Car tires contain hundreds of chemical additives that can leach into the environment and ultimately end up in crops, entering the food chain. Researchers at the University of Vienna's Center for Microbiology and Environmental Systems Science and the Hebrew University of Jerusalem have detected these chemical residues in leafy vegetables for the first time. Despite the low concentrations, the evidence is clear, echoing findings of drug residues in plant-based foods. The study was published in Frontiers in Environmental Science.

While drug residues in commercially sold fruits and vegetables have been widely studied, this new research highlights the presence of tire wear chemicals, known as additives, in the food chain. The international research team, led by Thilo Hofmann at the University of Vienna and Benny Chefetz at the Hebrew University of Jerusalem, examined vegetables from Switzerland and Israel. Some of these substances and their transformation products may pose ecological and toxicological risks.

Car tires are composed of a complex mix of materials that enhance performance and durability, including 5-15% chemical additives such as antioxidants, antiozonants, vulcanizing agents, and anti-aging agents. "The toxicity of tire and road wear particles is related to their organic additives and associated transformation products," explains Anya Sherman, a PhD student at CeMESS and the study's lead author.

These compounds can enter agricultural systems through atmospheric deposition, irrigation with treated wastewater, and the use of sewage sludge as fertilizer. "There they can be taken up by plants and thus also reach humans," adds Hofmann.
Residues of Tire Wear in Leafy Vegetables

The researchers analyzed the concentrations of tire additives in leafy vegetables from supermarkets and fields. They extrapolated these measurements to estimate daily intake levels. For example, the concentrations found were 238 nanograms per kilogram (ng/kg) for benzothiazole (BTZ) and 0.4 ng/kg for 6PPD, a substance whose transformation product, 6PPD quinone, is highly toxic to aquatic species like coho salmon. Depending on dietary habits, this leads to a daily intake of 12 to 1,296 ng for BTZ and 0.06 to 2.6 ng for 6PPD. These levels are comparable to drug residues found in food.

"The concentrations and daily intake are fortunately relatively low, but additives from car tires are still found in food, which is concerning," says Hofmann. The next steps should involve investigating the environmental and human health impacts of these findings.
From the Street, to the Plant, into the Body

In 2023, scientists demonstrated that additives from car tires can be absorbed by plants under laboratory conditions. The current study, however, analyzed whether this absorption also occurs in natural growing conditions. The team examined real samples from supermarkets in Switzerland and field vegetables from Israel. "We used high-resolution mass spectrometry to analyze sixteen tire-associated compounds in the samples," says Hofmann.

The leafy vegetables from Swiss supermarkets originated from Italy, Spain, and Switzerland, while the Israeli samples came directly from fields after harvest. The study confirms that tire additives can indeed be taken up by vegetables grown under natural conditions, highlighting a potential risk that warrants further investigation.