Microplastics are so pervasive that they're now found in our bloodstreams, bones, and — according to one alarming study — even our brains in enough quantities to make a plastic spoon.

But where do they all come from? One of the biggest sources may surprise you: car tires.

Or maybe it isn't that surprising, now that we've brought it up. Yet, the role of civilization's addiction to the automobile tends to go overlooked in these discussions in favor of more obvious forms of waste like plastic bottles. But cars are everywhere, and so are their rubber feet. And as these tires wear down, that material has to go somewhere.

That somewhere often tends to be nearby waterways, write Boluwatife S. Olubusoye and James V. Cizdziel, two environmental chemists from the University of Mississippi, in a new writeup for The Conversation. That runoff, they warn, is wreaking havoc on aquatic life like fish, crabs, and oysters. 

And little is being done to stop it, even though as much as 45 percent of all microplastic pollution may come from tire wear particles, according to the researchers. (Other estimates say it's closer to 28 percent, which is still a shocking proportion.)

"In Oxford, Mississippi, we identified more than 30,000 tire wear particles in 24 liters of stormwater runoff from roads and parking lots after two rainstorms," they write, referring to a study they published earlier this year.  "In heavy traffic areas, we believe the concentrations could be much higher."

Tires aren't solely rubber — they also contain a chowder of other chemicals. One, a compound called zinc oxide, is extremely toxic to fish, the pair note. Another, called 6PPD-Q, has been linked to a mass die-off of coho salmon in Washington in 2020.

Besides its ubiquity — tires shed some six million metric tons of microplastics per year, according to one estimate — what makes tire pollution so nefarious is that the particles are tiny enough to be present in the air, and thus can easily be inhaled. A 2022 study conducted by Chinese researchers, for example, found 6PPD in the urine of children and adults, and in even higher concentrations in pregnant women. The true extent of its hazardous effects isn't known, but the substance has been linked with organ damage. 

The good news, for our aquatic friends at least, is that Olubosoye and Cizdziel think they have a way of sparing aquatic life from being poisoned by tire waste. As detailed in a study published in the journal Sustainability, they designed a simple filtration system that uses pine wood chips and biochar, a type of charcoal made by heating biomass waste like leftover rice husks that's typically used in soil. The takeaway is that the materials are inexpensive and environmentally friendly.

In their experiments, they placed a biochar "filter sock" at the mouth of a drainage outlet and monitored it over the course of two months. By the end, they found that compared to runoff that didn't to undergo filtering, the concentration of tire wear particles had been reduced by as much as 97.6 percent.

"The unique elongated and jagged features of tire wear particles make it easy for them to get trapped or entangled in the pores of these materials during a storm event," the pair explained. "Even the smallest tire wear particles were trapped in the intricate network of these materials."

The advantage of this approach is that it's cheap and reuses agricultural waste — but it only addresses one vector of tire pollution, and there's significant questions about how feasibly it could be scaled up. In lieu of transitioning to a carless society or revolutionizing how we design tires, however, it could be a helpful band-aid.

More on microplastics: You'll Be Flabbergasted to Learn Which Contains More Microplastics: Plastic Bottles or Glass Bottles


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