Where Pollution Hits The Road: The Growing Environmental Hazard Of Rubber Tires

As ubiquitous as rubber tires are due to the many practical benefits they offer to cars, trucks, and other conveyances, they do come with a limited lifespan. Over time, the part of the tire that contacts the road surface wears away, until a tire replacement is necessitated. Perhaps unsurprisingly, the material that wears away does not magically vanish, but ends up in the environment.

Because of the materials used to create tires, this worn away material is counted as a microplastic, which is a known environmental pollutant. In addition, more recently it’s been found that one additive commonly found in tires, called 6PPD, is highly toxic to certain species of fish and other marine life.

There are also indications that these fine bits of worn-off tire contribute to PM_2.5 particulate matter. This size of particulates is fine enough to penetrate deep into the lungs of humans and other animals, where they can cause health issues and exacerbate COPD and similar conditions. These discoveries raise a lot of questions about our use of tires, along with the question of whether electric vehicles stand to make this issue even worse.

A Lot Of Dust

An obvious question that comes to mind when confronted with this issue is just how much material we are talking about. Kole et al. (2017) published a study that answers exactly this question using a collection of statistics and educated extrapolations. Their estimate comes down to between 0.23 to 4.7 kg/year, with a global average of 0.81 kg/year/capita. The estimated contribution of tires to the microplastics that end up in oceans annually would be 5 – 10%.

Most notable about these numbers is perhaps that this means that the particulate matter (PM_2.5) pollution produced by the wear on tires is significantly higher than that produced by the tail pipes of internal combustion engine vehicles (ICEs). After decades of regulating the exhaust gases from combustion engines, it would seem that it is now time to look at other sources of this type of pollution. Especially since this type of particulate matter is not one that will vanish as vehicles switch from ICEs to batteries and electric motors, as the latter will still have tires.

An awkwardly situated elephant in the room that further complicates this matter is that the weight of BEVs tend to be higher than that of ICE cars due to the weight of the battery pack. This may lead to BEVs wearing down tires faster and thus creating more microplastics. Whether thiss will this turn out to be an issue remains to be seen, as factors such as driving style and the use of regenerative braking have to be taken into account as well.

Considering that microplastics, particles < 5 mm in length, are now found practically everywhere on Earth, including in our own blood, it raises uncomfortable questions about how harmful they are exactly. This seems a valid question considering that one property of these plastics is that they take a very long time to degrade. If they hang around this long, then surely they do not interact with or harm ecosystems and the insides of our bodies?

As noted by XiaoZhi Lim in the Nature article, there are real, observable impacts from microplastics in the environment, with some obvious cases being fibrous microplastics interfering with zooplankton, and marine life discovered with significant amounts of microplastics in their digestive tract. Here the harm seems to mostly come from microplastics interfering with basic activities, such as foraging and digestion, rather than being outright toxic.

Although at this point in time there is no evidence to support the theory that micro- and nanoplastics are actively harmful to human health beyond PM_2.5 pollution, indications are that at least in the case of tire wear, the additives that leach out of the fragments into the environment  can do serious damage.

More Than Just Rubber

Tires are rather complex constructions. Rather than just natural, vulcanized rubber in a funny shape, their manufacturing involves the combination of natural and synthetic rubber, along with carbon black, silica and a range of antioxidants and antiozonants, the latter two serving to make the tire more resistant to UV and ozone exposure.

About half of a tire by weight is carbon black, which is a paracrystalline form of carbon. There is limited evidence (Group 2B) that exposure to carbon black may be carcinogenic, and at low levels it would seem to cause harm to the public’s health. The synthetic rubber in tires is usually styrene-butadiene rubber (SBR), which provides comparable properties to natural rubber when protected by additives, a common one being 6PPD (C₁₈H₂₄N₂), which acts as both an antioxidant and antiozonant.

Recent research (Tian et al. (2020), Brinkmann et al. (2022)) has shown that it is this commonly used 6PPD additive which ends up being highly toxic to marine species like the coho salmon (Oncorhynchus kisutch) with an LC₅₀ (median lethal concentration) of < 0.8 μg/L. The toxicity appears when the 6PPD molecule reacts with ozone to form a quinone form (6PPD-quinone). Multiple studies have now attempted to determine which marine species are the most sensitive to this 6PPD-quinone substance, with Brinkmann et al. finding that rainbow trout (LC₅₀ 0.59 μg/L) and brook trout (LC₅₀ 1 μg/L) are also very sensitive.

For these sensitive species, the displayed symptoms upon exposure to 6PPD-quinone in sufficient concentrations included increased ventilation, gasping, spiraling, and loss of equilibrium. Eventually the sensitive fish species in these studies would succumb and die. As noted by Brinkmann et al., 6PPD-quinone is found in significant concentrations in stormwater runoff and in surface waters along the US west coast at concentrations of ≤ 19 μg/L. Effectively this means that these waters are lethal to at least a number of species that are of ecological, economical and cultural importance in this area.

Fixing The Mess

Even as the ecological, environmental and health implications of microplastics are still hotly debated today, it is hard to argue with the well-documented health impact of PM_2.5 particulates, not to mention that of 6PPD-quinone on marine species. The human health impact of tire wear will be most visible in the occurrence and severity of COPD and similar conditions near roadways and similar areas where significant amounts of tire wear occurs.

For marine species the health implications seem significantly bleaker, however. While some fish species seem remarkably unimpressed by even full saturation levels of 6PPD-quinone, for many others even fairly low concentrations appear to be invariably lethal. Since many of these sensitive species are of commercial interest,  there is a real risk that the increasing presence of this molecule in marine environments may drive them into extinction, or at least make their commercial exploitation impossible.

Unfortunately, as noted by the U.S. Tire Manufacturers Association (USTMA), there is no known alternative to 6PPD that can easily replace this tire additive. In the meantime, mitigation methods are being examined, and motorists are being urged to maintain proper tire pressure to reduce tire wear. Hopefully before long we will find a replacement for 6PPD that addresses the toxicity issue.

As to the microplastics issue, this is an issue that sadly stretches far beyond tires. Even so, there are aspiring attempts to capture the particulates from tire wear at the source, using an electrostatic capturing method or air filtration attached directly near the tire. If successful, this might even help mitigate the 6PPD problem, though obviously not all tire wear particles are so easily captured, as demonstrated by the tire marks on every stretch of tarmac.