Tire Particles: Looking at The Big Picture Through (Very Small) Particles

As public concern over environmental pollution evolves and focuses on new areas, businesses continue to seek solutions. Some aspects of the problem, like cutting down on disposable plastic bottles or reducing emissions, have been studied extensively. Other areas, such as microplastics in the environment, are much newer and their implications are still being understood. 

Found in oceans and waterways, soil and land ecosystems, and even the air, microplastics are most commonly associated with degraded plastic packaging and microbeads used in personal care products. Less known sources of microplastics in the environment include synthetic clothing fibers, which shed when laundered and enter the water system, and the tiny particles of tires that are released through wear and tear as more than 1 billion vehicles travel the roads each year.

The particles generated during the normal use of tires are known as tire and road wear particles (TRWP). These distinct particles are an approximate half-and-half mix of tire tread material and road pavement material. 

Public awareness of TRWP and their potential impact on the environment and human health has been slow to develop. However, a report highlighted that TRWP might have an important impact on the presence of microplastics in the environment. Using a statistical model, the 2017 International Union for Conservation of Nature (IUCN) report estimated that “tiny plastic particles washed off products such as synthetic clothes and car tires could contribute up to 30 percent of the ‘plastic soup’ polluting the world’s oceans and — in many developed countries — are a bigger source of marine plastic pollution than plastic waste.” The IUCN report did not receive independent scientific validation, so it does not rank amongst the growing body of peer-reviewed scientific literature on microplastics; however, its publication marked an important increase in stakeholder interest in potential sources of marine microplastics.

Defining the known unknowns

Use of the phrase “could contribute” in that IUCN report is important. TRWP research is a relatively new field, and a complicated one. At its inception, TRWP consists of tire tread material combined with particles of road surface, resulting from friction. The particular composition of TRWP also complicates their potential “microplastics” label; with as much as 50 percent of each TRWP particle being road pavement material, TRWP are perhaps not a perfect fit with the definition of microplastics that we often read about in today’s environmental press.

However, TRWP can change as they move through the environment.

“The knowledge on particle properties is very limited, especially the possible changes of these properties during transport,” explains Dr. Stephan Wagner, who studies TRWP at Germany’s Helmholtz Centre for Environmental Research. “Our results suggest that … depending on the conditions of TRWP generation, we may expect different particle sizes and densities. Even more important is that our result suggests that these properties may change during aging of TRWP in the environment."

In addition, little is currently known about the environmental toxicity of TRWP, though an early study found that the risk of acute toxicity should be considered as low whether the particles are circulating in the aquatic environment or trapped in sediment. A more recent study has suggested that a degradation product of a tire chemical found in TRWP might be responsible for incidences of coho salmon mortality during storm events in the U.S. state of Washington, underlining the importance of continued research.

Growing the knowledge base

Dr. Wagner’s group is currently working to assess how quickly TRWP properties can change and identifying the factors that trigger these changes.

He also lists several other areas that are needed to develop TRWP research into a more mature science. Chief among the tasks ahead is to develop a consistent foundational platform for study. Researchers must validate and agree upon methods, tests, materials, and analytical tools, Dr. Wagner said. 

Building on that foundation, he suggests that one focus of future research centers on the changes that TRWP undergo as they reside in the environment and their long-term impacts on ecosystems and human health.

Getting ahead of the issue

Although public awareness of the TRWP issue is low, key stakeholders in the tire industry have been supporting TRWP  research over the past 15 years through the Tire Industry Project. 

The project includes a network of independent scientists and an Assurance Group, who are tasked with ensuring that the research is relevant and on point.

Dr. John Spengler of the Harvard T.H. Chan School of Public Health is an Assurance Group member. He notes that the Tire Industry Project’s work on tire materials and end-of-life issues informs a comprehensive, proactive approach regardless of the state of public awareness.

“The Assurance Group builds credibility into the system,” Dr. Spengler told us. [The Tire Industry Project] wanted the Assurance Group to make sure the right questions were being asked and the right protocols were being followed.”

One especially helpful development is the effort to identify a unique chemical signature for tires, which could better enable researchers to establish standard test methods to determine if particles found in the environment came from tires.

Aside from environmental impacts, there is an important public health angle, Dr. Spengler added. Most of the particles are too large to penetrate deeply into the lungs if inhaled, but they could still be transported into the body through nasal passages. “Just because they are large particles doesn’t mean they are safe,” Dr. Spengler said, while also noting that larger particles tend to fall on the roadside and do not travel far through the air.

Nevertheless, it is still important to quantify the potential impact, he said. “In Tokyo, in Korea, and in parts of the U.S., millions of people are living within 100 meters of a road,” Dr. Spengler explained. “So for a subpopulation, this is very important.”

Prospects for a solution

There are some indications that researchers studying the effects of TRWP may accelerate toward a solution in the coming years. The first study supported by Tire Industry Project was published in 2010. TIP has placed its research in the public domain, and this year, the organization is taking the additional step of making all its research open access, so it is easily available to anyone, online.

Dr. Spengler also noted that the Assurance Group has recommended ramping up its activity by meeting more frequently and ensuring quality by bringing additional topical experts into the research network. 

The initial study analyzed different methods for comparing particles to understand the specific physical and chemical characteristics of TRWP. Subsequent topics of TRWP research include additional work on testing methodology (to better identify and quantify TRWP in environmental sampling), as well as investigations on possible impact on aquatic life, the potential toxicity of inhaled particles, sediment and estuary studies, human health risk analyses, and studies relevant to urban environments. TIP’s study plan aims to complement the global state of knowledge on TRWP – seeking to answer questions raised by earlier research, delivered by TIP or others. 

Drivers are already well informed on the safety aspect of driving with tires with the right tread depth, but there are important opportunities for reducing tire wear rate through driving behavior and vehicle maintenance that consumers are perhaps less well acquainted with. 

TRWP is a topic that requires the engagement of multiple stakeholders, and scientific research provides industry, consumers, authorities and other actors with the evidence-base to better understand the potential impacts of TRWP, and consider what might be the most effective means of mitigation where it is necessary.

This article series is sponsored by the Tire Industry Project and produced by the TriplePundit editorial team. Members of the Tire Industry Project (in alphabetical order) are Bridgestone, Continental, Cooper Tire, Goodyear, Hankook, Kumho Tire, Michelin, Pirelli, Sumitomo Rubber, Toyo Tires, and Yokohama Rubber.

Image: Evgeny Tchebotarev/Unsplash