## 5 Conclusions  
To our best knowledge, this study is the first one to contribute, by the combination of culture-based, physicochemical, and molecular methods, to the understanding of factors affecting the occurrence of recurrent airway obstruction (RAO) in horses kept within stables. Moreover, we examined the airborne-related factors that may contribute to the occurrence of human allergic reactions and respiratory diseases, as horse stables are not only the working environment, but are also used in recreation by a variety of people. Our study demonstrated that the daily activities performed in horse stables, such as feeding or cleaning of horses, contributed largely to the increased levels of particulate matter and the fungal components of bioaerosol. The respirable fractions of fungal aerosol in many cases oscillated around 80% of the total fraction, which suggests the potential health hazard to the exposed people and animals. Out of the two examined stables in southern Poland (Krakow and Tarnow), the Tarnow stable was characterized by higher concentrations of particulate matter, whereas the total and respirable fractions of fungal aerosol were higher in the Krakow stable. Next generation sequencing conducted for the fungal community collected at the Krakow stable showed that _Wallemia sebi_, an important allergenic species, producing large numbers of tiny (1.5-3.0 \(\upmu\)m) easily dispersed spores that can penetrate deeply into the respiratory tract, predominated among the identified airborne fungi. Additionally, the second and third most prevalent species, i.e., _Aspergillus penicilloides_ and _Epicoccum nigrum_, are often described as causal agents of allergic reactions and potential contributors to the occurrence of respiratory diseases in horses. Having this in mind, it is highly recommended that the air quality measurements, including the assessment of particulate matter and fungal aerosol concentrations, coupled with the determination of the most prevalent fungal species, should be conducted in horse stables where there are cases of allergic reactions or respiratory diseases in workers or RAO cases among horses.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12189375/s1](https://www.mdpi.com/article/10.3390/app12189375/s1)]([https://www.mdpi.com/article/10.3390/app12189375/s1](https://www.mdpi.com/article/10.3390/app12189375/s1)), Table S1: Relative abundance of fungal species at OTU level.  
Conceptualization, M.T., A.L.-B., K.K., J.K. and A.B.; methodology, A.L.-B., A.B. and K.K.; software, A.L.-B.; validation, M.T. and J.K.; formal analysis, A.L.-B.; investigation, A.B. and K.K.; resources, M.T., J.K. and A.L.-B.; data curation, K.K.; writing--original draft preparation, A.L.-B.; writing--review and editing, M.T., K.K. and J.K.; visualization, A.B.; supervision, M.T.; project administration, M.T. and J.K.; funding acquisition, M.T. and A.L.-B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the statutory measures of the University of Agriculture in Krakow and granted for the Department of Microbiology and Biomonitoring.  
Not applicable.  
The data presented in this study are entirely available in this study.  
The authors declare no conflict of interest.