In our continuing series, Hermanson’s TAB/Commissioning/Controls Manager Brad Sharp, CHC, with assistance from Hermanson Senior Engineer, Pedro Alicea, are offering practical responses to questions related to building HVAC maintenance in the era of COVID-19. Their responses are based on years of experience and the guidance of industry recognized standards and best practices set for by ASHRAE, NAFA, FGI, and OSHA.

According to the CDC, COVID-19 is spread primarily from person-to-person through respiratory droplets produced when an infected person coughs or sneezes.

According to the Wells Riley Report  published by the National Air Filtration Association Foundation, (NAFA), particles larger than 10 um in aerodynamic diameter will settle out of the air rapidly (page 1). Particles smaller than 10 um tend to remain airborne for longer periods of time which increases the likelihood of airborne transmission and inhalation (page 2).  The National Institute of Health (NIH) has determined that COVID-19 virus is 0.06 um. 

This means that COVID-19, at 0.06 um, is well within the range of particles remaining airborne for longer periods.  Studies indicate that aerosols generated during coughing by infected patients remain suspended in indoor environments and contain the virus (page 6-7, Wells Riley Report). 

Therefore, the probability of exposure within a building is possible.  Studies have shown that the highest probabilities of exposure within a building are measles, followed by influenza and SARS-COVID-19, then Tuberculosis and Rhinovirus. (page 9, Wells Riley Report). 

According to ASHRAE Standard 62.1, a typical office experiences approximately 0.6 ACH (air changes per hour or 5 cfm per person x 0.06 cfm/sf).  An office worker would have approximately a 40% chance of acquiring influenza virus with the minimum ventilation rates defined by ASHRAE Standard 62.1 (page 10, Wells Riley Report).  Washington State Mechanical Code follows the ASHRAE standard 62.1.

A building’s HVAC system can play a role in minimizing exposure to COVID-19. 

One method is to increase the outdoor air ventilation to 4 ACH (air changes per hour).  This rate has shown to reduce the exposure rate down from 40% to 8% (Page 10, Wells Riley Report). 

If, however, an existing building system is not be able to handle the load of increased hourly air changes, HVAC filtration can help reduce indoor concentration of airborne particles coupled with current code outdoor air ventilation.

The rating system of filters is reported as Minimum Efficiency Reporting Values or MERV.  Per NAFA, the following MERV rating of filters can remove particles of a given size by a specific efficiency (Page 13, Wells Riley Report).

Studies have shown, using influenza as a sample virus in office settings, outdoor air ventilation will reduce the risk of exposure.   According to tests, assuming a 25% outdoor air supply and no filtration, the risk of exposure is reduced to approximately 25%. 

According to the graph below, by adding filtration, the risk of exposure begins to be reduced.  After MERV 13, the risk of exposure is not reduced until introduction of HEPA Filtration (page 19, Wells Riley Report). 

Using a combination of outdoor air ventilation (increased ACH) and filtration has proved to reduce transmission of infectious diseases within a building.

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The responses offered in this post are based on findings of industry recognized standards and best practices developed by ASHRAE, NAFA, CDC, OSHA and FGI.  They are offered as guidelines and do not account for the specifics of any building or its mechanical systems.  For specific information as to a given building or system, please contact Hermanson or a licensed mechanical engineer. Any actions taken based on the responses given are the sole responsibility of the party taking such actions.