Toxicity of tire wear particles rising up the industry agenda

Hanover, Germany - Issues around tire wear are now topping the agenda of the tire manufacturing industry, with much effort among leading players and industry bodies to establish standarised measurement protocols to guide future regulation in this increasingly critical area.

But while much of the attention is on quantifying the rubber ‘dust’ and tire & road wear particles (TRWPs) generated by abrasion on road surfaces, and how driving behaviour affects this, another topic – toxicity – could soon take centre-stage.

According to Nick Molden, founder & CEO of UK-based Emissions Analytics, each car generates 4kg/year of abraded rubber, equivalent to 6 million tonnes per annum globally.

These ultrafine – below 100nm – particles are airborne before eventually settling around roadways, with larger particles being washed into drainage systems, said Molden, who is also an honorary research fellow at Imperial College London.

This, in turn, can lead to tire wear reaching land and marine environments, he added in a presentation, titled ‘Independent VOC analysis of European and American market tire particles’, at the Tire Technology Expo 2023, staged 21-23 March in Hanover.

There are then multiple vectors for human inhalation or ingestion, with some research estimating that there is on average 135ng of rubber chemical-derived 6PPD (n-(1,3-dimethylbutyl)-n’-phenyl-p-phenylenediamine) and 6PPD-quinone in the urine of every adult.

Molden also noted that 6PPD-quinone, formed by the oxidation of rubber antioxidant 6PPD, has recently been linked with deaths of coho salmon and trout in the US.

In a recent research programme, Emissions Analytics analysed 228 ‘light duty’ tires sourced from retail outlets across Europe and North America.

The results showed significantly less human toxicity potential in US tires, due to lower aromatic and alkanes content. The marine toxicity levels recorded were much less significant in scale.

In similar testing of 55 ‘heavy duty’ tires, less aromatics and alkanes were found in tires sourced in North America, compared to Europe, though Molden reported “high marine toxicity potential from US tires, but mixed effects on human health.”

While cautioning that the results might “reflect more the OE- and product-mix than a like-for-like comparison,” he did conclude that the variations in toxicity reflected how tires on the US market have lower aromatics from all sources than tires in Europe.

Regional variation

Molden went on to present compound data available from both regions for seven OE tire manufacturers. This analysis showed variations ranging from 10% to 57% in the aromatic-content of tires supplied in Europe and the US.

The differences, he suggested, are linked to the focus on rolling resistance, grip, weather and noise in Europe, particularly as fuel-economy is a pre-eminent consideration in the region – compared to durability in the US.

With regard to the marine environment, “thousands of potential concerns can be seen at low concentrations,” said the Emissions Analytics expert, noting that “6PPD is not the only compound of concern.”

In summary, Molden said tire wear emissions, which are “all around and inside us” encompass larger and smaller particles as well as VOCs. And while there are more compounds of concern seen in European than in American tires, this likely due to market requirements.

Indeed, he added, this large variability would also be seen if the analysis was narrowed down to tires made in Europe or even by a single tire manufacturer.

Molden concluded by highlighting the need for the issue of toxicity of tire wear particles to be put in context: “We use tires all day long, and we’re not falling over dying.

“This is not [about] some acute industrial pollutants. It is a big chronic problem [but] your tire is not going to kill you.”

Tires, he continued, are being used “at vast scale by people around the world and emitting low concentrations of toxins of concern: that’s why it is hard to actually pinpoint them and understand their effects.”

This is still very much a work-in-progress, Molden adding that the issue of tire wear was now on the regulatory agenda with the arrival of the Euro7 standard.

The toxicity issue “is a challenge to get ahead on, because it is harder [than others] to solve and potentially more problematic. We’re here to help [industry] get ahead of these problems and collectively solve them.”