The Covid-19 pandemic has created intense interest in commercial building HVAC air filtration and purification. The novel coronavirus, SARS-CoV-2, is the virus that causes the COVID-19 disease. The SARS-Cov-2 virus is .15 micron (um) in size. Though viruses cannot survive by themselves in ambient air for long periods of time, they attach themselves to larger (mostly fluid) particles to remain viable. These particles are referred to as “aerosolized particles” and are immitted when people talk, sneeze or cough. Aerosolized particles range in size from .01 to 10 micron. Suspended particle matter (PM) generally refers to particles having a diameter of 10 um or less. Particles over 10 micron in size settle quickly, however particles 10 um and smaller can remain airborne for hours or days. A NIOSH airborne Influenza cough study published in 2010 reported that 42% of aerosolized particles immitted by a human cough were 1 um or smaller, 1-4um were between 1-4 um, and 35% were 4 um or larger.
CDC reports in the “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”, Oct. 5 2020, reports that although some viruses and bacteria (TB, Measles and Chicken pox) spread effectively through air borne transmission and at long distances over long periods of time, “Available data indicate that SARS-CoV-2 has spread more like most other common respiratory viruses, primarily through respiratory droplet transmission within a short range (less than 6 feet)”.
CDC also reported that; “Circumstances under which airborne transmission of SARS-CoV-2 appears to have occurred include:”
- Enclosed spaces where an infected person directly exposed others or where people were exposed shortly after the infected person left the space.
- Prolonged exposure to respiratory particles generated with expiratory exertion (shouting, singing, exercising etc.)
- Inadequate ventilation or air handling that allowed a build up of suspended respiratory particles or droplets.
ACT recommends that building owners and operators consider implementing the following HVAC upgrades to minimize the potential airborne spread of SARS-Cov-2 and other virus, bacteria and microbes.
- Upgrade the air filtration utilized in the building HVAC system. (CDC, ASHRAE and NIOSH recommend a minimum of MERV-13 but higher efficiencies should be considered).
- Increase air exchange rates within the building space. (Either through the existing HVAC system or with dedicated fan powered HEPA filter units)
- Install UVGI “Air Stream Disinfection”.
- Install portable of permanent fan powered HEPA filter units to operate independently of the HVAC system and to provide additional air exchanges through 99.99% HEPA filters.
AIR FILTRATION
The ASHRAE organization, CDC and NIOSH have all recommended installing a minimum of MERV-13 filters in all commercial building air handling systems. Unfortunately, the recommendation for MERV-13 filtration does not address the ASHRAE 52.2 test standard and the application of the Appendix J conditioning step to validate the MERV efficiency rating. Most MERV-13 filters do not maintain efficiency once they are installed. The passive electrostatic charge that provides the MERV-13 efficiency dissipates shortly after the filter is installed in an air handling unit.
The ASHRAE organization developed and implemented the Appendix J conditioning step to neutralize the passive electrostatic charge that all synthetic fibers have. Filters that maintain the MERV efficiency rating after being subjected to the Appendix J procedure are identified with the letter “A” after the MERV efficiency rating. Example; a filter that can provide a MERV-13 efficiency after Appendix J is labeled MERV-13A. This is applicable to all MERV efficiency ratings.
Furthermore, although MERV-13A filters offer better efficiency than typical MERV-8-12A filters, they do not provide significant removal of sub-micron particles. These sub-micron particles (<1 um) include viruses, bacteria, fungi and other
microbial contaminants. The SARS-COV-2 virus is .15 micron in size and therefore passes through most filters unless it is attached to a larger (mucus or other) particle.
ACT recommends that commercial building owners and operators install a minimum MERV-14A filters in all air handling units (AHU). MERV-14A filters are available in 2” and 4” thicknesses and therefore can be installed in virtually any air handling unit. Some AHU’s can accommodate 12” deep filters. In this case MERV-15A or 16A filters should be considered.
While MERV-13A filters remove only 50% of 1 um or smaller particles a MERV-16A filter removes over 90%.
HEPA filters are considered “absolute” and remove 99.99% of all particle matter. HEPA filters are also restrictive to the air flow and are difficult to retrofit into standard HVAC units. HEPA filters should be considered for use in free standing supplemental fan powered air purifiers to enhance air exchanges within interior spaces to minimize air borne contaminants.
ULTRAVIOLET GERMICIDAL IRRADIATION (UVGI)
Regardless of what type filter is utilized within an AHU, Ultra Violet Germicidal Irradiation (UVGI) purification should be installed to ensure that greater than 99% of airborne SARS-COV-2, that may be present within the HVAC system, is deactivated.
Ultraviolet Germicidal Irradiation (UVGI) UVC light systems reduce the risk of airborne infectious disease and have been proven to inactivate the SARS-COV-2 virus with the proper exposure time and UVC intensity. (SARS-COV-2 causes the COVID-19 disease). The ultraviolet light spectrum includes UV-A, UV-B, and UV-C light. UV-A and UV-B pass through the earth’s atmosphere but UV-C does not. Consequently, viruses, bacteria, mold and other microbials have no natural defense against UVC light. The very short 254 nm wavelength of UVC light is able to penetrate the cell walls of microorganisms and disrupt their DNA which inactivates them. Every commercial building can benefit from germicidal UVC disinfection. UVC can destroy virus, bacteria, mold and fungi as well as reducing biofilm on coils which results in significant energy and maintenance savings.
UVGI systems have historically been installed within air handling units to provide “surface disinfection” on the cooling coils, casing walls and other components to prevent microbial growth on surfaces. These systems, though very effective on killing microbial contaminants that are attached to surfaces, are not designed to destroy air borne microbials.
Inactivating viruses and other microbials that are “Airborne” require “Air Stream disinfection”. Inactivating air borne virus and microbials can be accomplished by installing Fresh-Aire UV Grid systems within the AHU (if adequate space is available) or installing Fresh-Aire UV ducted ADS systems within the supply air ductwork. It is critical to select the UVC systems so that the air stream contaminants are exposed to 254 nm of UVC light for as long as possible. This is considered the “Dwell time” of the UVGI system. The ADS systems provide superior performance with less power and are easily installed within the air handling unit or in the supply air duct. Most systems can provide greater than 99% inactivation rates for SARS-COV-2 and greater than 90% inactivation for Influenza-A.
HEPA FILTER- FAN UNITS
HEPA filters are considered “absolute” and remove 99.99% of all particle matter. HEPA filters are also restrictive to the air flow and are difficult to retrofit into standard HVAC units. HEPA filters should be considered for use in free standing supplemental fan powered air purifiers to enhance air exchanges within interior spaces to minimize air borne contaminants.