Lettuce Accrues Toxins From Our Car Tires, Study Proves

Salad greens in lab found to accrue unnatural chemicals, one of which has been causing mass salmon deaths in the United States

Ruth Schuster
Ruth Schuster
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Water droplets line a lettuce leaf in Imperial Valley, California. You should be more worried about what is now growing inside the lettuce.Credit: CAITLIN OCHS/REUTERS
Ruth Schuster
Ruth Schuster

Lettuce is usually touted as a healthy foodstuff rather than a source of hazardous waste. But it turns out it readily takes up toxic particles shed by car tires as we drive. Even worse, the plant can speedily break down these particles but some of the products of their breakdown are stable in the leaves – meaning, they stay.

As the plant’s exposure to the contaminant continues, these stable products can accrue in the leaves, warns new research done on lettuces grown in tightly controlled hydroponic conditions in a lab in Vienna.

The study involved basic research done in the lab, not a test of salad greens grown by roadways. However, it proves that lettuce can take up substances shed by vehicle tires that we then eat, and quantifies the problem under lab conditions.

We knew plants take up unnatural contaminants. We knew tires are a horror, environmentally speaking, and that as we drive, they shed material. In fact the Guardian reported in July that almost 2,000 times more particle pollution is produced by tire wear than is produced by car exhausts.

Now researchers at the Center for Microbiology and Environmental Systems Science at the University of Vienna have analyzed the lettuce-tire crud relationship in a clean environment, and reported their results in the prestigious journal of Environmental Science & Technology.

Why lettuce and why hydroponic? Because hydroponic cultivation (in water, without soil) is easy, and it’s easier to observe if something reaches the leaves from the root in comparison with, say, carrots, explains co-author Prof. Thilo Hofmann.

Also, lettuce grows fast: you can have plants to test in a couple of weeks, adds Prof. Benny Chefetz, head of the Soil and Water Sciences department at Hebrew University. He wasn’t involved in this paper but will be in the next phase of the research.

Tire contamination of crops was mimicked by spiking the water in which baby lettuces of the type Valerianella locusta L. were grown. The team added five types of chemicals, of which four serve in tire manufacturing, and cultivated the plants in tightly monitored conditions.

The fifth was a transformation product of one of the other four, 6PPD-quinone – a known toxin linked to mass deaths of salmon and trout in the United States.

The lettuces took up all the tire-derived compounds, according to Anya Sherman, the first author and a doctoral student at the university. The plants broke down the chemicals into stable elements that accrued in the leaves.

The lettuce might be able to rid itself of these unnatural additives over time, but in the lab the researchers kept spiking the water – again mimicking the reality, because driving hasn’t suddenly stopped.

If this happens in the lab, it is reasonable to assume it happens in “nature.” Vehicles are legion and their tires constantly leach particles; inevitably, the particles get blown about by the wind and water, and some reach fields.

So, aside from fossil fuel emissions, aside from traffic accidents, road rage, our physical condition and a thousand other reasons to eschew the car, we have salad.

Another major source of plastic particles in fields is using sewage sludge for fertilizer and recycled waste water or biosolids, as done a lot in Israel, Hofmann points out.

One might think that in the real world, small plastic or tire particles would land on the topsoil and stay there, but the study shows propensity to migrate to our meals. “Our measurements showed that the lettuce plants took up all the compounds we investigated through their roots, translocated them into the lettuce leaves and accumulated them there,” Sherman says.

An organic Romano lettuce.Credit: nblx / Shutterstock.com

Murky damage

Note that not all tire-production chemicals capable of roosting in plants are confirmed to be harmful.

So how dangerous are our tires to our lettuce? Hard to say: Some of the compounds that the plants produced from the tire particles were previously unknown, so we have no idea what they might do to us, explains team member Thorsten Hüffer.

But even if some compounds build up in the leaves because the lettuce “doesn’t know” how to handle them, our bodies may do. When we eat the tire-afflicted lettuce, the original chemicals could be reconstituted in our body, according to Sherman. Also, while the environmental harm is not in question, the harm microplastics do to our bodies is still under study.

“We expect this to happen for many other plants too,” Hofmann says. However, he cautions that results from the lab cannot be extrapolated to the supermarket.

Which brings us to the planned next stage of this research.

“The next stage will be to grow the same lettuce on actual soils, and the soils won’t be spiked with those compounds like in the Vienna lab but contaminated with real tire-wear particles,” Chefetz says. They will then test for real-world contaminant concentration in the leaves. This will be done at the Hebrew University’s faculty for agriculture in Rehovot, though Chefetz adds that the Israeli government has been reluctant to provide funding: “They try to ignore the problem rather than face it,” he says.

Make no mistake, the potential problem is huge. “Unlike ‘real plastic’ that needs a lot of time to break down into microplastics, the fact is that we’re using tires on a daily basis and they create microplastics all the time,” says Chefetz. “Our roads are polluted with microplastic on a daily basis, and washes off in the rain and from wind. There are also heavy metals. Tires are made of a lot of dirty stuff. It’s a real issue.”

And what are we laypeople supposed to do about all this? “Pressure politics and industry to search for better additives in tires, and ban toxic additives in plastic in general,” Hofmann advises. And in Israel, perhaps voters can help persuade the agriculture and environmental protection ministries to fund the research.

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