As combustion pollution diminishes, there is new focus on the rubber particles from tires.

Dan Carney

September 9, 2022

3 Min Read
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Image courtesy of Goodyear Tire & Rubber Co.

“Pollution,” in an automotive context, is normally associated with the smog-forming emissions from combustion engines that the Environmental Protection Agency terms “criteria emissions.” Another sort of pollution is particulate matter, the partially burned chunks of carbon that sweep out of the combustion chamber before they can be completely oxidized.

Particulate matter is barely a concern from modern gasoline engines, but it remains a significant hurdle for diesels, which is why compression-ignition vehicles employ particulate traps to prevent the release of this kind of pollution.

Researchers are finding that in an environment where combustion engines now release very little particulate matter, another source becomes prominent: tires. The friction between soft tire rubber and abrasive road surfaces creates Tire and Road Wear Particles (TRWP), which are a 50/50 mixture of tire tread fragments and road surface elements, according to the European Tyre & Rubber Manufacturers’ Association (ETRMA).

Considering the near-total elimination of particulate emissions from combustion engines, it is no surprise to learn that tires now contribute 1,850 times more particle pollution than engines do, according to research by the British firm Emission Analytics. The company’s report finds:

Related:Earth Day-Friendly Tires Made Using Recycled Water Bottles

“The fundamental trends that drive this ratio are: tailpipe particulate emissions are much lower on new cars, and tire wear emissions increase with vehicle mass and aggressiveness of driving style.  Tailpipe emissions are falling over time, as exhaust filters become more efficient and with the prospect of extending the measurement of particulates under the potential future Euro 7 regulation, while tire wear emissions are rising as vehicles become heavier and added power and torque is placed at the driver’s disposal.  On current trends, the ratio may well continue to increase.”

It is important to minimize or contain TRWP pollution, because some of these microplastics find their way into waterways, according to the ERTMA. “The study indicates that up to 5 percent of TRWP released may reach the estuaries and that the most important factors affecting the transportation of the TRWP to the estuary are their diameter and density,” the association said in a release on the topic. “The generation of TRWP is influenced by a variety of external factors including, but not limited to tire design. These include vehicle characteristics, road surface, weather, road topology and driving behavior.” 

At least this pollution doesn’t seem to get in the air as well as the water. The Tire Industry Project found that TRWP is a minor contributor to airborne particulate matter in three different cities studied. According to University of Applied Sciences and Arts Northwestern Switzerland physics professor Martin Fierz, tire particles are too big to hang in the air persistently or to be inhaled deeply into people’s lungs.

Related:Sensata Tire Tread Depth Sensor Automatically Warns of Worn Tires

In a Twitter thread commenting on this issue, Fierz explained that “Particles are not all identical. They have different sizes, different shapes, different chemical compositions - so some particles behave very differently than others."

The large chunks pose less risk of being inhaled, he continued. “Very large particles, bigger than about 10 micrometers, won't be inhaled deeply in our body, so they don't do much harm,” he said. “They are basically cleared out the next time you wipe your nose.”

“By far most of the tire wear particles are very large, so irrelevant for human health - we don't inhale them,” said Fierz. Because of this, it is the matter of tire microplastics getting into the water supply that is the more serious concern, he said.

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