A study by EPFL and the Swiss Federal Food Safety and Veterinary Office (FSVO) has found that tire additives enter into and pass through the food chain. Further research is needed to establish the implications for human health.
Traces of the additives typically used in tire manufacturing have been detected in all of the most common types of fruits and vegetables eaten in Switzerland. That's the key finding of the EPFL-FSVO study, published in Journal of Hazardous Materials. The scientists don't yet know the long-term implications of exposure to these substances for human health. Further research is needed to clarify this point.
The study follows on from two Austrian studies demonstrating that these additives were present in leafy vegetables. These earlier studies were cited in Swiss-German magazine K-Tipp in August 2023, causing quite a stir. The FSVO then contacted Florian Breider, who heads the Central Environmental Laboratory research group (GR-CEL) at EPFL, and asked him to investigate whether the same holds true for a wider range of vegetables sold in Switzerland. "We worked on the study together," says Breider. "We sampled around 100 of the most commonly eaten fruits and vegetables from a representative group of nine Swiss retailers, ranging from major supermarket chains to organic markets and small, local grocery stores."
After rinsing the fruits and vegetables and turning them into workable samples, the scientists tested them for 11 compounds typically found in tire additives. Using consumption data held by the FSVO, they were then able to calculate theoretical daily intake values for these substances. They found that 31% of the samples contained traces of the compounds, including 6-PPD and 6-PPD-quinone, with no difference according to where the fruits and vegetables came from or whether they were organic. "Since people's diets are broadly the same throughout Western Europe, there's a good chance that these findings are representative of consumers' exposure in neighboring countries," says Breider.
Unknown human-health implications
Previous studies have established that tire additives, especially DPG, 6-PPD and 6-PPD-quinone, are toxic to mammals. This research, which has so far been carried out only on rodents, found that these additives lead to decreased fertility in males and have neurotoxic and neuroinflammatory effects. The critical threshold for humans still needs to be established. "We don't know how these compounds are metabolized by the human body," explains Breider. "It's something we need to explore because we're all exposed to these compounds - and especially people in certain population groups, such as road workers." Scientists in China are also conducting in-depth research into the subject, analyzing human blood and urine for the presence of these substances. The GR-CEL team at EPFL plans to conduct its own studies in the coming months.
Inhaled and ingested
When tires wear against road surfaces, they release additives such as antioxidants and vulcanizing agents (which give rubber more strength, elasticity and durability). These particles, the toxicity of which is yet to be determined, disperse through the air, settle on the ground, and are transported in runoff water. Humans are exposed to them in two ways: by inhaling them and, as the EPFL-FSVO study shows, by ingesting them in contaminated food.
According to a paper published in 2017, six million metric tons of these additives are released into the environment every year. And, as a 2023 Norwegian study found, rural communities are also exposed to these substances despite their low road traffic. Research under way at EPFL has found that these compounds are even present in mountain lakes. "Our exposure to these additives is similar to that for other micropollutants," says Breider. "They're around us constantly, in every part of our environment. What we don't know is whether we need to introduce tighter controls, such as by phasing them out in tire manufacturing in favor of less toxic alternatives."
Decontamination and good driving practices
Scientists in Switzerland are currently exploring ways in which roads can be decontaminated to prevent tire additives from entering the environment. Several studies have shown that aggressive driving - with hard acceleration and braking - increases tire wear, making it more likely that these particles will transfer into the air, soil and surface water.
