With the knowledge that most viruses range in size from 0.004-1.0 μm (microns), some believe they are “too small” to be contained by even the best MERV or HEPA filters. This leaves many wondering about air filters’ effectiveness against pathogens, such as SARS-CoV-2. Can the virus be “caught” by an HVAC air filter in order to create cleaner, safer air for businesses, schools, and hospitals?
It’s been stated that increasing the HVAC systems’ minimum outdoor air ventilation rate is the best precaution to help limit the transmission of the disease, referring specifically, to indoor long-distance airborne transmission of the COVID-19 virus, which research has now proven to be possible.
Increasing outdoor air fractions can be helpful in diluting indoor concentrations of airborne particles, including viruses, with the understanding that the ventilation rate effectiveness (VRE) of the system will have a great impact on its overall efficacy. Air filtration (assuming the particulate of concern can make it to the air filter) has also been proven to substantially help mitigate subsequent health risks. Both ventilation and filtration are code-mandated requirements in hospitals as well as other buildings, and it is the careful implementation of both of these approaches that may garner the best result.
While the SARS-CoV-2 virus has been reported to be approximately 0.08-0.09 microns in size (very small), research has found the virus itself to be transported via airborne aerosol droplets that have appeared in two distinct size ranges, the smallest reported so far with an aerodynamic diameter of 0.25 microns. Small droplets like these are defined as “droplet nuclei” and can be the result of a larger droplet’s immediate desiccation after expulsion from an infectious individual (coughing, sneezing, talking, or breathing). These droplets, being comprised of liquid, will evaporate based on an attempt to reach an equilibrium with the moisture level of its new environment. Size reduction from larger droplet nuclei to smaller droplet nuclei can be substantial in ratios of 50:1 or more and can occur in a split second.
Recent research has confirmed that SARS-CoV-2 viral material has been disseminated throughout the environment by airborne droplet nuclei, and just because it has been desiccated does not render it nonvirulent. The SARS-CoV-2 virus has been found to retain its stability in aerosols and on surfaces for several hours to days,with similar findings substantiated in studies on other strains of respiratory disease. While the droplet transporting the SARS-CoV-2 virus (or other possible pathogens) may be very small, it remains larger than the individual virus itself, which has an impact on the ability of an air filter to help mitigate airborne transmission throughout the environment.
It’s mistakenly believed that traditional HVAC air filters function on the principal of capturing only those particles that are larger than the openings in the filter. In reality, specific filtration mechanisms, including sieving, inertial impaction, interception, and Brownian diffusion, are principals that determine what is removed from the airstream based on size. Impaction and interception are the dominant collection mechanisms for particles greater than about 0.2 microns, while Brownian diffusion is dominate for those smaller. Brownian diffusion is when the very small particles interact with air molecules causing them to react in chaotic or random motion. This forces them to collide with the filter fibers and allows the filter to capture particulate much smaller than openings in the media. When a filter’s efficiency is measured based on the most penetrating particle size (that most difficult to remove) then, for example, it might be 99.97% effective at removing particles 0.3 microns in size, but this is the filter’s lowest efficiency point. It will perform better at capturing both larger and smaller particles as a result of the mechanisms of filtration briefly defined.
This means that air filters are very capable of removing airborne viruses contained within small droplets (along with other particulate of concern) when applied properly in conjunction with an HVAC system design that balances airflow, air exchange, ventilation rates, ventilation effectiveness, and air-distribution along with other important dynamics that comprise a truly functional filtration system. The American Society for Health Care Engineering (ASHE) tells us that a MERV-16-rated filter has 95% or better efficiency for particles of 0.3-1.0 micron sizes, and, due to Brownian motion, particles smaller than 0.3 microns are trapped within filter media more efficiently than those of the 1.0-0.3 micron size. They state that “MERV 16 is the rating most effective in capturing the SARS-CoV-2 virus.”
The claim that air filters can only capture the bigger stuff that is strained-out by the openings between the filter fibers is false. This misinformation may lead to underestimating the role air filtration can play as an effective means to help keep us truly safe and healthy within buildings of all types.
Advanced IAQ technologies may be applied as a “bundled” approach and combined with traditional ventilation and filtration to help enhance overall efficiency and can be included in both new and existing HVAC systems. These may lead to improved results when striving for safer, healthier, and cleaner indoor environments in for the occupants residing within buildings and structures of all types, everywhere.
While not filters themselves, advanced IAQ technologies, such as Global Plasma Solutions’ needlepoint bipolar ionization (NPBI), can help enhance the efficacy of traditional ventilation and filtration systems. This technology is capable of agglomerating (combining) small particles in the space itself, making them easier to remove and filter.
That being said, it’s important to understand that ventilation and filtration are considered “dilution” strategies, which are based on removing or capturing particulates of concern from the air. Neither (alone or combined) is capable of “killing” or inactivating the virulence of a pathogen, such as a virus, for example. Both are effective only if the particulate (solid or gas) can be entrained in the airstream and removed by its individual or combined process.
1. ASHRAE Position on Infectious Aerosols.
2. ASHRAE Position Document on Airborne Infectious Diseases.
3. ASHRAE Position Document on Filtration and Air Cleaning.
4. ASHE COVID-19 Frequently Asked Questions (website).
5. David N. Schurk, Whitepaper: The HVAC System’s Role in Environmental Infection Control for Hospitals.
6. David N. Schurk, Whitepaper: Protecting Occupants in Non-Healthcare Environments from Airborne Infections.