The Microbes Between the Walls

People in industrialized countries spend 90% of their time indoors, yet the microbial ecology of indoor environments — the built environment microbiome — is poorly understood because it falls between the disciplinary boundaries of microbiology, building science, public health, and materials science. Building materials, ventilation rates, humidity levels, cleaning practices, and occupant behavior together shape indoor microbial communities that are distinct from outdoor environments, but no integrated framework connects building design decisions to microbial community composition to occupant health outcomes. The result is that building codes regulate temperature, ventilation, and air quality (particulates, VOCs, CO2) without considering microbial exposure, while microbiologists study indoor microbes without understanding how building systems shape their communities.

Indoor microbial exposure is associated with both protective effects (diverse microbial exposure in early childhood reduces allergy and asthma risk — the "hygiene hypothesis") and harmful effects (mold exposure, Legionella in water systems, pathogen transmission). Building tightness standards (driven by energy efficiency) have reduced ventilation rates and increased indoor humidity in some climates, creating conditions that favor moisture-dependent microbial growth. Hospital-acquired infections (HAIs) kill an estimated 99,000 Americans annually, and building design (room pressurization, surface materials, air handling) directly influences pathogen transmission — but HAI prevention focuses on clinical protocols rather than building systems. The disconnect between building design and microbial health means that buildings are optimized for energy, structural, and acoustic performance without considering the microbial environment that occupants actually breathe.

The Sloan Foundation Built Environment Program (2011–2017) established the field but funding ended before mechanistic understanding was achieved. 16S rRNA surveys have cataloged indoor microbial diversity across hundreds of buildings but these observational studies cannot establish causation between building parameters and health outcomes. Hospital studies have linked specific design features (single-occupancy rooms, copper surfaces, HEPA filtration) to reduced infection rates, but these findings have not been generalized to non-healthcare buildings. Building simulation tools (EnergyPlus, CONTAM) model airflow and pollutant transport but cannot model microbial growth, dispersal, or health effects. Antimicrobial building materials (silver, copper, photocatalytic TiO2) reduce surface microbes but may also eliminate beneficial microbes, and their health implications have not been studied.

Integrated building-microbiome models that connect building physics (temperature, humidity, airflow, light) to microbial community dynamics to health outcomes — enabling building designers to evaluate the microbial consequences of design decisions alongside energy and comfort. Longitudinal studies in instrumented buildings that simultaneously measure building parameters, microbial communities (using metagenomics, not just 16S), and occupant health, establishing causal pathways. Design guidelines that specify microbial exposure targets (diversity, specific taxa) alongside traditional indoor air quality parameters.

A team could instrument a campus building to simultaneously measure temperature, humidity, ventilation rate, and microbial community composition (using air and surface sampling) across rooms with different ventilation configurations, testing whether building parameters predict microbial community differences. An interdisciplinary team could conduct a literature synthesis mapping all published associations between building design features and microbial outcomes, identifying which building parameters have the strongest evidence for microbial community effects. Relevant disciplines: microbiology, building science, environmental health, mechanical engineering, materials science.