When one thinks about a diverse environment, indoor areas usually don’t come to mind. However, indoor environments are ripe for microorganisms, which can enter on humans and via exchanges with outdoor sources such as plumbing and ventilation.

Though some of the microorganisms do not survive, enough do to have significant implications on human health. And with the escalation of urbanization, the importance of understanding how humans interact with those indoor microorganisms is only increasing.

Northwestern Engineering’s Erica Hartmann explored this issue in “The Indoors, Microbiomes, and Human Health,” a review paper written with University of California San Diego professor Jack Gilbert published July 19 in the journal Nature Reviews Microbiology. The article examines the relationships within built environments – places where humans spend most of their lives – and their impact on our health. 

Hartmann – whose research focuses on understanding how microbial communities respond to anthropogenic chemicals and then using that information to influence real-world outcomes – is an associate professor of civil and environmental engineering at the McCormick School of Engineering.

“The indoor microbiome, a complex ecosystem of microorganisms influenced by human activities and environmental factors, plays a pivotal role in modulating infectious diseases and fostering healthy immune development,” Hartmann and Gilbert wrote. “Recent advancements in microbiome research shed light on this unique ecological system, highlighting the need for innovative approaches in creating health-promoting living spaces.”

Below are three takeaways from their paper.

Healthy buildings are a crucial goal

As architecture’s evolution continues, there is an accelerating focus on the concept of “healthy buildings,” which marry the well-being of occupants with environmental sustainability. Healthy buildings have superior indoor air quality, natural lighting, and are built from eco-friendly materials. Smart technology, which optimizes energy efficiency, is also beneficial.

One issue with constructing and maintaining these buildings is cost. Retrofitting existing structures, meanwhile, can be costly and logistically complicated, partially because there is no exact standard for what defines a healthy building.

As Hartmann and Gilbert write, efforts are underway to explore re-wilding, creating living buildings and plumbing systems, such as by reintroducing microbes into buildings’ various products. 

“Re-wilding urban space, and even indoor spaces, holds significant promise to improve microbial exposure. Scientific examination of the impact of outdoor time has accelerated in recent years,” the authors write. “We imagine outdoor time as being associated with exposure to soil, plants and animals, and their microbiota, but defining ‘outdoor time’ and ‘contact with nature’ is challenging. How much exposure to the microbial antigens in these systems is enough to facilitate a health benefit? Can built spaces and architecture be repurposed to increase the probability of such exposure? And can such contact with ‘nature’ exert meaningful health benefits, independent of other factors?”

Recent research shows that bacterial spores can be embedded into materials, either directly or via 3D printing. Spores in hydrogels remain viable and become active when exposed to certain pathogens. This approach could enhance indoor environments by reintroducing beneficial microorganisms, potentially improving health, plumbing systems, and building material integrity.

“While more research is needed to understand which microorganisms can be safely reintroduced, and how to, for example, create artificial assemblages of microorganisms that recapitulate the health benefits of outdoor living upon exposure indoors, the potential of this research is profound,” Hartmann and Gilbert write. “In an ever-increasingly urbanized environment, the value of creating healthy spaces through targeted re-introduction of microbes remains compelling.”

Understand the transmission of the indoor microbiome

Understanding how microorganisms transmit in indoor environments is vital for grasping indoor microbial ecology and its health implications. Transmission occurs through human-related pathways, where people carry microbes on their skin, breath, and bodily fluids, significantly influencing indoor microbial diversity. Airborne transmission, highlighted by the SARS-CoV-2 pandemic, involves microbes becoming airborne and dispersing via ventilation or activities like coughing, adding to the microbial load in the air.

Pets also impact the indoor microbiome by bringing outdoor microbes inside, enhancing microbial diversity. Surfaces in buildings act as reservoirs for microbial transfer, with regular human contact facilitating microbial movement. Additionally, outdoor microbes enter indoor spaces through doors, windows, and ventilation systems, shaping the indoor microbiome's structure and affecting its diversity and stability. Understanding and managing these pathways can help create healthier indoor environments, balancing microbial presence to promote health and prevent disease.

Occupants of built environments can experience negative outcomes such as:

• Sick Building Syndrome
• Metabolic disease
• Mental health challenges
• Allergic diseases

“Understanding the interaction between the built environment, its microbiome, and gut microbial primary and secondary metabolism, and lung microbiota is critical in asthma pathogenesis,” the authors write. “Gut dysbiosis can affect systemic immune responses and contribute to the development and exacerbation of asthma.”

The indoor microbiome has profound influences on health

Microbial exposure affects disease progression, immune system development, and overall health. Changes in microbial exposure can disturb the immune system and alter the host's microbiome, creating a feedback loop. Urbanization is associated with reduced microbial diversity and increased risks of various conditions like depression, obesity, and respiratory diseases, with gut microbiome diversity linked to the amount of nearby greenery.

The disparities in built environments, influenced by socioeconomic factors, have significant implications for the indoor microbiome and, consequently, human health. Erica Hartmann and Jack Gilbert

As health is increasingly viewed through a lens of holistic well-being, consumer products like cleaning agents must be scrutinized as rigorously as medical devices and drugs. Potential conflicts of interest in study design, similar to drug evaluation, also require attention. Research highlights show that changes in indoor microbiome diversity and composition are linked to infectious diseases, metabolic disorders, mental health issues, and allergic diseases.

Acute health concerns, particularly respiratory infections, are significantly affected by the microbial composition of built environments. The SARS-CoV-2 pandemic highlighted how the built environment can profoundly impact disease transmission, beyond direct human-to-human contact. Similarly, hospital-acquired infections arise from interactions between patients, healthcare workers, and the hospital's microbial environment.

“The disparities in built environments, influenced by socioeconomic factors, have significant implications for the indoor microbiome and, consequently, human health. In conclusion, this review highlights the critical role of the indoor microbiome in shaping health outcomes and emphasizes the need for a multidisciplinary approach to create healthier living spaces,” the authors wrote. “Future research should focus on developing practical, equitable interventions that leverage our growing understanding of the indoor microbiome to improve public health in an increasingly urbanized world.”