The Invisible Footprint of Climbing Shoes: High Exposure to Rubber Additives in Indoor Facilities

Abstract

There is growing concern about rubber-derived compounds (RDCs), predominantly originating from tire and road wear particles. Other consumer products, including sports equipment, also contain RDCs, and human exposure to these compounds is of particular interest due to demonstrated toxicity to animal species. In this study, we investigated RDCs intentionally incorporated into climbing shoes for enhanced performance. We found high concentrations of 15 RDCs in shoe sole samples (Σ₁₅ RDCs: 25-3405 μg/g), aerosol particulate matter (Σ₁₅ RDCs: 2.6-37 μg/g), and settled dust (Σ₁₅ RDCs: 1.5-55 μg/g) in indoor climbing halls. The estimated daily intake via inhalation/ingestion of Σ₁₅ RDCs for climbers and employees in some of these facilities ranged from 1.7 to 48 ng/kg/day, exceeding known intake levels of RDCs from other sources. 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. These findings reveal a previously unknown human exposure route of RDCs.

Figure 1

Schematic of a climbing hall, with photos of the four types of samples analyzed in our study. Specialized climbing shoes are worn with highly functionalized rubber soles (1, shoe soles). Friction between these shoe soles and the footholds generates rubber particles (2, foothold powder). Those can be aerosolized and inhaled directly upon generation, due to the brushing of holds, or by climbers falling onto mats and resuspending rubber particles that had settled (3, aerosol particulate matter). Eventually, aerosol particles also settle elsewhere as dust (4, settled dust).

Figure 2

Comparison of rubber-derived compound concentrations in climbing halls and other environments.Rubber-derived compound concentrations measured in the LRT and URT fractions of climbing hall aerosol particulate matter (left) and settled dust (right) compared to concentrations reported in the literature for various indoor (houses, vehicles, shopping malls, dormitories, parking lot, and sport halls) and outdoor (roadsides, city centers, playgrounds, recycling plants, and industrial sites) environments. Aerosol particulate matter and settled dust concentrations compiled from the literature represent a variety of size fractions. Details about literature values, including size fractions of particulate matter and settled dust, are provided in the Supporting Information as an Excel file. Statistical differences between groups were tested with the Wilcoxon signed-rank test (NS means p ≥ 0.05; * means p < 0.05; ** means p < 0.01; *** means p < 0.001).

Figure 3

Rubber-derived compound profile shift. Rubber-derived compound profile in foothold powder, settled dust, and aerosol PM. Foothold powder and settled dust compound profiles for each hall represent an average of triplicate samples, and aerosol PM represents an average of the URT and LRT fractions. Individual, absolute concentrations for each sample are presented in Table S4. Aerosol PM was only collected from halls 01–05, and in hall 05, the collected mass of aerosol PM was too low for accurate analysis of RDCs.

Figure 4

Rubber-derived compound concentrations in climbing shoes and foothold powder. RDC concentrations in 30 climbing shoe soles (left) and foothold powder from nine climbing halls (right). Concentrations of RDCs vary substantially among different shoe models. Foothold powder samples are representative of the variety of different shoe models.

Figure 5

Scanning electron microscopy images of rubber particles in settled dust. Representative scanning electron microscopy images of (A) chalk (magnesium carbonate powder), (B) a foothold powder sample, and (C) a settled dust sample collected in climbing halls. Rubber particles resulting from the abrasion of shoe soles are visible in the foothold powder and were tentatively identified in the settled dust samples (B and C). Rubber particles can be distinguished from chalk particles (A) due to their elongated shape and surface physical characteristics with a smooth carbon-based surface compared to chalk. The elemental composition of a rubber particle was determined with energy dispersive X-ray and is shown in Figure S7. Surface roughness appears to increase between recently generated rubber particles identified in the foothold powder samples (B) and particles found in settled dust (C).