Indoor air hygiene is a crucial aspect of human health and well-being. Previously, the concentration of carbon dioxide (CO₂) is used as a benchmark for assessing the hygienic quality of indoor air and ventilation strategies were based on this value. Nevertheless, the recent pandemic has once again demonstrated the significant impact that individuals can have on indoor air quality through the emission of bioaerosol particles. To date, the particles exhaled by humans that could be emitted into indoor air have not been extensively studied in the context of indoor air hygiene and occupational health and infection control.

The objective of this study therfore is to analyze the particles exhaled by healthy workers and to evaluate the influence of human bioaerosol particles on indoor air quality. In order to achieve this, human volunteers were instructed to generate respiratory particles from the lower airways using a specific breathing technique and then to breathe them into the exhaled particle collection device, designated as PExA (particles in exhaled air). Subsequently, the particles with a diameter below 7 µm were collected by impaction on filters. Following protein extraction and digestion, the peptides were analyzed using the Thermo Astral mass spectrometer with a Data Independent Acquisition (DIA) method. The data were subjected to metaproteomic analysis using an in-house protein database comprising predicted microbial and viral representatives of the respiratory tract and the human proteome.

The results of this study indicate the presence of a specific profile of human proteins that come from the lower respiratory tract into the exhaled human breath. These proteins are derived from the human immune system, particularly from neutrophil degranulation and haemostasis, as well as from the area of protein metabolism, including surfactant metabolism. Furthermore, a few proteins of the microbiome of the respiratory tract were identified in the exhaled air of the human test subjects. These belong, for example, to bacterial representatives that can be assigned to the phylum Pseudomonadota, as well as fungal representatives that belong to the phylum Ascomycota.

The results demonstrate that the exhaled air of healthy test subjects releases bioaerosol particles that are predominantly composed of human proteins, with a portion originating from microbial representatives of the respiratory tract. Further studies will concentrate on the quantification of specific proteins in exhaled air, as well as on the question of whether these proteins can be quantified in indoor air. The data will be employed as a foundation for an examination of the correlation between CO₂ concentration and the concentration of bioaerosol particles emitted. This will facilitate a reassessment of the recommendations for indoor ventilation, which were previously based on CO₂ concentrations. The objective is to enhance indoor air quality in the context of occupational health and infection control.