In the animal kingdom, humans are the ultimate artisans, fashioning a myriad of objects from the world around us. However, it’s only recently we’re beginning to learn the impact of such ingenuity. Tires can now be added to that list, with a team from EMPA finding that around 200,000 tons of micro-rubber – little particles from tire abrasion – are polluting the environment in Switzerland.
"The magnitude of the released micro-rubber is on another level than that of microplastics – it is an astonishing finding when we were finally able to compare the magnitude of the different flows," study author Bernd Nowack told IFLScience.
The wear and tear of tires on the road results in small particles skimming off and landing on the road or whirling into the air. On average, a Swiss passenger car loses about 10 to 30 percent of its tread rubber during lifetime use.
This tire abrasion makes up 97 percent of all micro-rubber in the environment, with artificial turf making up the rest, according to a study published in the journal Environmental Pollution. Around 75 percent of tire particles remain within 5 meters (16 feet) of roadsides, while almost 20 percent make their way into water bodies and 5 percent into the soils.
Hearing the word "micro-rubber" may conjure thoughts of microplastics, but the team are quick to note that they are different particles that cannot be compared to each other.
"Tires only contain 50 percent rubber, the rest are additives," said Nowack. "This is different to a polymer like PE that contains much less and completely different additives. In order to evaluate the environmental behavior and effects, we therefore need to consider different polymers separately. Nobody would evaluate the herbicide atrazine and PCBs as a group – different chemicals or materials each need a specific assessment."
He calculated that 93 percent of polymer-based microparticles are from tire abrasion, while only 7 percent of the seven most widely used commodity plastics are. "The amount of micro-rubber in the environment is huge and therefore highly relevant.”
To make such calculations, the team used a process called dynamic probabilistic material flow analysis (DPMFA), which allowed them to track mass flows over time under certain parameters to help them estimate the amount deposited into water bodies, soils, and on roads. They based the data on the import and export of tires from 1988 to 2018.
The team say the impact of rubber microparticles in the air on humans is likely low. In terms of surface water pollution, rubber from tires can get there via several sources.
"Almost half of the total rubber reaching surface waters (47 percent) originates from the separate sewer system. 34 percent of the rubber reaching surface waters has passed through [road runoff treatment plants] without being removed and thus flows into water bodies," write the team.
Starting in 2000, recycling guidelines for water and soil pollution prevention were tightened, which means some of the micro-rubber can be removed from the water via road wastewater treatment plants (SABA).
"The results provide a basis for further studies – both on the exposure side (e.g. for field studies or fate modeling)," said Nowack, "as well as the hazard side (e.g. what are the effects of micro rubber particles on soil organisms)."