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Toxic additives in tyre rubber can leach into soil, be taken up by plants and end up in our food, according to the latest study highlighting a major source of pollution that is largely unregulated.

鈥淚t鈥檚 too early to say there鈥檚 no risk or high risk from food at the moment, but this might change in the next five years,鈥� says at the University of Vienna in Austria. 鈥淭yre wear particles are a major environmental and health concern, in some cases bigger than other plastic pollution.鈥�

Tyre wear generates around 6 million tonnes of dust worldwide each year. It is estimated to make up half of the microplastics entering rivers, lakes and seas, and 80 per cent of the microplastics in air.

The inhalation of tyre dust is the biggest concern, says Hofmann. People near or on busy roads are breathing in tyre dust that ends up in their lungs or guts.

In addition to natural and synthetic forms of rubber, tyres can contain hundreds of other compounds. There is no regulation of what is added to tyres, he says, nor do tyre makers reveal what they add.

Some additives can leach out of tyre dust into water. 鈥淐hemicals leaching from plastics, including tyre wear, may be an even greater long-term environmental and human health concern than the physical properties of the particles themselves,鈥� says Hofmann. He points to a 2020 study, which found that a tyre additive was , as a reason for concern.

In many cases, little is known about what effects additives have on plants and animals. When his group started studying tyre wear pollution around a decade ago, they found to their surprise that hardly any research had been done. 鈥淭yres have been overlooked in the debate about plastic pollution,鈥� he says.

Tyre additives can end up on fields in three ways. Firstly, run-off from roads often goes into sewage systems, and in many countries, sewage sludge 鈥� euphemistically termed biosolids 鈥� is applied to fields.

Secondly, reclaimed water from sewage systems is increasingly used for irrigation in Europe, but the treatment plants don鈥檛 usually remove soluble compounds derived from tyres.

Thirdly, tyre dust from roads can blow directly onto fields.

To find out if tyre compounds in fields can get into food, Hofmann鈥檚 colleague , also at the University of Vienna, applied four tyre additives with very different chemical structures to lettuces growing in pots in a greenhouse and tested the leaves after three weeks. Significant levels of three of the four compounds were found in the leaves, with the levels varying according to soil type.

鈥淭he message here is very simple. These compounds are taken up under realistic growing conditions,鈥� says Hofmann, who presented the results at a meeting of the European Geosciences Union in Vienna in April.

鈥淭he risk to human health is unknown, but we need to act now and restrict chemical plastic pollution,鈥� he says.

鈥淚 find the presence of any chemical with toxic properties in foods and environmental systems concerning, and this study again emphasises the importance of only using non-toxic chemicals in tyres,鈥� says at the University of Washington in Seattle, whose team did the coho salmon study.

鈥淯ntil we can ensure that tyres are made from non-toxic chemical ingredients, I agree that we should be controlling the widespread dispersal of tyre dusts and other rubber particles,鈥� says Kolodziej.

There are three ways this can be done, says Hofmann. Firstly, manufacturers should alter tyres to minimise wear 鈥� this can be done without any impact on performance, his research has shown.

Secondly, filters in wheel wells that capture tyre dust should be introduced. Hofmann thinks they could be manufactured very cheaply.

Thirdly, the run-off from busy roads should be trapped and treated. A few newer roads in countries including the UK do now have retention ponds.

鈥淩educing the total number of cars on public roads and investing more into public transportation would also help,鈥� says H盲mmerle.

Hofmann says his group is in contact with regulators in both Europe and the US, and that both are considering introducing regulations controlling tyre ingredients. He thinks the best approach would be to allow only compounds thought to be safe 鈥� substitutes for toxic additives could turn out to be toxic too.

If nothing is done, the problem will get worse, say Hofmann. 鈥淏asically, more cars are being produced and more tyres are being abraded.鈥�