Rare occurrence of airborne bioparticles in a speleotherapy site: the case study of the Sant’Aloisio disused siderite mine (Brescia, Italy)

Caves and mines have a suitable microclimate in terms of temperature, humidity and air purity that could be exploited for speleotherapy. The aim of this study was to evaluate the current load of allergenic airborne bioparticles of a subterranean environment being considered as a potential site for treating respiratory allergies. Sampling was performed over 12 months (2015–2016) inside and outside the Sant'Aloisio mine by: (i) continuous aerobiological sampling with Hirst-type samplers and (ii) monthly air sampling with an on-plate volumetric sampler using three different selective culture media. The following protocols were applied for the two methodologies: (i) Samples were processed following the European standard (UNI CEN/TS 16868:2015) and analyzed by light microscopy to identify and count pollen grains and fungal spores; (ii) plates were incubated, colony-forming units (CFU) were counted and concentration of CFU was calculated. Spore and pollen load, as evaluated by the continuous monitoring, resulted in very low values inside the mine, with a total of 106 pollen grains and 41 fungal spores from two species (Alternaria and Cladosporium) collected over the entire year; the pollen and spore concentration in the subterranean environment was negligible when compared to the open-air sampling (0.2% for pollen; 0.02% for spores). Viable airborne microorganisms were less represented in the mine than in the open air (median: 7 and 83 CFU*m−3, respectively). The indoor air of the disused mine was confirmed to be suitable for hosting respiratory disease sufferers from an aerobiological point of view. The residual low quantity of pollen grains could be the result of unintentional human transport. The indoor fungal component of the viable airborne microorganisms was mostly derived from non-allergenic wood-decaying fungi