As noted in the first installment of this two-part series, When the Rubber Leaves the Road, approximately 296 million cars populate this country’s roads. Assuming a conservative average of four wheels per car, roughly 1.18 billion tires are actively on the road in the United States alone.
These tires, while in motion, break down and release particulate matter, which ultimately ends up in our food, our water, our air and even our blood; however, tires do not only emit pollutants when in motion. Every stage of the tire lifecycle — from production to disposal — has negative environmental consequences. Having explored the harsh ecological effects of in-use tires, the second part of this series analyzes the impacts of tire production and disposal on people and our environment.
The Effects of Tire Pollution
To support the vast population of vehicles in the United States and abroad, companies represented by the U.S. Tire Manufacturers Association (USTMA) produced 331.9 million original and replacement tires for passenger cars, light trucks and trucks in 2023; the USTMA projects this number will increase to 335.7 million tires produced in 2024. Modern tire compositions contain a broad range of materials such as textiles, steel and carbon black; however, the majority components are natural and synthetic rubbers, comprising roughly 56 percent of the tire by weight. Methods by which these rubbers are sourced must be considered in tandem with the carbon footprint of production.
This analysis of tire production environmental impacts begins with material sourcing methods. A study conducted by the Swedish Research Council for Sustainable Development found that to grow the quantities of natural rubber needed for global tire production, vast, diverse forests have been slashed and burned to facilitate monocultural rubber tree plantations. These monocultural plantations, established in historically biodiverse, tropical environments, have had disastrous impacts on biodiversity in the affected area.
Beyond destruction of the natural environment, the production of tires generates an immense volume of greenhouse gases. The Energy and Sustainability Research Institute at the University of Groningen determined that during the manufacture of a single tire, 243 grams of particulate matter are released into the air, and 0.19 grams of ionized ammonium and 0.69 grams of solids will be suspended in wastewater. Research by Lublin University of Technology finds that every tire produced will require six megajoules of electrical energy and 12 gallons of water. All told, these data show that production of a single tire produces more than 367 tons of carbon dioxide equivalent. For reference, the tailpipe emissions of a typical internal combustion engine car will emit roughly 5.07 tons of carbon dioxide equivalent a year, according to the U.S. Environmental Protection Agency (EPA).
When Tires Are No Longer Useful
Although appearing benign, out-of-use tires are still environmentally malignant as heavy metals, microplastics and carcinogens continue to leach from the material into the surrounding environment. The most prevalent mechanism for this emission is natural erosion. a review from YaleEnvironment360 reported on how the chemical structure within waste tire material, when exposed to wind, water and sunlight, deteriorates and releases harmful compounds into the surrounding environment. Further, the SCNU Environmental Research Institute determined that hazardous tire ingredients, such as 6PPD, react with each other and the environment to form and discharge additional, novel toxic compounds as the chemical makeup experiences erosion by air, rain and ultraviolet light.
According to Bridgestone, the second largest tire manufacturer globally, car tires should be replaced every three to five years. Tread loss, rubber degradation and other wear and tear necessitate this replacement. According to analysis by the U.S. Federal Highway Administration (FHWA), 280 million tires are discarded every year. Of this 280 million, 30 million are candidates for retreading and reuse, leaving 250 million tires, or 2.875 million tons of material, to be treated as waste.
What becomes of these waste tires? Internal research from the FHWA found that “seven percent of the 250 million scrap tires generated annually are exported to foreign countries, eight percent are recycled into new products, and roughly 40 percent are used as tire-derived fuel.”
The three largest importers of U.S. scrap tires are India, Pakistan and Colombia, according to research by the market analysis firm Volza. The U.S. EPA found that after receipt, the imported scrap tires are retreaded, or shredded into crumb rubber for construction or fuel. However, not all imported scrap tire is or can be repurposed. Some ultimately ends up in landfills alongside internally generated scrap tires, compounding the volume of tire waste in receiving countries.
Similar to how waste tires are used in importing countries, within the U.S., tire scrap is ground, shredded and chipped for various industrial applications. Tire end-of-life analysis conducted by the FHWA details the various destinations of scrap tire. Ground tire rubber is restructured into various rubber- and plastic-based products (such as seal coat, mud guards, or crack sealant) and as an addition to asphalt to improve its surface frictional resistance. Chipped and shredded rubber is used as a fill material for highway embankment construction and as aggregate in roadways and road basins, according to a representative from the Minnesota Pollution Control Agency. While the repurposing of these tires prevents them lying dormant in a landfill, their ultimate destination does not contain the harmful chemicals. From the same YaleEnvironment360 report, processing waste tires into useable materials releases the same toxic compounds as road abrasion. In addition, the repurposed tire scrap is degraded by sunlight, and water runoff leaches chemicals into their surrounding environment.
Shredded tire waste also has utility as a source of energy. From processes recorded by the USMTA, these tire shreds are incinerated for fuel in industrial processes such as cement and paper production. A report from the European Tyre and Rubber Manufacturers’ Association noted use of tire-derived fuels for urban heating and electrical power generation. According to a study conducted by the U.S. EPA focusing on tire-derived fuels, shredded tire fuel, per unit, produces similar quantities of energy as oil and more energy than coal and is comparably clean when compared to coal in heavy metal and nitrous oxide concentration in emissions. Regardless of any advantages in efficiency and relative cleanliness, research at Perm National Research Polytechnic University found that incineration of shredded scrap tire waste for power generation still emit carcinogenic particulates into the atmosphere when combusted in improperly constructed and filtered cement kilns and other industrial boilers.
The percentages of tires exported, repurposed and burned total only 55%; the remaining 45% are interned in landfills, stockpiles and illegal dumps. Although 48 states regulate the disposal of waste tires in landfills to some extent, regulation is not necessarily effective.
Minnesota, for example, has regulations in place to limit the number of waste tires in its landfills; however, these regulations still allow for 10,000 waste tires to be deposited in every landfill, per a representative from the MPCA. These tires are not necessarily whole; some have been chipped or shredded to provide infill and improve drainage. Yet, whole or processed, these landfilled waste tires are still subject to rain and sunlight degradation, allowing further emission of harmful compounds. In addition, waste tires stored in landfills are at risk of combustion. Research from the University of Iowa studied the effects of an uncontrolled, spontaneous fire in a landfill built over a drainage layer constructed with waste tire material. This fire burned for nearly a month, releasing particulate matter, aromatic hydrocarbons and carcinogens into the atmosphere at hazardous concentrations within a seven-mile radius of the landfill. The behavior of this fire was not unique among landfill tire fires.
Atmospheric Environment, Volume 104, 2015, Pages 273-283, ISSN 1352-2310,
What Can Be Done With Tires?
Once tires are created, they are impossible to dispose of perfectly. Regardless of processing method, negative environmental externalities will occur. But improvement on current methods is possible.
To reduce the environmental harm within existing systems of waste tire management, regulations on material processing need improvement. Landfills must plan for fire emergencies, as recommended by the study conducted by the University of Iowa; incineration facility construction must be monitored to ensure proper containment and management of emissions such that they do not exceed traditional fuels, as noted by the U.S. EPA; and, construction use of scrap rubber aggregate that would result in ground pollution must be curtailed.
Beyond expanding regulation of existing infrastructure, novel solutions founded in ongoing research show promise for improved waste tire management.
A review of literature by the University of Applied Sciences FH Technikum Wien highlights developing methods of rubber devulcanization. Vulcanization is the process of heating rubber with sulfurous compounds to improve the material’s elasticity and strength. This process is necessary for the tire to withstand the harsh use conditions; however, vulcanized rubber cannot be remolten and adapted for new purposes. The devulcanization process breaks down the sulfurous compounds that prevent the melting and recasting of recycled tire rubber. Although not currently an area of large-scale investment within the tire management industry, rubber devulcanization offers the ability to produce sustainable raw materials while maintaining their original mechanical properties.
The Botswana International University of Science and Technology (BIUST) reviewed literature discussing improvements in processes for tire-derived fuel generation. The current mass-implemented method of burning shredded and chipped waste material yields unusable byproducts; however, through pyrolysis, the process of breaking down a material in a pressure chamber, waste tire material deteriorates into usable non-condensable gas, pyrolysis oil and pyrolytic char. Both the gas and oil are useable as fuel in industrial processes and steam generation while the char has use in pigmentation and ultraviolet stabilization. Incineration of chipped and shredded tire material accounts for the vast majority of tire-derived fuel; yet, if built at scale, waste tire pyrolysis has potential to improve the energy recovery and cleanliness of deriving fuel from waste tires.
Ultimately, the environmental damage done through the generation of a tire cannot be completely undone by any means of recycling. For this reason, pollution is most economically and industrially efficiently managed through its prevention. The ultimate goal of tire pollution management can only be achieved by limiting the tire population to a level where each scrap tire generated is efficiently repurposed in some recycling method. This reduction in tires inherently requires an abatement of personal vehicle ownership — a proposal quite foreign to Americans. However, this goal can be achieved through an expansion of our public transit systems. Efficient and accessible public transit in cities and towns naturally creates conditions by which fewer people need to rely on personal vehicles for transportation.
The proposed Green Line extension, Blue Line extension and Gold Line Project would improve public, car-less mobility between downtown Minneapolis and St. Paul. To make the cities and surrounding areas fully accessible, however, the programs must expand to replace routes such as Interstate 94, I-35, I-394, MN-36, MN-280 and MN-55. These highways provide essential access to communities surrounding the city center, which are largely inaccessible without a personal vehicle. Meanwhile, the Minnesota Department of Transportation’s (MnDOT) Rethinking I-94 could provide an elegant solution to this problem. One at-grade option shows a mixed-use-zoned parkway featuring rapid public transit to replace the stretch of I-94 between Minneapolis and St. Paul. Though proponents have slim hope that MnDOT will favor this alternative, reconceiving the interstate would improve access to public transit along the busy corridor and provide dense, affordable housing and commercial districts for residents in communities within the project area. (See the Twin Cities Boulevard proposal as envisioned by Our Streets.)
For regional and interstate transit to be accessible, these solutions cannot be restricted to city metro areas; programs such as the Borealis Line and the Northstar Line provide residents across Minnesota access to affordable regional transit. Programs like these can be implemented in cities across the country, linking city to city and improving public transit access for rural communities. As communities across Minnesota, and the United States, become more walkable, as local and regional transit expand, and as commercial and residential areas synthesize, the necessity for a personal vehicle to accomplish daily activities and travel decreases. The natural consequence is a reduction in demand for personal vehicles and, by extension, tires.
Solutions for waste tire pollution are difficult to derive in isolation. Personal vehicles are so deeply engrained in the U.S. transportation system that a reduced ubiquity feels foreign and unattainable. However, as we better understand the impacts of the industrialized world on the environment and human health, we must remedy the underlying causes of the negative externalities. Considering production of goods in a circular manner, where goods are employed in some secondary and tertiary function after serving their original use, will inspire more thoughtful production and efficient use of resources.
Building new housing, railroads and bus lines and renovating highway systems will not innately lessen demand for vehicles and tires by directly restricting their use; rather, these actions will create conditions under which fewer people will require cars to buy groceries, to see their family — to live their lives. Environmental issues such as waste tire pollution necessitate a restructuring of our economic and infrastructural systems, a reconsideration of our relationship with resource management and industrial production, and a recognition of the inherent inequities in our society.
Each problem we face affords us the opportunity to work together to better our society technologically and socially and to build a better world for every member of our community.