As gas-guzzling cars are replaced by their electric counterparts, tailpipe emissions are on the decline. But cars have other negative impacts on environmental health, beyond what comes out of their exhaust pipes.
One of the bigger, and lesser known, problems is tire pollution — or "tire and road wear particles," in industry terminology.
Tires shed tiny particles with every rotation. Tire wear happens most dramatically during rapid acceleration, braking, and sharp turns, but even with the most conservative driving, particulate pollution is an unavoidable consequence of car use. And it's a problem that's poised to get worse as drivers transition to EVs.
"We're pushing for decarbonization by going to battery electric vehicles, and in doing so we're pushing up tire wear emissions … which is going to prove difficult to solve," said Nick Molden, founder and CEO of Emissions Analytics, a London-based company that performs independent tests on cars' real-world tailpipe and tire emissions. Molden pointed out that tailpipe exhaust is dramatically reduced by filters and catalytic converters, which use chemical reactions to reduce pollution. Meanwhile, tires are a fundamentally open system, so there is no viable way to capture the polluting particles that fly off of them.
Emissions Analytics found that a single car sheds almost nine pounds of tire weight per year, on average. Globally, that amounts to six million metric tons of tire pollution annually, with most of it coming from wealthier countries where personal car use is more prevalent.
Tire particulate is a toxic slurry of microplastics, volatile organic compounds, and other chemical additives that enter the air, soil, and water around trafficked areas.
The amount of tire pollution emitted per vehicle is increasing as more electric cars hit the road around the world — some 14 million of them this year, according to the International Energy Agency. EVs tend to be significantly heavier than gas-powered or hybrid cars due to their larger, heftier batteries. The average battery for an EV on the market today is roughly 1,000 pounds, with some outliers approaching 3,000 pounds — as much as an entire gasoline-powered compact car. Emissions Analytics has found that adding 1,000 pounds to a midsize vehicle increased tire wear by about 20 percent, and also that Tesla's Model Y generated 26 percent more tire pollution than a similar Kia hybrid. EVs' more aggressive torque, which translates into faster acceleration, is another factor that creates more tire particulate mile for mile, compared to similar internal combustion engine cars.
Tire particulate is a toxic slurry of microplastics, volatile organic compounds, and other chemical additives that enter the air, soil, and water around trafficked areas. The rubber, metals, and other compounds coming off tires settle along roads where rain washes them into waterways. Smaller bits of tire particulate linger in the air, where they can be inhaled, and the smallest of this particulate matter — known as PM 2.5, because each particle is 2.5 micrometers or less — can directly enter the bloodstream. A 2017 study estimated that tire wear is responsible for 5 to 10 percent of oceanic microplastic pollution, and 3 to 7 percent of airborne PM 2.5 pollution.
One particularly concerning chemical in tires is 6PPD, which is added to virtually all tires to prevent rubber from cracking. But in the environment, 6PPD reacts with ozone to become 6PPD-quinone, a substance that has been linked to salmon die-offs in the Pacific Northwest. A 2022 study confirmed the compound is also lethal to rainbow trout and brook trout.
Further research has shown that the chemical is absorbed by edible plants like lettuce and has the potential to accumulate in them. A study in South China found both 6PPD and 6PPD-quinone in human urine samples. The human health effects of the chemical are not yet understood, but other chemicals found in tires have been linked to problems ranging from skin irritation to respiratory problems to brain damage.
Given the intensifying realities of climate change, phasing out gas-powered vehicles rapidly is a must. But experts say the U.S. and other wealthy countries can accomplish this while also mitigating the environmental and health problems caused by EVs' increased tire wear — namely by curbing car use overall.
Foremost, local policymakers can take steps to make U.S. cities less cripplingly car-dependent. Although that might sound like a daunting task, there's historical precedent: The Netherlands used to be dominated by cars and experienced a higher rate of traffic fatalities than the U.S., until activist groups like Stop de Kindermoord ("Stop Child Murder") mobilized in the 1970s to let policymakers know that they wanted less traffic on their streets. According to Chris Bruntlett, the co-author of Building the Cycling City, policymakers created the low-traffic, bike-friendly Dutch cities we know today by instituting traffic-calming measures. "Officials started with speed-limit reductions, parking restrictions, through-traffic limitations, and lane narrowings and removals," Bruntlett told Grist.
David Zipper, a mobility expert and a visiting fellow at the Harvard Kennedy School, says that city leaders can also remove subsidies for car ownership, such as free residential parking on public streets. "Once car subsidies are removed, fewer people in cities will choose to buy and own them," Zipper said.
Of course, measures to reduce car use only work in tandem with investments in alternative transportation. The Infrastructure Investment and Jobs Act of 2021 provided some federal funding for transit and pedestrian and bike infrastructure, but making the most of these funds will require political will from state and local lawmakers. Zipper said that policymakers in some U.S. cities have begun to take positive actions — like Boston Mayor Michelle Wu, who has committed to expanding her city's bike lane network until 50 percent of the population lives within a three-minute walk of a bike lane.
One legislative solution to car bloat is introducing weight-based vehicle taxes, which encourage consumer interest in lighter cars and can be used to offset the cost of increased wear on roads caused by heavier vehicles.
Another way to reduce tire pollution is to trade big, heavy cars for smaller and lighter ones. Especially in the U.S., cars have grown significantly in size and weight in recent decades. Automakers began promoting SUVs in the 1980s, because a legal loophole allowed vehicles designated as "light trucks" to skirt fuel-efficiency regulations. Nine out of the 10 best-selling cars in the U.S. last year were trucks or SUVs, and the International Energy Agency has found that SUVs were the second largest cause of the global rise in CO2 emissions between 2010 and 2018.
One legislative solution to car bloat is introducing weight-based vehicle taxes, which encourage consumer interest in lighter cars and can be used to offset the cost of increased wear on roads caused by heavier vehicles. France implemented a weight-based car tax in 2021, charging consumers a penalty of 10 euros (about $10) for every kilogram above 1,800 (about 4,000 pounds) that their car weighs. This year, Norway also extended its weight-based vehicle tax to include EVs at a rate of a little more than a euro per kilogram above the first 500 kilograms (about 1,100 pounds) for EVs. Norway also taxes vehicles on their carbon dioxide and nitrogen oxides emissions. Taken together, these three taxes have the combined effect of dramatically incentivizing small electric vehicles.
In the U.S., some states already prorate vehicle registration fees based on weight, and Washington, D.C. recently overhauled its registration system to more heavily penalize larger cars. In D.C., owners of cars heavier than 6,000 pounds now have to pay $500 in annual fees. New York state lawmakers also recently introduced legislation that would similarly incentivize smaller cars.
Regulators can also take steps to minimize the harm caused by tire pollution — and in California, the process has already begun. In October, a new regulation implemented by the state's Department of Toxic Substances Control, or DTSC, will require manufacturers of tires on the California market to research safer alternatives to 6PPD. Manufacturers that sell tires in the state are obligated to notify DTSC about products containing 6PPD by the end of November.
Karl Palmer, deputy director of safer consumer products at DTSC, believes that making tire makers conduct an "alternatives analysis" will ultimately result in products that are safer for the environment.
"We're using California's market strength to say, 'If you want to park here, you've got to comply with our rules,'" Palmer told Grist.