A rubber tire is a continuous band composed largely of both synthetic and natural rubber that fits around the rim of a wheel and allows the wheel to roll on roads and other surfaces while minimizing rolling resistance, absorbing shock from uneven surfaces, providing a high-friction bond between the vehicle and the ground to improve acceleration and handling, and protecting the wheel from abrasion and other damage.
Although simple in appearance, tires are highly complex devices, typically composed of more than a hundred individual parts and can contain more than 400 chemicals, many of which are carcinogenic. Mixed into the rubber are carbon black to increase its strength, silicon dioxide powder to provide improved fuel efficiency and wet grip, oil to soften the rubber, sulfur to improve elasticity, and antioxidants. Among the additional materials are polyester, rayon, nylon, aramid fibers and steel.
Although an engineering marvel, tires are a major cause of the earth's rapid environmental destruction. This damage results not only from the use of tires in automobiles and other vehicles but also from their manufacturing and from their disposal together with the vast and ever-increasing volume of production, currently at more than 2.5 billion units annually.
Obtaining natural rubber for tires has contributed to large-scale deforestation in order to create rubber tree plantations, particularly in Southeast Asia and Africa. Tire factories release toxic gases and other wastes that can pollute the soil and water bodies. Moreover, the tire manufacturing process is energy-intensive, thus resulting in the burning of fossil fuels, with its inevitable greenhouse gas emissions.
When in use on road vehicles, tires release large quantities of ultra-fine particles due to friction with the road surfaces. These particles, which contain polycyclic aromatic hydrocarbons and heavy metals, come from both the tires and the asphalt road paving and are a major source of microplastics pollution. Industry sources estimate that about six million metric tons are released worldwide each year, contaminating the air, soil and water and eventually entering the food chain.
Although exhaust emissions from internal combustion engine road vehicles have been tightly regulated for many years, tire wear has been completely unregulated. The result is that pollution from tire use may now be equal to, or even exceed, that from the engines.
Used tires are extremely difficult to recycle safely and economically because of both the the large number of different materials in them and the energy-intensive processes required. Although some attempts are being made, including by removing the steel and pulverizing the rubber for use in such applications as road paving, artificial grass and playground equipment, there is concern that these products will likewise continue to release toxic substances.
Thus, used tires are often stored in huge piles, deposited in landfills, or just scattered around the countryside and dumped into waterbodies. Because tires are highly flammable, it is not uncommon for such piles to catch fire, in which case they can burn for extended periods, emitting large volumes of harmful substances. When disposed of in landfills or elsewhere, they can leach harmful substances, including heavy metals, into the surrounding soil and contaminate the groundwater.
In addition to the ever-growing production of tires and accumulation of used tires, environmental damage from tires is also continuing to worsen because of the trend towards heavier vehicles. This trend is the result of consumer preferences, especially for SUVs and small trucks rather than conventional passenger cars. But it is also because of the increasing use of electric vehicles, which weigh substantially more than comparable internal combustion engine vehicles because of the heavy weight of the batteries.