The COVID-19 pandemic forced the building industry to reexamine how indoor environments influence human health. Architects, engineers, and building owners have long focused on structural safety, fire codes, energy efficiency, and ventilation standards. But a viral pandemic introduces a different category of risk, one that calls for a fresh look at how buildings interact with the microorganisms that inhabit them. Research from the University of Oregon’s Biology and the Built Environment (BioBE) Center suggests that the goal should not be to eliminate all microbes from indoor spaces, but rather to cultivate healthy microbiomes that support human well-being. This approach connects directly with broader building envelope design considerations, including how we manage airflow and moisture barriers. For instance, understanding how weather resistive barriers influence building envelope performance becomes especially relevant when designing interiors that can respond to airborne pathogen threats.
Understanding the Indoor Microbiome
Every indoor space hosts a complex microbial ecosystem made up of bacteria, fungi, viruses, and protozoa. These organisms do not simply float about harmlessly. They interact with occupants in ways that can either support or undermine health. The Old Friends Hypothesis, also known as the Hygiene Hypothesis, proposes that reduced exposure to diverse microbes in modern built environments contributes to rising rates of autoimmune disorders, asthma, and allergies. When buildings seal out the outdoor environment too effectively, they may also seal out the microbial diversity that humans evolved alongside.
Each person carries their own microbiome, a personal collection of microorganisms as unique as a fingerprint. These microbes help regulate digestion, influence mood, affect weight, and provide immunoprotective effects through the skin. The indoor microbiome interacts constantly with these human microbiomes, creating a dynamic exchange that building design can shape. Research has shown that aggressive antimicrobial cleaning practices can have unintended consequences. Studies from the BioBE Center have found correlations between the use of antimicrobial compounds and increased abundance of antibiotic-resistant bacteria indoors. Excessive sterilization creates selective pressure that favors hardier, potentially more dangerous microbes.
This understanding shifts the conversation from disinfection to microbial stewardship. Rather than attempting to sterilize indoor spaces, building professionals should aim to manage microbial communities in ways that favor beneficial organisms over harmful ones. This principle applies across building types, from homes to hospitals. Retrofitting existing structures for better microbial management often involves upgrades that improve overall building performance, similar to how structural strengthening methods for seismic upgrades improve building resilience in a different context. Both approaches recognize that proactive design yields better outcomes than reactive fixes.
Architectural Features That Shape Microbial Communities
Architects have more control over indoor microbial ecosystems than they may realize. Several design parameters directly influence which microbes thrive inside a building and how they spread. These include space configuration, occupant density, interior material selection, window placement and glazing type, electric lighting spectrum and intensity, and air movement strategies.
Natural systems such as daylight and natural ventilation have been shown to support more diverse indoor microbial ecosystems while reducing the abundance of potential pathogens. These strategies also reduce energy consumption and support general human health, making them multi-benefit design choices. The selection of interior surfaces matters as well. Natural unfinished wood surfaces have been demonstrated to reduce the abundance of certain viruses more quickly than stainless steel or plastic. This finding has implications for high-touch surfaces in commercial and healthcare settings.
Site conditions also play an indirect role. Buildings situated on challenging terrain or in dense urban areas face different microbial management challenges than those in open, well-ventilated locations. As noted in discussions about building on challenging lots, the site itself constrains what design strategies are feasible. A building tucked into a hillside may have limited window placement options, affecting natural ventilation and daylight penetration. Every design decision either expands or constrains the toolkit available for managing indoor air quality and microbial ecology.
Humidity, Ventilation, and Pathogen Control
Among the most powerful tools for managing indoor microbial health is humidity control. Most indoor environments become extremely dry during the heating season. While dampness can produce mold and other moisture-related problems, very dry air creates its own set of health risks. Dry air dehydrates mucus membranes and skin, and it allows fine particles to travel deeper into the respiratory tract. Both effects leave occupants more susceptible to infection.
Research indicates that maintaining indoor relative humidity between 40% and 60% provides multiple benefits. This range reduces viral infectivity by disrupting the outer membrane of viruses. It also decreases the amount of time that ultra-fine particles remain aerosolized, reducing the risk of airborne transmission. Dry air below 40% relative humidity allows particles to stay suspended longer, increasing the window of exposure for building occupants.
The importance of targeted disinfection techniques varies by setting. The table below summarizes key environmental strategies and their applications across different building types:
| Strategy | Residential | Commercial Office | Healthcare |
|---|---|---|---|
| Increased outdoor air intake | Open windows, HRV operation | Adjust AHU damper settings | Pressure cascade management |
| Humidity control (40-60% RH) | Portable humidifiers or whole-house system | Building-wide humidification plant | Zone-level precision humidification |
| Natural ventilation | Operable windows in each room | Atrium and stack effect design | Limited; filtered mechanical preferred |
| UV-C light disinfection | Not typically applicable | Upper-room UV in common areas | Air handler UV and surface UV-C |
| Daylight exposure | South-facing glazing | Open floor plate with light wells | Atriums with UV-filtering glazing |
These strategies align with the broader principle that buildings should work with natural systems rather than against them. Just as period-accurate architectural columns rely on classical proportional systems to achieve both beauty and structural function, healthy building design relies on an integrated understanding of how environmental factors interact with biological processes. The proportions of airflow, humidity, and light determine whether a space supports or undermines occupant health.
Practical Building Operations During Health Emergencies
Based on research conducted during the COVID-19 pandemic, the BioBE Center developed practical guidelines for reducing indoor viral transmission that apply to homes, medical buildings, and other critical infrastructure. These strategies do not require expensive retrofits or specialized equipment in most cases. Simple operational changes can produce meaningful reductions in transmission risk.
Key operational recommendations include:
- Increase the proportion of outdoor air admitted into the building. Most HVAC systems can adjust outdoor air damper settings to reduce recirculation.
- Increase the air exchange rate. More frequent exchanges dilute airborne pathogen concentrations.
- Maintain relative humidity between 40% and 60% throughout the building.
- Open windows where possible to provide natural ventilation and flush out indoor spaces, even during colder months.
- Increase access to natural daylight, which has been shown to reduce pathogen survival on surfaces.
- Implement targeted disinfection using UV-C light in high-risk settings such as healthcare facilities.
For residential settings, the recommendations become more personal. If someone in the household is infected or symptomatic, they should self-isolate in a room adjacent to a bathroom with an exhaust fan that can run continuously. This configuration creates negative pressure that pulls air from the rest of the home through the infected space and exhausts it directly outside. This principle mirrors how fresh paint transforms historic building interiors by addressing both aesthetic and environmental conditions. In both cases, attention to airflow, material condition, and space utilization creates healthier indoor environments.
Historical precedent supports these approaches. During the 1918 influenza pandemic, high ventilation rates proved effective at reducing contagion within homes and hospitals. Building practices of the 1920s reflected this lesson. Heating systems were deliberately oversized to accommodate the extreme ventilation loads created by keeping windows open year-round. The resulting overshoot and comfort problems led to innovations in radiator design and control, including low-emissivity paints and air-restrictive covers. These adaptations show that the building industry has responded to pandemic-driven ventilation needs before and can do so again with modern tools and materials.
Managing Residential Humidity and Envelope Performance
Residential buildings present unique challenges for indoor microbial management because they typically lack the sophisticated HVAC systems found in commercial or institutional buildings. Yet homes are where most people spend the majority of their time, especially during pandemic conditions. Achieving healthy humidity levels in a home requires attention to both mechanical systems and building envelope performance.
Weatherstripping, air sealing, and insulation all affect how well a home retains conditioned air and resists moisture intrusion. A leaky envelope makes humidity control nearly impossible, while an overly tight envelope without adequate mechanical ventilation can trap pollutants and pathogens indoors. The balance point differs by climate zone and building type. Understanding how bedroom humidity and building envelope best practices interact provides homeowners and builders with actionable guidance for creating healthier indoor spaces. Simple measures like adding weatherstripping to windows and doors, sealing ductwork, and installing exhaust fans in bathrooms and kitchens can dramatically improve a home’s ability to maintain healthy humidity levels.
Portable humidifiers can help during dry heating seasons, but whole-house humidification systems integrated with forced-air heating offer better consistency. Dehumidification becomes more important in humid climates, where excess moisture promotes mold growth and dust mite proliferation. Both conditions affect the indoor microbiome, though in different ways. Dry conditions favor dust mites and viral survival, while damp conditions favor fungal growth and bacterial proliferation. The 40-60% relative humidity target represents the sweet spot where both risks are minimized.
Future Directions in Healthy Building Design
Looking beyond the immediate pandemic response, researchers are working to define what constitutes a community of beneficial microbes in indoor environments. The BioBE Center is developing real-time microbial monitoring technologies that could support better building operating practices and improve contact tracing during outbreaks. These sensors would allow building managers to detect pathogens early and adjust ventilation, filtration, or occupancy patterns before widespread transmission occurs.
Some cleaning product manufacturers are exploring the concept of adding beneficial microbes to indoor environments to outcompete harmful pathogens. This probiotic approach to building maintenance avoids the scorched-earth strategy of traditional antimicrobial cleaners, which rely on caustic and volatile ingredients that can themselves pose health risks. The idea is promising but requires robust research and regulatory oversight from agencies such as the Environmental Protection Agency, which regulates antimicrobial products classified as pesticides.
The architectural design and construction industry has spent decades developing standards for energy performance, structural safety, and fire protection. The next frontier is developing equivalent standards for microbial health. This means treating the indoor microbiome as a design parameter alongside thermal comfort, daylight access, and acoustic performance. Presentations and findings from recent industry gatherings have highlighted how building science research from symposia and industry events is converging on a set of actionable strategies that professionals can implement today. The path forward involves designing buildings that not only protect occupants from pathogens but actively cultivate the microbial diversity that human immune systems need to function properly. By shifting the focus from microbial elimination to microbial stewardship, the building industry can create interiors that support health in normal conditions and offer resilience during emergencies.