LAUSR

Abstract Natural passive ventilation (windows opening) is frequently used in many houses and old buildings to renovate the air, remove unpleasant odors and dust, and reduce the physicochemical pollutants indoor. However, little is known about the effect on biological particles such as pollen grains and fungal spores (both allergenic) or bacteria (potentially infectious and pathogenic). In the present research, the bioaerosols composition in a small room naturally ventilated was analyzed by high-throughput DNA sequencing. Pollen grains were the most abundant particles outdoors while microbial phyla Actinobacteria, Firmicutes, Proteobacteria and Ascomycota were predominant indoors. The main divergences in bioaerosols between indoor and outdoor environments were caused by the different abundance of the biological particles rather than the different taxa composition. Keeping the window open for 2 h did not change significantly the microbial community present indoors, although there was a tendency to mix the components of both environments. The abundance of human-related and potentially harmful microorganisms was higher indoors and was not remarkably affected by natural ventilation. In our study, natural ventilation through window opening had a poor effect on removing these microorganisms from the atmosphere indoor, suggesting that additional mechanisms such as air filtering systems would be required in order to improve the air quality of these environments from a microbiological point of view.