Improved indoor HVAC system can help prevent airborne respiratory viruses

In a response to requests for more research, The WHO has acknowledged that in the case of COVID-19, airborne transmission through respiratory aerosols could not be ruled out in crowded and poorly ventilated settings. Scientists across the world continue looking for clarifying evidence as it may affect many guidelines on how to mitigate risk indoors. Previously, The WHO had emphasized studies and evidence detailing the transmission of COVID-19 from infected people to others as primarily occurring by those in close contact through respiratory droplets, either by direct contact with the infected person or by close contact with contaminated objects and surfaces.

The Centers for Disease Control (CDC) adjusted their findings to report the virus has been identified to spread in one of four ways:

  • Person-to-person in close contact with one another.
  • Through large respiratory droplets inhaled or deposited on mucous membranes when a nearby infected person coughs, sneezes, sings, talks, or even breathes.
  • Airborne through smaller droplet nuclei or aerosols.
  • Contacting contaminated surfaces.
Two methods involve transmission through the air by way of either large respiratory droplets or smaller airborne aerosols. These droplets are aqueous solutions that are forced out of the respiratory system when exhaling and could contain a quantity of the virus if expelled from an infected person.

The virus does not travel through the air by itself, but rather is contained within these droplets. The dividing line between large respiratory droplets that fall quickly to nearby surfaces and the much smaller aerosols which can remain suspended in the airstream for hours, is broadly set at 5 microns.

However, the size of droplets does not always remain steady. Droplets begin to dehydrate once exposed to the environment which reduces both their size and mass. This creates a grey area between droplets large enough to fall quickly and droplets small enough to remain in the airstream for longer periods of time.

When considering risk mitigation as it relates to air filtration, the recommended strategy is to put systems and air filters in place that have a higher likelihood of capturing both droplets and aerosols that exist outside the social distancing zones before the contaminants make contact with healthy people.

This infographic shared by BBC explains the difference between the droplet and airborne transmission.


What can be done?

When taking steps to mitigate risks through a building’s HVAC system, it’s important to take a comprehensive approach with your air filtration solution and consider the entire heating and ventilation systems as well as the environment. ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) recommends increasing outdoor air ventilation in order to reduce the percentage of air recirculated. Additional considerations should be to evaluate exhaust air for possible re-entrainment and investigate natural ventilation as a means to dilute any virus particles. 

When it’s time to consider the proper particle capture efficiency of the air filter, it’s critical to understand the size of the particle creating the risk. In the case of minimizing the risk of COVID-19 infections, defining the size of the respiratory droplets and aerosol that could possibly contain the SARS-CoV-2 virus is the first step. 

Other diseases are spread by respiratory droplets and aerosols, not just COVID-19. As such, the size and dispersion of human exhaled droplets and aerosols have been well researched. While the infectious dose of COVID-19 is not known, a 0.5-micron size aerosol particle is likely capable of containing a sufficient quantity of viruses to be considered a risk. That’s the small end of the scale. On the larger end of the scale, are the droplets not quite large enough to fall within the six-foot social distance zone from the source. Between those, one size distribution analysis showed a peak at 2.5 microns for those particles. As such, an air filtration solution that offers particle capture efficiency of 95%+ within these size ranges and is able to scale upwards based on risk factor, would be a sufficient risk mitigation strategy.

Standard Risk Areas

Based on that size range, for standard risk areas such as commercial office and retail buildings, schools, airports, manufacturing facilities, and areas within healthcare facilities not occupied by COVID-19 patients, if the current configuration of HVAC ventilation system allows, the minimum efficiency recommended is a MERV-15A. To extend the service life, a prefilter of a lesser MERV rating should be installed upstream if possible. Camfil products achieving this level would be the Camfil 30/30® Dual 9 as a prefilter followed by the MERV-15/15A Durafil® ES2. Below are particle capture efficiency percentages for MERV-13A through MERV-16A.

Higher Risk Areas

For higher risk areas within healthcare facilities such as treatment or isolations rooms designated for COVID-19 patients or facilities that regularly house individuals in the CDC high-risk category, if the current configuration of the HVAC ventilation system allows, the minimum efficiency recommended is a 99.97% HEPA filter with appropriate prefiltration of lesser MERV value. Camfil products achieving this level would be the Absolute® VG 99.99% HEPA with a Camfil 30/30 Dual 9Hi-Flo® ES pocket filter or Durafil ES2 in lesser MERV ratings as a prefilter depending upon system configuration.

Stand-Alone solutions

In situations where the HVAC system lacks the capability to accept higher MERV-A rated filters or is incapable of providing adequate ventilation, consider installing stand-alone air purifiers as supplemental filtration. These products can be positioned to target specific areas where improved air quality is needed.

In all situations where risk mitigation is undertaken that involve ventilation and filters, it is strongly recommended a qualified HVAC specialist is consulted. An air filter is a single component within a larger and more complex HVAC system designed to accomplish the proper ventilation of a building. Increasing particle capture efficiency is not necessarily a singular solution to lowering the risk of infection. Air filters with higher capture efficiency ratings often have higher pressure drops which may reduce air changes per hour (ACH) and influence humidity as well. These filters may also be physically larger than lower-rated filters, therefore it is important to verify that the HVAC system is equipped with the frames necessary to ensure an airtight seal and that the fan is capable of overcoming the added resistance.



Can HVAC systems help prevent transmission of COVID-19? - Published by McKinsey

WHO rethinking how COVID-19 spread in the air – Published by BBC

High humidity leads to loss of infectious influenza virus - Journal

Transmission of COVID-19 virus by droplets & aerosols – Study & Journal

Created Friday, November 6, 2020
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