## 4 Discussion
These results differ from the ones obtained by [PERSON] and [PERSON] [28] in the Krakow Zoological garden, who observed a positive correlation between fungal aerosol concentrations and relative humidity of air. The aerosol particles in livestock premises (including horse stables) are mostly organic (above 90%) and may include, among others, water droplets, animal secretions such as saliva and feces, fragments of fodder, hair, straw, soil, and manure, as well as various types of microorganisms [29]. According to [PERSON] et al. [30], exposure to the aerosol particles and its compounds from the livestock premises can cause death of animals and severe damage to the respiratory tract of people spending significant time inside the contaminated premises. However, the exposure level and thus the degree of hazard created by biological contaminants in indoor environments is controlled by a number of factors. Among these, the concentration, size distribution, microclimatic parameters of the indoor environment (including temperature and humidity), and ventilation are the most important ones [31]. Regarding the fact that there is no clear pattern of associations between microbial aerosol concentrations with particulate matter and microclimatic parameters that can be revealed neither from our studies, nor from the studies performed by other authors, other factors, such as particle size and species composition, need to be taken into account [18]. For this reason, the combination of classical and molecular identification approach was used in this study to determine the composition of the most prevalent fungal genera and species in the studied horse stables. The next generation sequencing-based assessment of fungal community composition, conducted at the Krakow stable, showed _Wallemia sebi_ to be the most abundant among the fungal species, with relative abundance of 35.16%. These fungi are very well adapted to the environments characterized by low water activity. They can produce a number of bioactive metabolites, including tricyclic dihydrosgesquier-penes: wallimidione, walleminone, walleminol, and azasteroids. There are rare reports on the involvement of _Wallemia_ spp. in cutaneous and subcutaneous human infections, but the representatives of this genus are important allergens, as they produce massive amounts of tiny conidia (1.5 um to 3.0 um), ideal for aerial dispersal, and thus can easily enter the host lungs [32]. Moreover, conidia of _W. sebi_ are covered with a layer of hydrophobins (small cell-wall proteins) which helps pathogens avoid detection by the immune system cells by masking them [32], making this fungus even more potentially dangerous. Importantly, with respect to our observations, numerous studies showed high concentrations of _Wallemia_ spp. in outdoor and indoor air as well as in agricultural environments [33,34]. _W. sebi_ has been frequently reported as the cause of respiratory allergies and atopic diseases in asthmatic individuals, and has also been identified as one of the causal agents in the farmer's lung disease [35]. The second and third most prevalent fungal species, i.e., _Aspergillus penicillinides_ and _Epicoccum nigrum_, have been found among the causal agents of allergic rhinitis in humans [36,37]. Out of the ten most abundant fungal genera observed in the Krakow stable, five are listed as the most common causal agents of allergies [37]. These genera are _Aspergillus_, _Epicoccum_, _Trichoderma_, _Alternaria_, and _Cladosporium_. Airborne fungi are described as being among the main groups of organisms involved in the pathogenesis of equine RAO [12]. Members of _Aspergillus_ species have been detected in high concentrations in tracheal aspirates of RAO-affected animals [12]. The findings of our study corroborate this observation, as the Krakow stable has been dealing with a number of RAO cases among horses. Other fungal genera, demonstrated by [PERSON] et al. [12] as predominant in the respiratory tract of RAO-affected horses, included _Alternaria_, _Cladosporium_, and _Penicillium_.