<p>The impact of particulate matter (PM) on respiratory health is a multifaceted issue, contingent on factors such as particle size and composition. The present study set out to compare the concentrations of size-resolved PM and selected gases (VOCs, CO, and NO₂) in the underground treatment chamber in the “Wieliczka” Salt Mine (a UNESCO-listed tourist attraction and health resort, not an active industrial mining site), and in the adjacent outdoor environment. The study also assessed the variability of these concentrations with respect to occupancy. The results showed that the mean indoor-to-outdoor ratio (I/O ratio) for fine particles (PM₁) was less than 1 (i.e., 0.623). In contrast, the mean I/O ratio for larger fractions (PM₄–PM_tot) was greater than 2, likely due to the mechanical resuspension of settled dust by visitors. VOC concentrations were particularly low in the underground environment; the mean I/O ratio was 0.063, and the mean I/O ratio for NO₂ was also low (0.62), indicating limited infiltration of anthropogenic pollution. CO levels were moderately elevated indoors due to restricted ventilation but remained below the strictest applicable regulatory thresholds. Overall, the underground microclimate is characterised by internally generated aerosol and low levels of gaseous pollutants. The underground chamber was characterised by markedly lower concentrations of anthropogenic pollutants than the outdoor environment. However, episodic increases in concentration highlight the need for continuous monitoring, as well as appropriate ventilation and occupancy management in subterranean spa facilities.Kindly check and confirm the corresponding author of the article and the first/last name of the authors are correctly identified.I confirm</p>