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. In this article, we will discuss the Minimum Efficiency Report Value (MERV) rating system, how it relates to virus filtration, and other air filtration technologies that can help protect you from contracting COVID-19. The MERV rating system is used to measure a filter's ability to capture particulate matter. Filters with a MERV-13 rating or higher are capable of trapping smaller particles, including viruses.
The COVID-19 virus attaches to droplets that are released when a person coughs or sneezes. These atomized particles range in size from 1 to 100 microns, so any filter with a Merv 9 rating or better would be effective at trapping them. Can building air filtration protect me from contracting COVID-19? Leakage in building heating, ventilation and air conditioning (HVAC) systems may be part of a general risk mitigation approach, but it is generally not considered a solution on its own. 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 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 properly designed for the building in which they are used.
More importantly, in most buildings and in most situations, filters can be significantly 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 people in my building from COVID-19? 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., ultraviolet (UV) lamps), and have dedicated staff who operate and maintain this equipment to provide maximum benefit. What about ultraviolet (UV) lamps? Do they work? UV lamps are quite effective at maintaining the cleanliness of HVAC coils, drain pans, and other damp surfaces. Properly designed systems can also 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. What about portable air cleaners? 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. What precautions should I take when changing filters? 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 conclusion, while there is no direct scientific evidence that air filtration can protect you from contracting COVID-19, it may still be part of a general risk mitigation approach. Filters with MERV 9 ratings or higher are capable of trapping particles containing viruses like COVID-19. Hospitals have specially designed mechanical systems that can adapt to the levels of filtration they need. UV lamps are quite effective at maintaining the cleanliness of HVAC coils and other damp surfaces.
Finally, when changing filters it is important to take precautions such as wearing gloves and respiratory protection.