Environment: ENEA studies concentrations of microplastics from tires into the air

An international study published in the journal Atmospheric Environment, conducted as part of the European POLYRISK^[1]project, in collaboration with ENEA, shows that concentrations of microplastics generated by tires in the air can be up to five times higher in urban areas with heavy traffic, where vehicles brake and accelerate frequently.

"In cities, one of the main sources of microplastic pollution is the tiny particles generated by the friction of tires on asphalt during normal vehicle traffic. So far, only a few international studies have quantified the atmospheric concentrations of these particles," explained Maria Rita Montereali, a researcher at the ENEA Laboratory for Impacts on the Territory and Developing Countries in the Sustainability Department and co-author of the study with Laura Caiazzo and Sonia Manzo from the same department. "We wanted to measure the presence of these microplastics and evaluate their variations compared to other primary traffic pollutants, analysing areas characterized by different traffic conditions. In the future, the data collected can be used to verify possible associations with health effects" she said.

In the period 2022-2023, the POLIRISK project team monitored three sites characterized by different traffic flows and speeds: an urban road with a stop-and-go traffic pattern, an highway section with heavy but flowing traffic, and a city park located 50 meters from the nearest (secondary) road, conducting sampling campaigns for PM10 atmospheric particulate matter ^[2]in Utrecht (Netherlands), where it is estimated that between 880 and 2900 tons of plastic particles are released into the air each year due to tire abrasion on roads.

To measure air quality, the researchers used synthetic and natural rubber markers-the main components of tires-and a marker for benzothiazole, an additive used to harden rubber. Concurrently, other traffic-generated pollutants from brake wear (metals such as iron, copper, chromium, and manganese) and engine emissions were also monitored to verify their correlation with the microplastics studied.

The lowest concentrations of microplastics from tires were measured in the park (3.1-5.1 nanograms per cubic meter), while the highest concentrations were detected near the highway (7.8-18.1 ng/m³) and to a greater extent in the "stop-and-go" traffic area where vehicles stopped and started frequently. Therefore, compared to the park, the levels of synthetic and natural rubber in the air were on average up to 3 times higher on the highway and almost 5 times higher in stop-and-go traffic area.

Benzothiazole concentrations were also higher near the highway (2.4 times higher) and in the stop-and-go area (4.6 times higher) than in the park. Benzothiazole is a substance used in the rubber vulcanization process for tire production. Although it can be found in other products- antifreeze, corrosion inhibitors for the paper industry, intermediates in dye synthesis-we identified it as a marker of particular interest since it is used in in vitro and in vivo toxicity tests and in our samples showed a strong correlation with other indicators of the presence of rubber particles in the air," explained ENEA researcher Laura Caiazzo.

In general, metal concentrations resulting from brake wear were three to eight times higher in busy areas than in the park. Furthermore, compared to the highway, the area characterized by intermittent 'stop-and-go' traffic had the highest amounts of metal elements in the air (2 to 4.8 times more).

In the samples collected, the amount of microplastics from tires accounted for an average of 0.45% of PM10 particulate matter ^[3]: a relatively small fraction, but one whose weight could increase in the future. "The adoption of increasingly stringent air quality standards, which set even lower limits for exhaust emissions, may lead to a relative increase in the contribution of non-combustion particles to PM10 particulate matter, a trend that is likely to continue in the coming years," pointed out ENEA researcher Sonia Manzo, who coordinated the study for ENEA. "The spread of electric vehicles" she said "will partially reduce emissions of combustion-related pollutants, but it is likely to lead to an increase in the levels of microplastics produced by wear and tear and contact with asphalt tires, due to greater friction caused by the increased weight of these vehicles".