Researchers are just beginning to unveil the hazardous mix of chemicals, microplastics, and heavy metals concealed within car and truck tires. Experts warn that emissions from these tires significantly contribute to air and water pollution, posing a serious threat to both human health and wildlife. As tires wear, they release pollutants through atmospheric, aquatic, and terrestrial pathways, and evidence shows that wear and tear on tires and brakes now generate more particle pollution by mass than car exhaust systems in various real-world and test scenarios.
The US Environmental Protection Agency recently held a webinar on the emerging impacts of tire wear particles. The presentation was given by Dr. Paul Mayer, a research ecologist with the EPA’s Office of Research and Development in the Center for Public Health and Environmental Assessment. In his presentation, Dr. Mayer discussed the fate and transport of tire pollutants, the potential risks to human health and the environment, how the potential risks can be remediated, and some EPA-supported research on tires and tire-related pollution.
Much of the information for this presentation is taken from the paper, Where the Rubber Meets the Road: Emerging Environmental Impacts of Tire Wear Particles and Their Chemical Cocktails, recently published in the journal, Science of the Total Environment. Over 30 authors contributed to the publication, which synthesized published information about tire pollution from scientific literature and includes about 400 references cited in the paper.
Dr. Mayer emphasized the ongoing need for further research and remediation efforts in this field, despite the recent publication of the paper. He pointed out that the paper is likely already outdated due to the continuous influx of new information in the scientific community on tires and tire-related pollutants.
Tires as pollution
Tires are a complex nonpoint source of pollution due to the vast number of vehicles on the road and the extensive miles driven. The production of tires is energy and resource-intensive, involving materials such as latex from trees and petroleum-based products. As tires wear down, they release particles and chemicals into the environment, contributing to pollution in various forms.
Tires contribute to pollution in multiple ways, including air and water pollution. As tires wear out, tire particles end up in the environment as large macroscopic particles down to small nanosized particles. These particles can enter the air and water systems, where they can be consumed by animals and marine life, leading to potential health issues for both wildlife and humans. Tires are disposed of after they are worn and can end up in tire graveyards, where they start to break down in the environment. Constituent chemicals can leach from those tires and be transported through various pathways, such as stormwater.
Dr. Mayer noted that the exact formulation of tires is proprietary. “So we don’t always know what goes into the construction and manufacture of tires. And sometimes some of those chemicals are either undescribed or unknown.”
The scope of the problem
- Every year, 3 billion new tires are produced globally, enough to reach to the moon and nearly back again.
- An estimated 800 million tires become garbage every year because they wear out and become waste tires.
- As tires wear, an estimated 10 to 16% of the mass of the tire is shed as particles, which can represent 2-15 pounds of rubber lost, depending on the size and use of the tire.
- There are an estimated 1000s of chemicals and compounds in tires. The paper includes several summary tables listing many of the chemicals found in tires and their chemical and toxicological characteristics.
- Tires are sometimes disposed of illegally in forests or the ocean; some end up in tire graveyards, which are large areas where used tires are stored. These tire piles themselves can present health risks as they can become habitats for disease-carrying organisms such as mosquitos or rats. When tire piles catch fire, it can be difficult to extinguish, and the emissions from tire fires can be very toxic.
- With normal tire wear, tire particles fly off the tires can be anywhere from 10PM to 2.5PM, or even smaller, nanosized particles that end up in the environment.
- Electric vehicles wear through tires much more quickly than comparable gas-powered cars due to batteries, battery sizes, and the torque forces produced by electric vehicles.
“Tires are a highly engineered product that contains 1000s of chemicals and materials,” said Dr. Mayer. “They can include components of rubber fibers, textiles, and metals. There are steel belts in tires. There’s zinc in tires. Tires are usually a combination of natural, latex, artificial, and synthetic rubber from petroleum-based products. They can contain other polymers, fillers, and plasticizers. They are highly engineered for particular uses, based on use, wear characteristics, ride characteristics, and so forth.”
Tire wear particles and emissions include heavy metals, pHs, and other toxic elements, which can be transported to waterways. Emissions can be in the form of particles and can be coarse, fine, or ultra-fine. An estimated 3-7% of roadside particulate matter is in the range of PM 2.5 – PM 10, which are sizes of particles that are small enough to be inhaled and can cause health problems. Up to 10% of the mass in roadside dust was estimated to be tire particles, with much of those being in the ultra-fine range.
While not the only source, tires contain metals and zinc. Because tires are petroleum-based products, they contain polycyclic aromatic hydrocarbons (PHs) and volatile organic compounds (VOCs). They also contain plasticizers, which have been identified as endocrine disruptors. Tire particles, by definition, are technically microplastics, which are becoming an increasingly significant component of pollution found everywhere on the planet.
Fate and transport of tire pollution
With so many different components and compounds mixed into tires, analytical methods for understanding the fate and transport of tires in the environment can be really complicated. The diagram below illustrates that depending on where we want to sample in the environment for these contaminants, the sampling methods would differ across the different pathways: sediment, water, hair, or animal tissue. In addition, the analytical techniques can be very tricky and costly.
“While I’m not an expert in this, it does take an expert to analyze these materials, and some of the methods are still in development,” said Dr. Mayer. “We are in the midst of having an emerging issue with tires as a form of pollution and trying to better understand the fate and transport of those chemicals. The fate and transport of tire particles and chemicals are very complex because once these emissions end up in the environment, there are multiple pathways that those toxic particles and the toxic chemicals might take. So, understanding the totality of tire wear particle fate and transport is not a trivial thing.”
One approach to understanding the fate and transport of these chemicals is using a ‘chemical cocktail’ concept. If we take a sample in a stream, we will find trace metals, nutrients, organic matter, and ions; together, these represent a mixture of chemicals dubbed a ‘chemical cocktail.’ Examining them together is important because these chemicals can behave and be co-mobilized together, making it easier and more insightful to understand how these chemicals interact.
The slide below shows data not from tire wear particles but environmental samples in different urban rivers. The triangular graphs represent correlation matrices across various chemicals, with the colors indicating the chemicals that co-mobilize or move together.
“The value of such insightful information is that because chemicals in tires are difficult to analyze and to monitor, so if we can identify a chemical or process or a condition like pH or specific conductivity that correlates well with other chemicals that we measure, we may be able to develop proxies that we could measure in real-time across a broad geographic space, either temporarily or spatially,” said Dr. Mayer.
The slide below shows data collected in Baltimore of chloride, calcium, magnesium, sodium, and specific conductance in a stream across its flow path.
“Those concentrations change as that water moves downstream. In this case, we were interested in road salts and understanding what effect roads might have on these chemicals,” he said. “We can see that at one point here where there’s an arrow pointing to an increase in concentrations across these chemicals. That’s where this little stream crosses the 695 Beltway in Baltimore. This is a very heavily traveled highway and a source of these chemicals, and there are sinks of these chemicals as well. So here, we might better understand hotspots of those chemicals, and we would do so similarly for tire-related chemicals.”
Potential risks to human health and the environment
Tire related chemicals can follow various exposure pathways: inhalation, ingestion, or dermal. Concern about dermal exposure comes from tire crumb, which is used in various products, including the construction of turf and soccer fields.
Based on the nation’s road infrastructure, at least 72 million people in the US live close to major roads; 17,000 schools in the US are very close to heavily traveled roads. Demographically, populations near major roads are more likely to be people of color and lower socioeconomic status. The most significant racial disparities are found in urban counties with larger roads and more densely packed roads.
Tire particles are deposited in soils on roadsides; they can be transported downwind, and many tire compounds can be readily absorbed in plants. There may be potential risks from consumption if those compounds are absorbed in the foods that we eat.
Tire particles account for a significant component of the microplastics found in the ocean. The San Francisco Estuary Institute took samples from the San Francisco Bay and found that most of the microplastic particles in the bay are actually derived from tires. And many tire compounds have been shown to be toxic to aquatic species.
Methods to remediate tire pollution
The graphic shows the different methods that could be used to manage tire wear particle pollution from a report from a report from the San Francisco Estuary Institute. The management options are laid out along a continuum from prevention to remediation.
There are efforts across the tire industry to create airless tires, which may be longer lasting and have less wear. Though they’re still in the experimental stage, there have been some experimental efforts to install collection systems on tires to collect the wear debris as it flies off the tires. There have been efforts to collect tire particles by street sweeping, which is somewhat effective. At the end of the continuum is filtration of stormwater runoff or sending stormwater through a wastewater treatment plant, which are the most expensive means of dealing with pollution.
“The adage here applies: An ounce of prevention is worth a pound of cure,” said Dr. Mayer. “Prevention is a more cost-effective approach to dealing with pollutants of all sorts. And in this case, prevention might include voluntarily reducing driving hours or ensuring that your tires are kept at a proper pressure.”
Tire disposal and management
The chart shows how tire waste management has changed over the years. In the 1960s, people sent worn tires to the landfill. As the number of waste tires increased, some were recycled into asphalt production or ground up into tire crumbs used for various products. Tires have a lot of highly dense energy-based materials, so now, a large portion of waste tires are used as fuel for combustion, either burned in industrial kilns or pyrolyzed for gas collection and production.
Recycling tires has to happen on an industrial scale, given the number of tires produced each year and those that become waste. The infrastructure for doing so exists here in the United States and Europe, but such infrastructure only sometimes exists in developing countries.
Tire chemicals and salmon
Tires are a very complex form of pollution. The properties that make tires very durable also make them very difficult to get rid of. Tires represent a form of pollution across phases that include whole tires, particles, and chemicals. Those emissions are pollutants of various forms that follow air, land, and water pathways.
One chemical in particular has caused concern in the Pacific Northwest because it is a chemical that has been shown to induce mortality in coho salmon. Salmon are an important economic resource, recreational resource, and cultural resource, in particular, for the 250 recognized Native American tribes that exist in the region.
The paper, A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon, published in 2020 by a group from the University of Washington and supported by EPA, finally solved a long-running mystery of why salmon swimming upstream to spawn were dying.
“What seemed to be perfectly healthy salmon filled with eggs ready to go spawn met with an untimely death as they got closer to urban areas,” said Dr. Mayer. “There was an understanding that there’s some relationship of this mortality to location and the existence of roads. But it wasn’t until this team from the University of Washington and Washington State identified this chemical, which was undescribed previously, and then went further to identify it as a very toxic chemical to coho salmon. After decades of observing this phenomenon in urban streams in Washington and elsewhere, only then did we know that it was a chemical called 6PPD that is added to tires as an anti-ozonant to keep them from breaking down into ozone that was causing these mortality events.”
6PPD is an anti-degradate that is added to rubber tires. It’s designed to scavenge oxygen, so as it becomes oxidized, it turns into 6PPDQ. That leeches into the water in the form of the chemical linked to acute mortality and coho salmon in the Pacific Northwest. However, he acknowledged that if tires didn’t contain 6PPD, they would break down much more quickly and create another problem.
Interestingly, 6PPDQ is very toxic to salmon and related species, but not toxic for all fish species. “Some fish species seem able to practically drink it and not be very affected,” said Dr. Mayer. “The mechanism of action is really mysterious, and there’s a lot of effort within EPA and elsewhere to understand what is going on with this chemical and why it’s toxic.”
The human health risks of 6PPDQ are unknown. In a study in China, 6PPD and 6PPDQ were detected in adults, pregnant women, and children, and in a recent study in China, 6PPDQ was shown to cause liver damage in mice.
EPA-supported research
The EPA is working to identify the fate and transport of tire chemicals and address the potential toxicological effects and or effects on human health and the environment.
Research by Jankowski, Labiosa, and Gockel
In this study, the researchers are focusing on where 6PPD and 6PPDQ exist in the environment and working to identify hot spots or hot moments. This chemical follows the same path as other roadside chemicals, such as road salt or oil that accumulate on roads. During precipitation events, researchers followed the discharge in the stream where, at some point, the chemical increases in concentration over time and then declines in concentration.
Data from another team in China published in ES&T letters found 6PPDQ in various aquatic systems, stormwater, and storm melt. Further research from China found the chemicals in the environment along flow paths into estuarine systems. There have been efforts to identify 6PPDQ in Washington, California, Michigan, and elsewhere. And data from Zheng et al 2023 found hotspots of 6PPDQ in urban rivers, a central source of roadside pollutants, with lesser amounts in estuaries, along the coasts, and in the deep sea.
Research: Baldauf and Joerger
6PPD is not only transported in water, but it’s also transported in air. Researchers Baldauf and Joerger have focused on measuring 6PPD and 6PPDQ in the air and understanding the toxicological effects of 6PPD and 6 PPDQ in relevant bioassays.
They are sampling around Washington DC in areas with heavy road use and developing methods for collecting and identifying 6PPDQ from air filter samples.
“A lot of what I’m describing here is a form of methods development,” said Dr. Mayer. “Because this is a new chemical and an emerging issue for us, a lot of the research is going into understanding how we identify the chemical, where we can identify it, and where we can find it.”
Research: Deycard and Wan
Deycard and Wan are looking at 6PPDQ in stormwater, sampling in areas in Florida, and working to understand where the chemical is moving through the stormwater system.
“In their preliminary analysis, they have shown that 6PPDQ does differ between stormwater influent and effluent, suggesting that something is happening in the sediments in those stormwater features,” said Dr. Mayer. “That’s actually a promising result because it might suggest to us that there are ways of attenuating 6PPD in stormwater features based on transformation and retention of 6PPDQ in those sediments.”
Research: McKane and Halama
McKane and Halama are using a grid-based fate and transport model to understand the fate and transport of various chemicals on large geographic scales.
“They are using data from the epicenter of this issue, Seattle, and combining different geographic data layers to identify hotspots in the city based on road structures, stormwater structures, and water runoff patterns,” said Dr. Mayer. “Their publication, which came out this year, shows very promising results identifying and predicting the concentrations of 6PPD in stormwater runoff.”
Research: Mayer, Fritz-Endres, King
Work that Dr. Mayer and colleagues have been working on in Corvallis, Oregon, is focused on remediation.
“We are leveraging the observations from other studies that show that stormwater filtered through soil reduces salmon mortality from 6PPDQ to virtually nothing,” said Dr. Mayer. “That’s very encouraging and very exciting. Our research is focused on understanding how we can mimic the effects of this filtration in green infrastructure. So we’ve set up some clever lab experiments to replicate what’s happening in the environment and focused on how we can supercharge soil media by adding components like coconut core, mulch, biochar, and other components that might retain or transform it.”
One thing they are trying is using biochar to supercharge the soil filtration. Biochar is created through the pyrolysis of biomass; they use biochar created from mill waste, which is abundant in the Pacific Northwest from the timber industry.
“It’s a win-win situation where that waste can be turned into something that becomes a cleanup product, based on its ability to retain contaminants, and biochar is very effective at retaining and containing heavy metals,” said Dr. Mayer. “The purpose here is that if and when we identify that supercharged, optimized filter material, can we scale it up to add to grey infrastructure features like rain gardens, bioswales, semi-permeable paved surfaces, and so forth, to optimize the capture and attenuation of those tire related pollutants.”
EPA’s approach to research
Collectively, the research across the EPA is taking a three-pronged approach: identifying where the chemicals are found in the field, identifying fate and transport hotspots and hot moments, and identifying where the problems are and aren’t. That is combined with lab experiments, where they can control the constituents that can potentially attenuate 6PPDQ and integrate those data to parameterize the models that can identify where across the landscape to focus efforts to get the most significant results in terms of remediation and application of best management practices.
In conclusion …
“Billions of tires are produced each year,” said Dr. Mayer. “Hundreds of millions of those tires become waste. Tires are a complex source of pollution, including whole tires, particles, compounds, and chemicals. As they wear, tires emit those pollutants, and they can transport them across atmospheric, aquatic, and terrestrial pathways. Tire pollutants represent potential environmental and human health risks. Comprehensive cleanup solutions are needed to reduce that risk.”
“This is not a trivial endeavor. Tires are a complex pollutant. And the remedies, the attenuation approaches, and remediation approaches are going to have to be equally complex, clever, and comprehensive.”
