Fig. 1

The pan-microbiome of the built environment (BE) is influenced by various factors. a Mode of ventilation within the BE facilitates both the introduction (natural ventilation) and the limit (mechanical ventilation) of outdoor microbes into the BE. The choice of ventilation mode also modulates environmental parameters including temperature, humidity, airflow, and carbon dioxide levels, affecting the survival and growth of specific microbes in the BE. b Building design affects the microbiome of the BE by the spatial relationships between different spaces within the BE, thereby influencing the flow of microorganisms within the indoor space. Also, special building types, such as screen doors and aboveground/underground rail lines associated with subways, may be associated with changes in the microbiome of the subway BE. c Humans in the BE release occupant-associated microbiomes by involuntary and voluntary means associated with physiology and activities. Occupant movements also re-suspend particles and microbes from surfaces and floors. Components a, b, and c therefore describe general factors shaping the microbiome of the BE (grey boxes). However, d outdoor sources of microbiomes may differ depending on the geographical location, as adjacent soil, plant, and water environments, land use, and level of urbanization will affect the microbial community composition of the immediate outdoors. Also, geography-based climate variations will also shape the microbiome of the outdoors, by affecting the survival and growth of certain microbes, and also influencing dispersal of microorganisms through outdoor spaces. e Population-based variations in microbiomes of human gut, oral cavity, and skin have been documented. Furthermore, lifestyle differences such as diet, activities, and living conditions will also affect what human-associated microbes are emitted into the BE. As a result, components d and e are major forces (purple boxes) that contribute to a global pan-microbiome of the BE, which is greater than the microbiome of any single BE