As the world continues to grapple with the COVID-19 pandemic, many are wondering what else they can do to help “flatten the curve”. While hand washing and staying at home remain the most effective means of limiting the spread of the virus, there is conflicting information about the role and effectiveness of air filters. Can air filtration in a building protect us from COVID-19? Filtration in building heating, ventilation and air conditioning (HVAC) systems may be part of an overall risk mitigation approach, but it is not generally considered a solution in itself. There is no direct scientific evidence of benefit, but reduced exposure can reasonably be inferred based on the ability of some filters to remove particles containing the SARS-CoV-2 virus.
For filters to have any impact on the transmission of infectious diseases, transmission must occur through the airway, 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. More importantly, in most buildings and in most situations, filters can be considerably less effective than other infection control measures, such as social distancing, isolation of known cases and hand washing. What filter should I use to protect those in my building from COVID-19? Hospitals (and many healthcare facilities) have specially designed mechanical systems that can be adapted to the filtration levels they need. They are often based on other control systems and strategies (for example, MERV 11 filters). The most important thing is that they have dedicated staff who operate and maintain this equipment so that it provides maximum benefit. What about ultraviolet (UV) lamps? Do they work? What about ionizers, ozone generators, plasma and other air cleaning technologies? None of these technologies have been shown to reduce infection in real buildings, even if they show promise based on tests conducted in a laboratory or in an idealized environment.
Some of them are considerably concerned about secondary issues (such as ozone production).What about portable air filters? 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, as it generally does not address transmission by surface contact or by close contact between people. However, the distinction between airborne and droplets is particle size. We know that (a) droplets can remain in the air for long periods of time (for example, MERV 11 filters), and (b) DNA and RNA from other viruses have been found, which are usually associated with droplets in the filters used. What precautions should I take when changing filters? In general, it is advisable to assume that filters contain microbiological active material.
It is not known whether this represents a significant risk of infectious virus disease, but the precautionary principle suggests that care should be taken. This becomes particularly important in any building (including a home) where infectious diseases, including COVID-19, are known or likely, and also extends to portable air purifier filters and vehicle cabin air filters. Filters should be replaced with the system turned off, with gloves, with respiratory protection if available, outdoors if possible and disposed of 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 prior to continued use of filters within ventilation systems. While UV systems are quite effective in maintaining the cleanliness of HVAC coils, drain pans, and other damp surfaces, properly designed systems can be quite effective in on-the-fly inactivation of microorganisms in moving air streams.
These systems generally require more lamps, so they can provide significant UV doses in a short period of time. A typical single-pass inactivation efficiency is 85%, very similar to a good particulate filter, but systems can also be designed for greater than 99.9% inactivation. In addition, a well-designed UV air disinfection system within an HVAC system, and located next to 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 a radiated area above the occupant and disinfect air in the space as air circulates naturally, mechanically or via the HVAC system. This type of system has been approved for use in tuberculosis control by the CDC for almost 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 and other spaces. Systems such as these are commonly used in unoccupied spaces due to concerns about occupant exposure.
The three types of systems may be relevant depending on the type of building and individual spaces within it. The design and size 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. Duct systems are further complicated by the configuration of the air handling unit and ducts and surface reflections that can help. In conclusion, while there is no direct scientific evidence that MERV 11 filters can protect us from COVID-19 infection directly, they may play an important role as part of an overall risk mitigation approach when combined with other infection control measures such as social distancing and hand washing.