Indoor climbing may build muscles and confidence—but it might also come with a less obvious downside: airborne chemicals from your shoes.

A new study led by researchers from the University of Vienna and EPFL Lausanne has found that climbing shoes, specifically the rubber soles, shed microscopic particles and chemicals into the air during use. These compounds—many of which are also found in car tires—accumulate in indoor climbing gyms and can be inhaled during physical exertion.

And in some gyms? The concentration of these particles was as high as what you’d breathe standing on the shoulder of a major highway.

Published in Environmental Science & Technology Air, the research confirms that the familiar smell of rubber in climbing gyms is more than just part of the ambiance—it’s a warning sign.

“The soles of climbing shoes are high performance products, just like car tires,” explains first author, Anya Sherman.

These soles are made durable and grippy thanks to chemical additives—compounds that enhance elasticity and toughness. But as shoes grind against holds and walls, fine rubber particles and these additives get scraped off, settle on surfaces, and become airborne.

“We were familiar with the black residue on the holds in climbing gyms—the abrasion from the soles of our shoes. Climbers wipe it off to get a better grip, and it gets kicked up into the air,” Sherman says.

Breathing Like a Climber, Measuring Like a Scientist

Using a specialized device called an impinger—which mimics the airflow through human lungs—they collected air samples in five climbing gyms across Vienna, and analyzed dust samples from gyms in France, Spain, and Switzerland.

The impinger inhaled air at 60 liters per minute, separating particles just as they would be filtered (or not) by the respiratory system.

“Air pollution in the bouldering gyms was higher than we expected,” said senior author, Thilo Hofmann. “The levels we measured are among the highest ever documented worldwide, comparable to multi-lane roads in megacities.”

Among the chemicals identified in 30 pairs of climbing shoes were 15 different rubber additives, including 6PPD, a compound used in tires that has been linked to mass salmon deaths in rivers.

So… Is It Dangerous?

That part’s still unclear. The health effects of long-term exposure to airborne rubber additives in climbing gyms haven’t yet been fully studied. But Hofmann doesn’t think we need to wait for perfect data to take action.

“These substances do not belong in the air we breathe,” he says.“It makes sense to act before we know all the details about the risks, especially with regard to sensitive groups such as children.”

Keep Climbing—Smarter

Sherman hasn’t hung up her shoes—far from it.

“I will continue to climb, and I am confident that our research will contribute to better conditions in climbing gyms,” she says.

So no, you don’t need to give up your gym membership. Climbing remains a form of fitness and physical activity for many, and it is still better to have some fitness than none. But it’s probably time to open a window—and maybe ask your gym managers and gear brands what they’re doing to clean up the air.

A Sherman et al. The Invisible Footprint of Climbing Shoes: High Exposure to Rubber Additives in Indoor Facilities. Environmental Science & Technology Air (2025). DOI: 10.1021/acsestair.5c00017