Watch your step — a world of trouble may be underfoot.
New research suggests that recycled material widely used on playgrounds, sports fields and running tracks could be quietly releasing toxic chemicals into the environment.
And those substances don’t just wash away. They can singer in soil and water for years — threatening ecosystems and raising concerns about potential risks to human health.
In the study, scientists in Poland focused on crumb rubber, the tiny black granules made by grinding up old tires.
Crumb rubber is often sold as an environmental win, keeping millions of worn-out tires out of landfills. It’s also prized for its springy, shock-absorbing surface, which can cushion falls far better than hard asphalt.
It’s no wonder the long-lasting, low-maintenance material has become a go-to for playground surfacing, filling in synthetic turf and creating cushioned, slip-resistant tracks, all marketed as a way to protect kids and athletes from injury, among other benefits.
But when researchers examined the rubber granulate itself, focusing on three commonly used particle sizes, they uncovered a troubling downside.
The material contained high levels of toxic compounds known as polycyclic aromatic hydrocarbons, or PAHs.
Over time, this group of chemicals may pose risks when inhaled, swallowed or absorbed through the skin. They have been associated with complications during fetal development, as well as liver and reproductive damage, and some are known carcinogens.
That’s part of the reason why experts already recommend washing hands after playing on playgrounds with crumb rubber, avoiding eating on turf and limiting time on the fields during extreme heat.
To better understand the potential risks, the team measured both the total amount of PAHs in the recycled material and the fraction that is bioavailable, or the portion that can dissolve in water and be absorbed by living organisms.
“Our results show that recycled tire rubber contains very high concentrations of PAHs, especially in the smallest particle sizes,” Patryk Oleszczuk, the study’s corresponding author, said in a press release.
“These fine particles are more reactive and release a greater share of toxic compounds into water and soil, increasing the risk to ecosystems and potentially to human health,” he explained.
The study found total PAH levels ranged from 49 to 108 milligrams per kilogram depending on particle size, with the smallest granules consistently packing the most toxins.
Even more alarming, the bioavailable portion of these chemicals was also highest in the tiniest particles, reaching levels higher than those typically found in many contaminated soils and sediments.
To test the real-world danger, the researchers exposed tiny soil-dwelling invertebrates called springtails, along with garden cress plants and glowing marine bacteria, to both solid rubber granulate and water that had been soaked with it.
All of the organisms showed adverse effects. While larger rubber particles caused less harm, the smallest granules significantly reduced their chances of survival, growth or biological activity across multiple tests.
“Chemical data alone do not tell the whole story,” said Oleszczuk. “By combining chemical analysis with ecotoxicological tests, we were able to demonstrate that the compounds released from rubber granulate are not just present, but biologically active and harmful.”
The researchers also detected potentially toxic metals such as zinc and copper in water extracted from the rubber. In some cases, metal levels exceeded drinking-water guidelines, adding to the overall toxicity.
That’s especially troubling because crumb rubber is often used in places where people — particularly children — come into direct contact with it.
And the problem can worsen over time. Sunlight, heat and moisture can further break down the rubber, potentially releasing even more hazardous substances into the environment.
“Our findings suggest that the environmental safety of tire derived rubber depends strongly on particle size,” Oleszczuk said. “Fine granulates pose a disproportionate risk and should be carefully regulated, particularly in public spaces like playgrounds and sports facilities.”
But whether regulators will act is another question. There is still fierce debate among researchers and public-health experts over whether the level of exposure most people experience with crumb rubber can actually cause significant harm, and a large body of research has produced mixed results.
In 2024, for example, a federal multi-agency research initiative released a report finding no significant differences in PAH exposure between people using synthetic turf fields with crumb rubber infill and those on natural grass fields.
The Polish scientists stressed that recycling tires is still an important environmental goal, but maintained that not all recycled materials are inherently safe.
“This study highlights the need to rethink how recycled materials are evaluated and applied,” Oleszczuk said. “Sustainability should not come at the expense of environmental or human safety.”
To cut the risks, the researchers are pushing for size-specific regulations on recycled rubber, along with long-term safety monitoring and the development of safer alternatives.
