Published on medium.com
Written by Devin Malone
COVID-19 has changed the way we think about our indoor spaces. People are concerned about the health risks of being in closed areas with one another and the increased risk of exposure to viruses and bacteria. How do building owners reassure occupants and let them know action has been taken to keep their environment as safe as possible?
As an engineering firm specializing in mechanical, electrical, and plumbing systems, our answer to this question begins with establishing an understanding of how pathogens such as COVID-19 spread, and how building systems can contribute to reducing the risk to occupants. There is a lot we don’t know about COVID-19, but the healthcare industry has been working to understand and prevent the spread of infection for decades. We can use these infection-prevention strategies in combination with what we do know about COVID-19 to reduce the transmission of infections in buildings.
How is infection spread?
The information given here, while specific to COVID-19, is generally true of most infectious viral and bacterial transmission.
COVID-19 is understood to be spread by droplets and aerosols containing the SARS-CoV-2 coronavirus. An infected person releases droplets and aerosols while coughing, sneezing, shouting, singing and, it is now believed, while talking at a normal volume. Generally, there are three primary modes of transmission:
1. Inhalation or direct contact to eyes, nose, or mouth with droplets. Droplets are defined as larger particles that drop out of the air after traveling a short distance, usually 3 to 6 feet.
2. Indirect contact with droplets that have contaminated surfaces and are then transferred to the eyes, nose or mouth typically by contaminated hands.
3. Inhalation of aerosols (droplet nuclei). These are smaller particles that remain airborne and can travel in the air longer distances than droplets.
The first two modes of transmission have been widely accepted as contributing to the spread of COVID-19. Significant efforts are being made to eliminate droplet and contact transmission by encouraging social distancing, frequent handwashing, and disinfection of surfaces. Administrative controls are also being used to change the layout of workspaces and to reduce the number of people in a contact group. The third mode of transmission, airborne contamination, is less understood and has been largely left unaddressed. There is an increasing call for more attention to be given to indoor air quality and aerosol transmission.
How your building systems impact transmission
The most effective way to eliminate transmission in buildings is to not have people in them to begin with. Many have taken this measure already by restricting access to buildings, allowing employees to work from home, and in some cases, this will remain a reasonable strategy. But as we consider how and when to allow students to return to school and employees and the public back into buildings, how can we improve indoor air quality to make them safer?
Every building is unique, with different systems and capabilities. There is no one-size-fits-all approach and implementing strategies should be done with a complete engineering assessment of the impact and effectiveness of changes to your building. In general, below are HVAC strategies that can be used to impact air quality, and thus transmission, in your building.
HVAC and Indoor Air Quality Recommendations:
Humidity and temperature — both humidity and temperature have been shown to impact airborne transmission and overall human immune system health. A dry environment allows droplet nuclei to remain airborne and viable longer. A dry environment has also been shown to increase a person’s susceptibility to infection. While most HVAC systems can regulate temperature for comfort control, many buildings and systems were not designed to maintain acceptable levels of moisture and humidity. The recommended range for indoor relative humidity is between 40–60% and an occupied temperature range of 68–72 degrees.
Ventilation provides dilution of airborne pathogens and the extraction of pathogens from a space. Many buildings have inadequate ventilation because of systems not operating correctly, buildings relying on natural (e.g., window) ventilation, outdated systems, or user tampering (e.g., blocking vents, closed outside air dampers). Buildings should, at minimum, meet code-required ventilation rates. Buildings with at-risk occupants or that have a high occupant load should consider increasing towards 100% outside air. There can be adverse effects from this strategy, particularly in more extreme climates, and this should be done under the advice of a professional.
Improving HVAC filtration can reduce the amount of airborne contamination in spaces that share recirculated air. Minimum Efficiency Reporting Value (MERV) is a standard that rates the overall effectiveness of air filters. Higher value MERV rating equates to finer filtration, meaning fewer airborne contaminants can pass through the filter. Consider adding the highest MERV level filters appropriate for your system’s capability. In some instances, a portable, free-standing high-efficiency filtering system may be an effective way to reduce risk in a single space.
Important note: HVAC and building systems are complex engineered systems. Any changes should be undertaken with a full understanding of the implications to the whole system, climate, and building occupancy. For instance, humidity levels above 60% can result in unintended mold problems if not properly implemented, and adding advanced filtering can inhibit airflow reducing the air exchange in a space.
What measures disinfect and improve indoor air quality?
Ultraviolet light has been widely used for years to inactivate virus and bacteria, particularly in hospital settings, or in the prevention of mold in HVAC systems where moisture can be a problem. Currently, there is much discussion about its ability to be effectively used as we consider ways to equip our buildings to combat aerosol pathogen transmission.
In air handlers While UV light has been effectively used to address fungal growth at the cooling coil in air handlers that remain wet for a period of time, the use to eliminate aerosols is more complicated. To be effective, the exposure time must be sufficient to inactivate the bacteria or virus. To ensure appropriate implementation of this UV strategy, system evaluation and design will be required.
In-room UV systems Upper air UV can directly address the concentration of virus or bacteria in a single space. Units are mounted high up on a wall and continuously disinfect the upper air, known as the disinfection zone. To be most effective, these systems require air circulation to bring contaminated air into the disinfection zone. Current recommendations for this technology are for spaces with a large occupancy, such as waiting rooms. In our experience, we have found that because ventilation and air circulation are generally inadequate in many buildings, improving these measures should be taken on prior to or in combination with upper air UV disinfection.
Bi-polar ionization units have been used in HVAC systems to eliminate airborne particles, odors, and airborne pathogens. The ions created act like scavengers, attaching themselves to any airborne particle they can find, forming a larger particle that will fall to the ground or more easily be filtered. Bi-polar ionization has been shown to be effective at inactivating a virus or bacteria particle, including COVID-19. It is important that the appropriate technology is chosen, as some produce ozone, which can be harmful to health. However, this can be a cost-effective air cleaning strategy when implemented correctly.
Where to start?
The first step to identifying what strategies can be employed in your building is understanding your systems and ensuring that they are working as designed by optimizing controls and checking system components.
Next, consider a professional evaluation of your facility to help you identify what measures are most appropriate based upon current systems, climate, building occupancy, effectiveness at mitigating transmission, and cost.
It is important to note that while COVID-19 is on everyone’s mind, indoor air quality considerations apply to the transmission of other viruses like the flu, to bacteria, and to mold and other air contaminants. Addressing indoor air quality in a sustainable manner is critical to providing a healthy environment for building occupants at any time.
Disclaimer: Willdan is an engineering firm. We have expertise in HVAC systems, especially in the measures discussed here, and experience working in healthcare environments; however, we are not medical experts. The information in this article presents our reading of the current literature and our interpretations on how to best act on the available information.