Driving a car produces tire wear, which ultimately ends up on farmland via wind and rain. From there, the organic pollutants contained in them could first end up in the vegetables and finally on our plates, as a study now shows. According to this, lettuce plants absorb all of the chemical compounds examined, some of which are toxic, and accumulate them in their leaves. When eating, we could then also eat the pollutants.
Contact with the road releases fine abrasion from car tires, which contains, among other things, microplastics and toxic pollutants. The particles of abrasion get into the environment with the wind and with the rain into water bodies and sewage sludge. The latter is often used as a fertilizer in agriculture, which means that the abrasion contained in the sludge also reaches the upper layers of arable soil. Since the roots of crops are also located there, the chemicals could get into plant foods from there.
Salad in the pollutant bath
In order to find out whether vegetables actually absorb toxic substances from tire abrasion, researchers led by Stephanie Castan from the University of Vienna carried out laboratory experiments. To do this, they added five chemicals to the nutrient solutions of lettuce plants. Four of these organic compounds are already used in tire production, among other things they serve to speed up the vulcanization of the plastic, as antioxidants, plasticizers and processing aids. The fifth chemical, an antioxidant, is only produced when the tire is actually in use. This so-called 6PPD quinone has been proven to be toxic and is associated with the mass die-off of salmon in the USA, for example.
During the cultivation of the test plants, the researchers regularly took samples of roots and leaves. Using high-resolution chromatography-mass spectrometry techniques, Castan's team then used these samples to determine how much of each of these five chemicals was being taken up by the lettuce plants. The scientists also tracked what the lettuce did with the pollutants once they entered its pathways and tissues. Where does he store them? What compounds does it metabolize them into?
Pollutants and their derivatives on the plate
The result: "Our measurements showed that the lettuce plants took up all the compounds we examined via the roots, transferred them to the lettuce leaves and accumulated there," reports Castan's colleague Anya Sherman. The plants also drew chemicals from the nutrient solution when they were still in the coarser tire granules. The analyzes also revealed that the lettuce uses the various chemical compounds for its metabolism and therefore chemically transforms and breaks down. “The plants processed the substances and also created compounds that have not been described before. Since we do not know the toxicity of these metabolites, they pose an unpredictable health risk," emphasizes Thorsten Hüffer, who is also part of the research team.
The products metabolized by the lettuce plant are also very stable. This means that they will most likely be preserved until our dinner plate. When eating lettuce, the substances then enter our body and are broken down there. "So if someone eats a contaminated lettuce like this, the original chemicals could be released back into the body," Sherman explains. In follow-up studies, the research team now wants to find out how and whether lettuce plants also absorb tire abrasion from the soil in the field. The team explains that the processes there are probably different than in the laboratory.
Source: University of Vienna; Specialist article: Environmental Science & Technology, doi: 10.1021/acs.est.2c05660