Air Purity and COVID-19

By: Gary B. Xavier | Jan 05, 2022

The pandemic known as COVID-19 is all around us, never more than a miniscule moment away from our conscious thought. There is little doubt that particles within the air can carry disease. The transmission of germs from one human to another through the air we breathe is now readily recognized as a fact of life. 

It was not an accepted theory, however, until Irish physicist John Tyndall proved it in the 1870s, a century and a half ago, through his vast experiments with microbe growth in the air. 

The virus causing this massive loss of life around the world is known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and the respiratory illness it causes is called COVID-19. 

Much research, thought, discussion, and controversy has arisen because of this outbreak. The virus is, in fact, carried by moist particles in the air that can spread over a wide breadth. As a result, maintaining air purity in occupied spaces has become a challenge for building owners, managers, and maintenance staff. 

ASHRAE, the American Society of Heating, Refrigeration, and Air-conditioning Engineers, has spent vast resources on research to determine the best practices for maintaining building air purity, both before and during the pandemic. The ASHRAE Epidemic Task Force recently issued its Core Recommendations for Reducing Airborne Infectious Aerosol Exposure 

According to their research, the findings are ‘…based on the concept that within limits ventilation, filtration, and air cleaners can be deployed flexibly to achieve exposure reduction goals subject to constraints that may include comfort, energy use, and costs.’  (ASHRAE Epidemic Task Force, October 19, 2021)  

It is prudent, therefore, to study their findings and implement corrective measures as necessary. The ASHRAE recommendations are paraphrased here. 

  1. Public Health Guidance – Follow all current regulatory and statutory requirements and recommendations, including vaccination, wearing of masks and other personal protective equipment, social distancing, administrative measures, circulation of occupants, hygiene, and sanitation. 
  1. Ventilation, Filtration, Air Cleaning

2.1 Provide and maintain at least required minimum outdoor airflow rates for ventilation as specified by applicable codes and standards.  

2.2 Use combinations of filters and air cleaners that achieve MERV 13 or better levels of performance for air recirculated by HVAC systems.  

2.3 Only use air cleaners for which evidence of effectiveness and safety is clear.  

2.4 Select control options, including standalone filters and air cleaners, that provide desired exposure reduction while minimizing associated energy penalties.  

  1. Air Distribution – Where directional airflow is not specifically required, or not recommended as the result of a risk assessment, promote mixing of space air without causing strong air currents that increase direct transmission from person-to-person. 
  1. HVAC System Operation

4.1 Maintain temperature and humidity design set points.  

4.2 Maintain equivalent clean air supply required for design occupancy whenever anyone is present in the space served by a system.  

4.3 When necessary to flush spaces between occupied periods, operate systems for a time required to achieve three air changes of equivalent clean air supply. 

 4.4 Limit re-entry of contaminated air that may re-enter the building from energy recovery devices, outdoor air, and other sources to acceptable levels. 

  1. System Commissioning – Verify that HVAC systems are functioning as designed.

Particular attention should be given to Item 2.2 above concerning the MERV-13 filtration recommendation. The MERV ratings – Minimum Efficiency Reporting Value – establish guidelines for determining the effectiveness of particle removal by various filter densities. 

The MERV 13 filter has been determined to remove over 85% of particles in the 1–3-micron size, has a dust spot efficiency of 89-90%, and an arrestance of >98% of droplet nuclei (sneeze particles). Arrestance is the measure of the ability of an air filtration device to remove larger particles, such as dirt, lint, hair, and dust from the air. However, if a ventilation system was designed and balanced for a filter of lower density, say, for example, a MERV 8, raising the filter density to a MERV 13 will reduce the airflow.  

The adaptation of a building ventilation system to provide a greater degree of filtration may result in the need for a change in airflow (cubic feet per minute, CFM) and a necessity to have the ventilation system reassessed and balanced to incorporate the filtration improvement.  

[Gary B. Xavier, member ASHRAE, has over 50 years of experience in the HVACR field and has written numerous textbooks and training programs. He can be contacted at] 


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