Many home HVAC systems will have a MERV-8 filter, which is designed to capture particles that are larger than 1 micron in size. While this type of filter is effective for capturing large droplets, it is not effective for capturing smaller airborne droplets, which are the primary way that COVID-19 is transmitted. Public health guidelines suggest that transmission of COVID-19 is predominantly associated with large droplets, so air filtration is only a small part of a solution. It generally does not address transmission by contact with the surface or by close contact between people.
DNA and RNA from other viruses that are usually associated with droplets have been found in used filters. Therefore, it is important to take precautions when changing filters. If chemical disinfectants are used, they should only be applied with the HVAC system turned off. In addition, disinfectants should not be applied to ventilation filters before continuing to use the filters within ventilation systems.
The effects of disinfectants on filter performance are unknown. Filters should only be treated with disinfectants if they are to be removed from service and disposed of. UV systems are quite effective at maintaining the cleanliness of HVAC coils, drain pans, and other damp surfaces. Properly designed systems can be quite effective in inactivating microorganisms in moving air streams on the fly.
A typical one-pass inactivation efficiency is 85%, just like a good particulate filter, but systems can also be designed for inactivation greater than 99.9%. In addition, a well-designed UV air disinfection system within an HVAC system, and located adjacent to the cooling coils, can also provide surface disinfection benefits. Another way to install UV is in a “top air” configuration. Specially designed wall-mounted fixtures create an irradiated zone above the occupant and disinfect the air in the space as the air circulates naturally, mechanically, or through the HVAC system.
Mobile UV systems are frequently used for terminal cleaning and surface disinfection in healthcare facilities and other spaces. The design and sizing of effective ultraviolet disinfection systems can be a complex process due to the need to determine the dose delivered to a moving air stream or to an irradiated region of a room. Accredited system manufacturers and designers can help by making the necessary calculations and designing specific systems for individual spaces. With more than 57,000 members from more than 132 countries, ASHRAE is a diverse organization dedicated to promoting the arts and sciences of heating, ventilation, air conditioning and refrigeration to serve humanity and promote a sustainable world.
ASHRAE recommends a MERV rating of 13+, which is designed to capture 1 micron sized particles that would be effective for capturing COVID-19 virus particles indoors. For HVAC systems, ASHRAE recommends a filter with a minimum MERV rating of 13, but MERV 14 or higher is preferred. MERV 14 and higher can capture particles that are even smaller than previous MERV ratings, ranging from 0.3 to 1.0 microns in size.