Exploring and defining concepts of microbiome stewardship for the indoor built environment.

Microbiome stewardship (MS) aims to maintain microbiome diversity and functioning to support human and ecosystem health. However, the built environment presents a unique challenge, as human-made indoor spaces lack a native microbial baseline. This review addresses this challenge and proposes a framework for MS for the built environment that bridges indoor exposure science with socio-ecological, engineering, and policy insights. We consider indoor spaces as active exposure ecologies shaped by ventilation, filtration, and moisture control. While mitigating harmful pollutants, dampness-related microbiota, and pathogens is an ethical and practical prerequisite for MS of the built environment, it is insufficient for achieving MS. We must shift from traditional concepts of sterility toward a model of "healthy porosity" through intentional microbial cultivation. Synthesis of current evidence highlights the health benefits associated with taxonomically rich, environmentally sourced microbial consortia, such as outdoor-, soil-, dog-, and plant-associated taxa. Moreover, moisture control, dust-reservoir maintenance, appropriate filtration, and supportive occupant behaviors are needed to favor benign and beneficial microbial communities. To transition building operations from pathogen suppression to intentional cultivation of health-supportive indoor microbiomes, we outline actionable stewardship goals that prioritize human health, favor benign environmental inputs, and promote occupant behaviors that sustain resilient microbial communities.