and 6PPD-q in the environment is therefore highly dependent on that of TRWP.
TWRP are made up of particles from tire rubber (called tire wear particles, or TWP) and particles from the road surface, which may include pavement fragments, debris, and other vehicle-generated contaminants such as brake dust (ITRC, 2024). TWP consists of rubber compounds, 6PPD, and other polymers and additives. TWP are considered one of the most prevalent types of microplastics in urban stormwater runoff, and their large surface area allows for significant leaching of 6PPD and other chemicals into the environment (ITRC, 2024). The fate and transport of TRWP in the environment is impacted by the physical and chemical properties of both the TWP and the road wear particles.
While tire wear from vehicle operation is the most common source of tire particles, TWP are also generated when used tires are repurposed in other sectors. Additional sources of TWP include construction materials, crumb rubber in synthetic turf surfaces (athletic fields, playgrounds, etc.), rubberized asphalt concrete (RAC), rubber-modified asphalt (RMA), tire-derived aggregate, and other industrial, commercial, and residential uses of old tires. (ITRC, 2024). Discussion of tire particles in this document will primarily focus on TRWP generated from vehicle tires on roadways and other paved surfaces.
6PPD and 6PPD-q in TRWP migrate into the environment through several transport pathways that will be further discussed in this document, including aerial deposition, soil and sediment, and stormwater runoff. Additionally, humans may be exposed to 6PPD and 6PPD-q through airborne particulates, roadway dust and other sediments, rubber products, and indoor dust.
The presence of 6PPD and 6PPD-q in the environment, and especially in surface waters, is a global urban stormwater containment concern because of its proven toxicity to certain aquatic species and the widespread use of tires and repurposed TWP worldwide. Of the aquatic species studied to date, coho salmon have the highest sensitivity to 6PPD-q with mortality ranging from 20% to 90% before spawning when returning to freshwater streams in urban watersheds to spawn, labelled as urban runoff mortality syndrome (URMS) (ITRC, 2024). Research has indicated that URMS caused by 6PPD-q could result in extinction of coho populations within the next decade in regions where mortality is high; some populations of coho are currently listed as threatened or endangered. While coho salmon have been identified as the most impacted species to date, further research is needed to evaluate mortality rates in other susceptible salmonids, such as brook trout and rainbow trout (steelhead). Due to the universal use of 6PPD in tires, 6PPD-q has been detected or investigated by multiple countries including Australia, Canada, China, Japan, New Zealand, and throughout Europe.
Salmon are considered a keystone species, meaning they have a profound impact on their ecosystems and are necessary for the survival of many organisms. Salmon species, including coho, provide an important food source for animals such as bears, birds, and whales; additionally, they transport crucial marine-derived nutrients to fresh water and terrestrial habitats when they die naturally during spawning.
Though there have been studies on the population-level impacts of coho mortality due to 6PPD-q, there is not yet enough research to determine ecosystem-level impacts of the lethal and sub-lethal effects of 6PPD-q on coho and other salmonids.
These salmon-sustained ecosystems are culturally, recreationally, and economically valuable to surrounding communities, especially indigenous communities. Declines in the populations of sensitive species such as coho, rainbow trout/steelhead, lake trout, and brook trout will have both ecological and societal impacts. Until more studies are conducted on the toxicity and population effects of 6PPD and 6PPD-q on other aquatic and terrestrial species globally, the broader impacts to the natural and human world cannot be fully understood.
