Beyond exhaust, non-exhaust emissions from tires and brakes are a growing source of urban pollution affecting all vehicles. An international study reveals their localized impact.
International study sheds light on tire and brake wear emissions
It’s not just the direct emissions from cars, those resulting from fuel combustion in the engine. There are also non-exhaust emissions from abrasion, which have a purely mechanical origin. Some examples? Tire wear emissions produced by friction on the asphalt, road surface wear, and consumption of braking materials.
While exhaust emissions from cars are regulated by increasingly precise legislation and must be reduced by 90% in Europe by 2035, abrasion emissions represent a growing share of urban particulate matter. And they concern both combustion and electric vehicles indiscriminately. In fact, battery-electric cars, being heavier, accelerate component wear.
Tire emissions: lichens used as sensors
An international study, which brought together researchers from the University of Siena, the National Institute of Geophysics and Volcanology, and Trent University in Canada, investigated the dispersion of non-exhaust emissions along 150 meters from Highway 401 in Toronto, North America’s busiest road. To analyze the impact, lichen transplants were performed at various distances from the roadway; this procedure is a biomonitoring technique used to assess air quality in areas where lichens do not grow naturally due to high pollution or where the impact of a specific source is to be measured. The reaction of the lichens determines the level of pollution.
Microplastic dispersion and fragment weight
Researchers placed lichen transplants at increasing distances from the roadside to understand how far the toxic footprint of traffic extends. The study confirmed that microplastics do not disperse uniformly in the air; their concentration dropped significantly just a few meters away, as most rubber fragments, being relatively heavy compared to gases, fall to the ground or onto lichens in the immediate vicinity of the roadway.
Tire and brake wear emissions: correlation between components
The study revealed that at just 35 meters from the highway, the amount of heavy metals – such as copper, iron, and barium, mainly from the wear of brake pads – detected in lichens drops by 70%. The highest risk zone for inhalation or soil contamination is very narrow and close to the infrastructure. Data show that where there is a lot of tire microplastic, there is also a lot of metallic brake particulate matter: this confirms that the source is unique, namely road transport. Every time a car decelerates or turns, it simultaneously releases both types of pollutants.
Towards Euro 7 regulation: a legislative turning point
The Euro 7 regulation also includes tire and brake wear emissions, in addition to exhaust emissions.
For the first time, it introduces standard methods for measuring particulate matter released by tires.
“The new Euro 7 provisions represent a turning point in automotive emissions legislation, regulating friction emission limits, i.e., from brakes and tires, for the first time,” states Aldo Winkler, head of the paleomagnetism laboratory at INGV. “In this regard, this article introduces the application of magnetic methodologies to tire microplastics, expanding on the approach that in 2020 demonstrated the decisive role of automotive brakes in the spread of metallic particulate matter in urban areas.”
