Defining a Healthy Microbiome of the Built Environment

Built structures host complex and dynamic microbial communities whose assembly is shaped by occupancy, ventilation, building materials, cleaning practices, connections to outdoor environments, and other factors. These communities, collectively described as the microbiome of the built environment (MoBE), are increasingly recognized as critical to human health, yet there is currently no consensus definition of what constitutes a healthy MoBE. This gap impedes the development of engineering efforts to balance microbial diversity and immune support with the minimization of infectious disease risks. We propose three pillars that together define a healthy MoBE: (i) pathogens below context-specific risk thresholds; (ii) non-sterility, as sterility is likely to be suboptimal for human health in most contexts; and (iii) resilient diversity, as more diverse communities may resist pathogen colonization and recover more rapidly from perturbations. We synthesize evidence for each pillar, propose a framework for feedback-controlled MoBE engineering, and discuss how pillar priorities should be weighted across building types and occupant populations. We identify critical research priorities to advance actionable performance standards in support of healthy buildings.