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Indoor climbing facility air contains levels of lung-toxic rubber-derived compounds far above reported values, according to a recently published study.

Concentrations of several rubber-derived compounds (RDCs) in the air of indoor climbing facilities substantially exceed previously reported values, according to a study published in ACS ES&T Air.

“Particulate matter exposure during indoor climbing is hypothesized to be the driver for acute decline in lung function of climbers,” wrote first author Anya Sherman, Center for Microbiology and Environmental Systems Science, University of Vienna, and colleagues. “Total daily intake of RDCs for individuals visiting or working in these facilities exceeds exposure via all other known routes.”

According to the authors, 6.36 million people in the US participated in indoor climbing in 2023, of which approximately 20% participate regularly, often spending multiple hours per visit. Prior monitoring studies have revealed highly elevated concentrations of particulate matter (PM) in indoor climbing facilities, primarily attributed to climbers’ chalk. The team investigated whether abrasion of engineered rubber climbing shoes was also contributing to poor air quality, generating airborne rubber particles bearing chemical additives. “This is of concern, since RDCs such as 6PPD-quinone are also toxic to human lung cells with inhalation hypothesized to be a major route of exposure,” the authors wrote.

Sample Collection, Chemical Analysis & Exposure Assessment

The team collected aerosol PM, settled dust, climbing‑hold powder (foothold powder), and climbing‑shoe sole samples between February 2023 and June 2024 in nine European climbing halls. In five Vienna halls, aerosol PM and triplicate settled dust and foothold powder were sampled; four additional halls in Switzerland, France, and Spain contributed triplicate settled dust and foothold powder. Thirty shoe‑sole samples representing major brands were obtained. Aerosol PM was captured using glass liquid impingers separating upper respiratory tract (>6.4 μm) and lower respiratory tract (LRT, <6.4 μm) fractions at 60 L/min over 3-hour peak‑activity intervals. Dust was scraped from floors and wood surfaces 5 to 10 m from walls; foothold powder was collected directly from holds. Shoe‑sole rubber was excised, cryo‑milled, and extracted.

After accelerated solvent extraction, the team analyzed the samples for benzothiazoles (BTZs), aniline, diphenylguanidine, melamine derivatives, and phenylenediamines (PPDs) and their quinones (PPDqs). Blanks and reference office samples were used to assess contamination and background levels.

The authors calculated estimated daily intakes by inhalation and ingestion (EDIinh/ing) of RDCs for regular climbers and hall employees using measured aerosol PM concentrations, inhalation rates, exposure durations, and frequency adjustments.

High Concentrations Revealed

Concentrations of most RDCs in dust and aerosol PM far exceeded levels reported in other indoor and outdoor environments, the researchers reported. Settled dust in some halls contained BTZs at concentrations two orders of magnitude above typical house dust, with LRT aerosol fractions showing PPD and PPDq levels comparable to urban roadside pollution events. Shoe‑sole samples exhibited cumulative RDC concentrations ranging from 25 to 3,405 μg/g (mean, 711 μg/g), with 2-mercaptobenzothiazole comprising 67% of total RDC mass. Foothold powder mirrored shoe‑sole profiles, and scanning electron microscopic imaging confirmed a respirable fraction (<10 μm). Ozone‑aging experiments demonstrated transformation of parent RDCs under elevated ozone, consistent with particle‑surface chemistry.

According to the study, EDIinh/ing revealed that employees incurred greater exposures than climbers, exceeding benchmarks from industrial and roadside settings by orders of magnitude.

New Human Exposure Route

“Abrasion powder resulting from friction between climbing shoes and footholds is the likeliest source of high concentrations of RDCs observed in aerosol particulate matter and settled dust [in indoor climbing facilities],” the authors concluded. “These findings reveal a previously unknown human exposure route of RDCs.”

The authors advocated for the inclusion of consumer products—particularly climbing shoes—in future research and regulatory frameworks aimed at identifying safer alternatives to toxic RDCs.