Plastic microparticles released into the environment from common road tires should be treated as a “high concern” pollutant, that may exceed chronic safety limits in some heavily contaminated environments, new research has shown.

A team of researchers from the University of Plymouth and the University of Exeter in the UK looked at the chronic toxicity of particles and chemical leachates found on a series of popular tire brands. They then looked at the effect these particles and chemicals would have on small planktonic crustaceans, the water flea (Daphnia magna).

They discovered that the plastic pollutants from the tires showed a distinct effect on both the reproduction and development of the water flea, which also displayed visible particle uptake within their digestive tract.

When looking at the leachates—liquid that has passed through the tire material, taking some of the harmful chemicals with it—they found a strong presence of zinc, titanium and strontium as well as many organic chemicals.

Overall, of the numerous organic chemicals present during the test, more than 50 were found across all five tire brands, with a significant number of those chemicals classified as very toxic.

This new research shows that tire particles are hazardous pollutants, and should be treated as a particular concern close, to or possibly above, chronic environmental safety limits in some locations, researchers say.

The research, published in an open-access paper the Journal of Hazardous Materials, is part of the tire-LOSS: Lost at Sea – where are all the tire particles? project being led by the University of Plymouth with funding from the Natural Environment Research Council.

Our previous work has shown that road debris is a major source of microplastics in the environment. In recent years, we have been working with partners across research and industry to determine how those particles distribute and their potential to cause harm. This new study is of key importance because it demonstrates the potential for harmful effects on an aquatic invertebrate species, at concentrations similar to those we have recorded near to roads in the UK. It is clearly an area we need to explore further.

—Professor Richard Thompson

Professor Richard Thompson OBE FRS, Head of the International Marine Litter Research Unit at the University of Plymouth, leads the tire-LOSS project and is a co-author on the new study.

The use of tires by cars, buses and other vehicles is a ubiquitous feature of modern life, all served by a mass global tire industry that is forecast to grow around three per cent each year.

Tire-tread particles generated during roadway use are a major environmental pollutant, with a significant proportion being washed away into waterways near traffic networks. It is estimated at around 18% of these particles eventually reach freshwater, while 2% reach estuaries.

However, while previous research suggested there may be concentrations of these particles in the environment, the toxicological effects they have on aquatic organisms were less well understood.

For this study, the team used several popular tire brands from around the world to generate a tire tread microparticle mixture. The toxicity of both particles and chemical leachates were then studied on the test species, the water flea.

They found that, over a three-week period, pristine tire tread microparticles were more toxic (concentration being lethal for 50% of the population, LC50, is 60 mg.L^‑1) than chemical leachates alone (LC50 542 mg.L^‑1).

Boisseaux et al.

The results suggest the particles were more toxic to water flea than the more commonly studied polyethylene microplastics.

Resources

• Paul Boisseaux, Cassandra Rauert, Pradeep Dewapriya, Marie-Laure Delignette-Muller, Robyn Barrett, Lee Durndell, Florian Pohl, Richard Thompson, Kevin V. Thomas, Tamara Galloway (2024) “Deep dive into the chronic toxicity of tyre particle mixtures and their leachates,” Journal of Hazardous Materials doi: 10.1016/j.jhazmat.2024.133580