Phase three trials for one or more COVID-19 vaccine candidates may be completed by the end of this year, proving a vaccine is safe, effective and ready to be licensed by the FDA and administered to the public in 2021. However, most credible health experts suggest that even with a vaccine, we will likely be reckoning with this pandemic though much of next year. In the interim, to regain some semblance of normalcy in workplaces, schools, restaurants and other gathering places, we will need to do more than health screening, enhanced cleaning, social distancing and mask mandates.
These are all helpful measures (please wear a mask!), but to reduce transmission risks in indoor spaces, multiple lines of defense are required, including effective strategies to address airborne pathogens. Scientists now recognize that aerosolized respiratory particles containing SARS-CoV-2 can travel hundreds of feet and remain in the air for extended periods after being expelled by an infected person, even one who is asymptomatic/pre-symptomatic.
To address ongoing COVID-19 transmission risks over the next year, and to make our buildings more pathogen resilient against the next likely outbreak, as well as common colds and seasonal flu, we need comprehensive approaches, including air cleaning and disinfecting technologies. In response to this growing demand, building owners and managers are being bombarded by manufacturers of various technologies promising solutions to address airborne pathogens. Determining which ones are truly effective and picking the best solution (or combination of solutions) for unique building conditions is challenging. The following is a brief summary of the most common air cleaning solutions being promoted and the pros and cons of each, based on ASHRAE’s independent assessments and analysis conducted by Engineering Economics Inc. (EEI).
Dilution Ventilation – The simplest and most straightforward means to reduce virus-laden airborne particles is to introduce more fresh air. The aerosolized contaminants released by an infected person can be diluted quickly by increasing the air changes per hour (ACH), a measure of the air flow relative to the volume of the indoor space. Since COVID-19 infection potential increases with the intensity and duration of exposure, reducing the concentration of airborne pathogens as quickly as possible following an infected person’s expiratory event (e.g., cough) is an effective means to reduce transmission risks. Significantly increasing outside air ventilation rates in some existing HVAC systems can increase energy use, especially in more extreme climates.
Filtration – Similar to dilution ventilation, enhanced filtration directly removes aerosolized pathogens from indoor air. While the SARS-CoV-2 virus measures approximately 0.1-micron, aerosolized respiratory particles range from 1 to 10 microns which can be captured in MERV 13 filters (90% capture > 1 micron). MERV 13 filters have a higher cost, require more frequent filter changes, and may increase fan energy, but are a quick and effective upgrade that can be reversed after the pandemic subsides. High Efficiency Particulate Air (HEPA) filters remove over 99% of particles but are more costly and would create excessive pressure drop in many existing HVAC systems. However, supplementing (or in place of) higher efficiency filters in HVAC equipment, portable HEPA air cleaners can be deployed to effectively remove airborne pathogens. Generally, the HEPA filters in these air cleaners are highly effective by themselves, and devices that include additional virus deactivation technologies do not have increased efficacy.
Ultraviolet Germicidal Irradiation (UVGI) – The UV-C range of the ultraviolet light spectrum has been proven effective at pathogen deactivation, from tuberculosis-fighting application in the last century to the current pandemic. The intensity of UV-C light required to deactivate virus can also cause severe damage to our skin and eyes, requiring extreme caution in where it is deployed and how it is maintained. One common application of UVGI is inside HVAC systems, typically in return air ducts or air handling equipment. The UVGI device must have enough intensity to sanitize the volume and rate of air flowing past it. Another common application is upper-room UVGI, where devices are mounted on walls or pendants and function like indirect light fixtures, directing light upward with shielding to prevent line-of-sight exposure of the light source to the eyes of occupants. Either of these applications can be an effective supplement to ventilation and filtration strategies, or as a primary strategy in spaces where increased ventilation is not feasible.
Photocatalytic Oxidation (PCO) – This deactivation strategy seeks to decompose organic molecules via oxidation using a UV-activated catalyst such as titanium dioxide. While some devices have been shown to remove harmful airborne contaminants, there is a lack of independent research demonstrating the effectiveness of PCO for airborne pathogen deactivation. Its potential efficacy is dependent on the design and placement of the system, and there is some concern that devices may generate ozone or partially decompose molecules, creating other harmful compounds.
Ion Generation – This technology relies on the generation of negative and/or positive ions that convey a charge to airborne particles, leading to agglomeration, making them more readily captured by filters. One form of ion generator being promoted as a virus deactivation technology is called needlepoint bipolar ionization (NPBI). Some laboratory studies have shown that prolonged exposure of pathogens to high concentrations of ions result in deactivation, however, there is little evidence that these ion concentrations can be achieved in typical building applications. EEI estimates that even a best-case extrapolation of these laboratory virus deactivation rates would result in a pathogen concentration reduction equivalent to implementing 4 ACH of dilution ventilation. The ASHRAE Epidemic Task Force states, “Relative to many other air cleaning or disinfection technologies, needlepoint bi-polar ionization has a less-documented track record in regard to cleaning/disinfecting large and fast volumes of moving air within [HVAC] systems.”
Bottom line: buyer beware! It is best to focus on vetted strategies and keep it simple. Scientists tell us that we will be living with COVID-19 for a while, and that this will not be the last major outbreak in our lifetimes. As we invest in making pandemic-resilient buildings, supportive of health and wellness, we should consult independent expert advice to ensure that we get the expected results.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. To learn more, follow Scott on Twitter @alanscott_faia.