The Role of Microbes in Indoor Air, From Pathogens to Protectors

A microbiological perspective on indoor environments: understanding the invisible biosphere shaping air quality, immunity, and ecological balance

Abstract

Microorganisms in indoor air are often feared as threats,  carriers of disease, allergens, and contamination. However, emerging research shows that indoor air microbes are not merely pathogens, but essential regulators of ecological and human health. This article explores the dynamic microbial ecosystem of indoor environments, explaining how bacteria, fungi, and viruses become airborne, interact with humans and surfaces, and contribute not only to illness but also to immunological resilience and air purification. By understanding this invisible biosphere, we can shift from pathogen avoidance to microbial balance as the new frontier of indoor air quality.

1. Introduction

When we think of indoor air pollutants, we typically imagine smoke, dust, or volatile chemicals. But a major component of indoor air,  arguably the most biologically active,  is microbial. Every breath we take indoors contains thousands of bacterial and fungal particles, ranging from harmless skin commensals to spore-forming molds and opportunistic pathogens. Yet not all airborne microbes are harmful. In fact, the microbial landscape of indoor air is shaped by occupancy, surface materials, ventilation, and humidity,  and in turn, shapes human health in complex ways. This article explores a new framing of microbes not only as vectors of disease, but as participants in a living indoor ecosystem whose composition and behavior we can influence,  and must learn to understand.

2. Origins and Composition of the indoor air microbes

Microbes in indoor air originate from a range of sources. Human occupants shed bacteria from skin, oral, and gastrointestinal microbiota with every movement. Pets contribute fur, dander, and associated microbes. Plumbing, HVAC systems, building materials, and even outdoor air introduce additional species. A study by Meadow et al. (2014) revealed that every person emits over 37 million bacterial genomes per hour into their immediate indoor environment, effectively creating a personalized microbial cloud. Indoor surfaces act as both reservoirs and secondary emitters, as biofilms can form on HVAC coils, damp carpets, and water-damaged walls, later releasing microbial fragments or spores into the air during disturbances or air movement.

3. Airborne Microbes as Pathogens,  and as Part of the Solution

While some microbes are pathogenic,  such as Staphylococcus aureus, Aspergillus fumigatus, or norovirus,  the majority are neutral or beneficial. The indoor microbiome, much like the gut microbiome, plays a role in immune system development and modulation. Children exposed to a wider range of indoor microbes in early life have lower rates of asthma and allergies, according to findings from the LISA and GABRIEL cohort studies in Europe. Moreover, competition among microbial species can suppress the dominance of harmful organisms. This phenomenon, known as microbial antagonism, suggests that promoting diverse, stable indoor microbiomes may reduce pathogen load naturally,  without antibiotics or biocides. As pointed out by Gilbert and Stephens (2018), wiping out microbial diversity through excessive sterilization may paradoxically create ecological vacuums that favor opportunistic invaders.

4. Environmental Factors Shaping the Indoor Microbiome

Humidity, ventilation, temperature, and surface chemistry all influence microbial viability and distribution. High humidity promotes mold and bacterial growth, particularly in porous and nutrient-rich materials like drywall, insulation, or wood. In contrast, dry environments tend to increase the proportion of desiccation-resistant species like Gram-positive bacteria. HVAC systems play a dual role,  either filtering out airborne microbes or becoming breeding grounds for biofilms if poorly maintained. Airflow patterns also affect microbial dispersion; turbulent airflow can dislodge biofilms from surfaces and increase bioaerosol exposure. More nuanced is the role of light: ultraviolet radiation, even at low intensities, can inhibit certain microbial populations, influencing the microbiome at the architectural level. A 2020 study in Science Advances found that rooms exposed to daylight had significantly lower viable bacterial loads than those kept in darkness.

5. Bioaerosols and the Human Body

Bioaerosols,  airborne particles of biological origin including bacteria, viruses, spores, pollen, and fragments of microbial cells,  interact directly with human physiology. Upon inhalation, they can trigger immune responses, either protective or inflammatory. The size of these particles matters. Larger particles are filtered in the upper respiratory tract, but particles below 5 µm can reach the alveoli. Some microbial fragments are small enough (<0.5 µm) to cross into the bloodstream, especially during chronic exposure. Research by Douwes et al. (2003) demonstrated that chronic exposure to microbial cell wall components such as β-glucans and endotoxins was associated with elevated inflammatory markers and respiratory symptoms, but that controlled low-level exposure could induce tolerance or immune training.

6. Designing with Microbes: From Sterility to Stewardship

Traditional building hygiene emphasizes disinfection and sterilization, but this approach often disregards the complexity of microbial ecology. New approaches are emerging that aim not to eliminate all microbes, but to cultivate healthy microbial ecologies indoors. These include the use of probiotic surface treatments, biophilic design principles, and the incorporation of materials that support beneficial microbial diversity while resisting pathogen colonization. The field of microbial design architecture is advancing the idea that buildings should be viewed as living systems with microbial dynamics, not just inert containers. For instance, the integration of living plant systems with bioactive substrates may regulate not just chemical pollutants, but microbial balance through competitive exclusion, humidity regulation, and metabolic exchange. This represents a shift from wariness of microbes to an informed relationship with them.

7. Conclusion

Indoor air is not sterile,  and should not be. The microbial world that circulates through homes, offices, schools, and hospitals is complex, adaptive, and critical to human health. While some microbes are undoubtedly harmful, the majority are benign or beneficial, forming part of a symbiotic relationship with our bodies and buildings. As science uncovers the roles microbes play in immunity, pollutant degradation, and air ecosystem balance, a new model is emerging: not pathogen eradication, but ecological stewardship. The future of healthy buildings may lie not in eliminating microbes, but in designing with them,  cultivating air that supports life in all its necessary and beneficial forms.

To explore how living systems are being designed to support microbial balance and indoor ecological health, visit: www.justbreathe.in