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Before the pandemic, budget-cutting downgrades to HVAC and other building systems that could compromise indoor air quality (IAQ) were often tolerated because the health consequences to occupants were not immediate or obvious. The greater concerns were energy conservation and occupant comfort rather than indoor health.
The airborne contagion of COVID changed that mindset as facility managers had to race to make their buildings safer for returning occupants. Suddenly, facility managers had to seek solutions for mitigating viral transmission through ventilation, filtration, humidification, and other air-cleaning strategies.
A silver lining in the pandemic is that these strategies will ultimately benefit facility managers and their organizations economically when they factor in the positive impact of IAQ investments – and cost savings – on occupant health and building reoccupation. And the impact is supported by a growing body of research which reveals that the indoor built environment establishes its own microbiome that impacts occupant health in the same way our gut’s microbiome plays a major role in maintaining human health.
Controlling airborne transmission
The pandemic helped bring to light the risks posed by the transmission of the COVID-19 virus in buildings. Science has known for some time the powerful influence that IAQ has on our well-being. But for decades, facility managers have focused more on mitigating individual and perceivable factors.
For example, volatile organic compound (VOC) off-gassing can be detected by chemical odors, and has therefore been a target in “sick building syndrome” complaints. Missing in the list of key indoor constituents which contribute to occupant health or disease transmission are invisible factors such as outdoor air, humidity, and ozone. For transmission of an infectious disease to occur, the disease-causing virus or bacteria must be able to both travel and find a susceptible host. The indoor environment affects both the pathogenic microbe and the susceptibility of the host, therefore plays an important role in enhancing or suppressing infectious disease spread. Unfortunately, there has been little progress in regulating the indoor environment to help protect the health of the building occupants.
An ever-increasing body of scientific research now confirms the impact of these IAQ factors on the transmission of viral respiratory infections and respiratory, immune system response. As we witnessed during the first COVID-19 winter season, cases increased as the outdoor temperature fell, heating systems were turned on, and more people spent more time indoors.
That’s because viral respiratory infections are almost exclusively transmitted from person to person indoors. Transmission pathways in indoor settings are through direct contact, indirect contact, droplets, and airborne aerosols. Droplets and airborne transmission via aerosols, in particular, are the most common infection pathways in close-range environments.
COVID-19 particles from an infected individual that are breathed in by another person enter the respiratory mucous membranes. During breathing, coughing, talking or sneezing, infectious viruses present in a person’s respiratory system can be emitted in droplets of saliva and mucus.
Droplets travel only up to a distance of about 1.5 to 2 meters. They can also be spread by contact with contaminated surfaces – such as when these surfaces are touched, and the viruses then come into contact with mucous membranes via the hands. Airborne transmission of aerosols is more worrisome. Virus particles of less than 5 micrometers in diameter can travel over long distances through the air before infecting people. They have the potential to escape gravitational forces and remain airborne indoors for several hours. Even if indoor air remains relatively still, tiny infectious aerosols can spread through the air in large spaces over a long time.
Relative humidity is key
The spreading of viruses by airborne aerosols hinges on the indoor climate in buildings. Among all the contributing factors (fresh air, temperature, humidity, etc.), indoor relative humidity plays an essential role in reducing the risk of infection. Science now shows that the lowest risk of transmission is achieved with a relative humidity of 40 to 60 percent. At the same time, this is also the range in which the human immune response is most effective.
Some earlier research done in hospitals clearly pointed to a correlation between low relative humidity in patient rooms and an increase in bacterial and viral infections. Subsequent studies in nursing homes, schools, and offices have shown that people are much healthier, obtain fewer infections, have increased productivity, and sleep better at night with this range of humidity.
Relative humidity affects the ability of viral aerosols to remain suspended in indoor air. Unlike the larger and heavier infectious droplets produced by coughing or sneezing, which fall to the ground after a few seconds, lighter and smaller aerosols can stay suspended in the air for hours at a time.
A relative humidity under 40 percent causes aerosols to lose their moisture and dry out. The result is a build-up of dry aerosols that are smaller and lighter, and can float through indoor air for longer. Unlike larger droplets, their lower water content also makes them less "sticky," and so they cannot bond together so readily. Air flows and people's movements in a building also cause dry aerosols to be swept off surfaces more quickly, enhancing their ability to spread further.
Humidity also plays a massive role in the contagiousness of COVID-rich droplets. At less than 40 percent relative humidity, aerosols dry out so much that the salts they contain crystallize. These salts protect the viruses, and they remain infectious for longer. When breathed in, the crystallized salts dissolve once more in the moist environment of the respiratory tract. The viral particles, still contagious, are released onto the mucous membranes, where they can trigger an infection. If relative humidity is within the optimum range of 40 to 60 percent, however, particles only dry out to an extent where salt concentrations rapidly inactivate viruses rather than protecting them.
Tackling infectious aerosols with fresh air
Bringing as much fresh air into a facility as possible is another effective method for removing viral aerosol particles from indoors. As the proportion of fresh air rises, the viral aerosol particles in room air are increasingly diluted.
The simplest option is just to open a window. The volume of air that flows through an open window depends on the temperature gradient, the wind speed/direction, and the angle at which the window is opened. An effective strategy for facility managers is to use short but ample ventilation, with windows being fully opened for several minutes at least every hour. But there are limits to the effectiveness of using windows for ventilation. In summer, the temperature gradient between outdoor and indoor air is often too low, and air exchange is minimal. In winter, energy losses and sharp drops in relative humidity are arguments against the constant use of open windows.
Mechanical ventilation and filtering are necessary as well. HVAC systems can move the required volumes of fresh air and used air into and out of an indoor environment in a controlled manner. As air changes increase, the risk of infection grows smaller. But facility owners should be aware that higher rates of change can lead to an increase in energy consumption and a lowering of relative humidity levels. Specialized filters can also remove even the smallest aerosols from the air. They are particularly recommended for HVAC systems where air is frequently recirculated.
Enhancing a facility’s microbiome
While it may seem counter-intuitive, keeping the indoor environment ultra-hygienic and germ-free in defense of the COVID-19 virus is harmful to our immune system. A facility must also enable interaction with the good microbes (e.g., viruses and bacteria) present in the environment to help maintain human immune responses. In other words, it is vital to suppress the microbes that make us sick while boosting the health of occupants through exposure to healthy microbes.
Science now shows that communities of helpful microbes, called a microbiome, reside in us and are essential for our survival. Buildings need to be understood as living ecosystems with their own microbiomes. To achieve a balanced diversity of microorganisms, facility managers should use porous-free, smooth synthetic materials only sparingly, and on surfaces that are to be frequently cleaned – such as handrails, door handles, taps, and keyboards. For walls, ceilings, and furniture, natural materials with porous surfaces are preferred, offering an amenable environment for diverse communities of microbes. On these natural surfaces, water and nutrients are in plentiful supply for bacteria and viruses.
Because of the pandemic, facility managers are starting to look at their building environments from a different perspective. No longer can they just focus on real estate value or energy consumption. The new focus will be on designing buildings with occupant health – not just comfort – in mind. Gaining this visibility through careful monitoring, and analyzing of data through a health lens will also be important. Doing so will not only help facility owners address future waves of COVID-19 and other viral outbreaks, but it will also be extremely beneficial to businesses, healthcare services, and the national economy.
Dr. Stephanie Taylor is founder and president of Building4Health.