As we all socially distance and stay at home amid the COVID-19 pandemic, some may wonder what else they can do to “flatten the curve”. While hand washing and staying at home remain the most effective means of limiting the spread of the virus, conflicting information is emerging about the role and effectiveness of air filters. Can building air filtration protect us from contracting COVID-19? What filter should we use to protect people in our building from COVID-19? Why shouldn't we use the most efficient filter we can find? What about ultraviolet (UV) lamps, do they work? What about portable air filters? Doesn't filtration require droplets that cause COVID-19 to be in the air? What precautions should we take when changing filters? What about ionizers, ozone generators, plasma and other air cleaning technologies? Where can we go for more information? Minimum Efficiency Report Value (MERV) ratings measure a filter's ability to capture particulate matter. Filters with MERV-13 ratings or higher can trap smaller particles, including viruses.
Many home HVAC systems will have a MERV-8 filter installed by default. The direct scientific evidence of benefit is limited, but reduced exposure can reasonably be inferred based on the ability of some filters to remove particles containing a SARS-CoV-2 virus. For filters to have any impact on the transmission of infectious diseases, transmission must occur through the air route, filters must be properly installed and maintained in systems suitable for treating recirculated air, and filters must be designed appropriately for the building in which they are used. Hospitals (and many health centers) have specially designed mechanical systems that can adapt to the levels of filtration they need.
They are often based on other control systems and strategies (e.g. social distancing, isolation of known cases, and hand washing). Most importantly, they have dedicated staff who operate and maintain this equipment to provide maximum benefit. None of these technologies have been shown to reduce infection in real buildings, even if they are promising based on tests conducted in a laboratory or in an idealized environment.
Some of them have substantial concerns about secondary issues (such as ozone production). Yes, most public health guidelines suggest that transmission of COVID-19 is predominantly associated with large droplets. This is why air filtration is only a small part of a solution, since it generally does not address transmission by contact with the surface or by close contact between people. However, DNA and RNA from other viruses, which are generally associated with droplets, have been found in the filters used.
In general, it is prudent to assume that filters have microbiological active material on them. It is not known if this represents a significant risk of infectious disease from viruses, but the precautionary principle would suggest that care should be taken when changing filters. This becomes particularly important in any building (including a home) where there are known or probable cases of an infectious disease, including COVID-19. Filters should be replaced with the system turned off, with gloves, with respiratory protection if available, outdoors if possible and discarded in a sealed bag. 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. While 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.
These systems generally require more lamps, so they can provide significant UV doses in a short period of time. 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 mentioned above. Another way to install UV is in a “top air” configuration.
Specially designed wall-mounted fixtures create an irradiated area above the occupant and disinfect the air in the space as it circulates naturally or mechanically through the HVAC system. CDC has approved this type of system for use in tuberculosis control for nearly 20 years and there is guidance from NIOSH on how to design them. Finally, mobile UV systems are frequently used for terminal cleaning and surface disinfection in healthcare facilities and other spaces due to occupant exposure concerns.