Preventing Disease Transmission Through HVAC Systems Expert Article

What is the standard of care for minimizing the risk of disease transmission through building HVAC systems? Prior to 2020 this was an esoteric question largely reserved for academics, but it has since been sprung upon the masses with increasing relevancy for business owners, facility operators, and workers alike.

In this article, mechanical engineer Rene Basulto will address this question from the perspective of a forensic investigator. He will discuss some of the aspects of building HVAC systems that can help to minimize the spread of airborne pathogens. He will also address broader building systems factors that may be relevant to investigating pathogen exposures.

Disease prevention hvac expert

Preventing Disease Transmission Through HVAC Systems

The standard of care for indoor air quality is changing and HVAC systems are at the forefront of this change. In a post-COVID-19 world, the emphasis for engineers when designing, maintaining, or upgrading HVAC systems will go beyond the traditional measures of temperature and humidity control to include considerations for minimizing the spread of airborne pathogens. Particularly in environments where people work or congregate, there will be an expectation that pathogen control be optimized alongside efficiency and other traditional performance markers.

It’s impractical to conceive that all facilities will immediately overhaul their building systems to address changing needs, but from a forensic standpoint there are system calibrations and minor modifications that can be implemented in most facilities at a nominal cost. It is likely that many forensic investigations following exposure to an airborne pathogen will look at the adequacy of performed maintenance, system calibration, and the application of reasonable modifications that are effective at preventing the spread of dangerous pathogens.


Dilution is achieved through the introduction of outdoor air into an air conditioning system. As clean outdoor air is added to the re-circulated air already in the system, the concentration of pre-existing contaminants within the system is reduced. Dilution is effective at decreasing the Parts per Million (PPM) of pathogens, but is not sufficient for preventing the transmission of all airborne pathogens.

There are monetary and system limitations to how much outside air can be introduced for dilution ventilation. There is a significant energy cost associated with conditioning outside air. Prohibitive costs aside, the existing HVAC equipment may not have sufficient capacity to accommodate the added heat or cooling load associated with treating outside air. Therefore, filtration and other non-traditional air cleaning strategies also need to be evaluated and considered to minimize the potential for transmission.


The effectiveness of an HVAC filter to remove particles from the air depends on routine maintenance as well as the Minimum Efficiency Reporting Value (MERV) rating of the filter. MERV values vary from 1 to 16. The higher the MERV value, the more effective the filter will be in trapping airborne particles. Typical residential and light commercial air handling system filters have a MERV-4 rating. With less than 20% efficiency, MERV-4 filters out pollen, dust mites, dust, and carpet fibers.

Prior to the Covid-19 pandemic, The American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) recommend the use of filters rated at MERV-6 or higher. MERV-6 filters have efficiencies of 35% to 49%, filtering out smaller particles like mold spores, hair spray, and cement dust. Post Covid-19, ASHRAE is recommending improving filtration to MERV-13 or higher. MERV-13 filters have efficiencies of 90% or better for particles between 3 to 10 microns, and are able to filter out much smaller particles like most bacteria and droplet nuclei (sneezes).

Increased filtration is not a panacea due to the limitation to filter submicron bacteria and the much smaller viruses (0.02 to 0.4 micron) and aerosols. Additionally, filters with higher MERV ratings have an adverse effect on system airflow. Many systems were not designed to accommodate the higher efficiency filters, and making a modification without assessing other system parameters can restrict airflow through the system to levels that may be inadequate for equipment operation. Therefore, a balance must be achieved between removing the majority of larger airborne contaminants and maintaining adequate airflow.

Increased ventilation and filtration may still not be enough to reduce some airborne particulates such as submicron bacteria and viruses to safe levels. Other air purification strategies like ultraviolet germicidal irradiation or Bipolar Ionization of the air stream may need to be implemented as well.


Ultraviolet Germicidal Irradiation (UVGI) is a disinfection tool that utilizes UV-C or short wavelength light to inactivate microorganisms. UV-C destroys the nucleic acid in the microorganism and disrupts their DNA, rendering them virtually inert. The safety of UV-C is well known. It does not penetrate deeply into human tissue, but it can penetrate the very outer surfaces of the eyes and skin, with the eyes being most susceptible to damage. Therefore, shielding is needed to prevent direct exposure to the eyes.

Bipolar Ionization technology was first used in the US in the 1970s as a tool to control pathogens in food manufacturing. The technology only moved into the HVAC industry in recent years. Integrated into HVAC systems, the technology converts oxygen molecules from the air into charged atoms that cluster around micro particles, surrounding and deactivating harmful substances like airborne mold, bacteria, allergens, and viruses. The atoms also attach to expelled breath droplets and dust particles that can transport viruses, enlarging them so they are more easily caught in filters. It’s an active process that provides continuous disinfection.

Building Evaluation

A holistic approach must be taken when evaluating readiness of the built environment for reoccupation in a post-COVID-19 world. An in-depth evaluation of the existing HVAC system by a competent professional should be performed before fully reoccupying buildings. Proper system operation needs to be confirmed to ensure adequate ventilation is being provided in accordance with ASHRAE guidelines. Not following these guidelines may leave employers, building owners, building managers, and facility operators liable for problems encountered during re-occupancy of the workplace.

After a thorough evaluation is performed, the parties may find that modifications to the HVAC system are necessary in order to properly minimize the risk of transmission. These may include augmentation of the ventilation system and additional air cleaning strategies beyond traditional filtration.

In April of 2020, the ASHRAE published a position paper on infectious aerosols. The position paper indicates that design teams for facilities of all types should follow, as a minimum, the latest published standards and guidelines. Based on the risk assessment or owner requirements facilities may need to beyond the minimum requirements. Additionally, the position document indicates that the following modifications to building HVAC system operation should be considered:

  • Increase outdoor air ventilation as indoor and outdoor conditions permit. This will drastically improve the dilution of contaminants, but may have adverse effects on energy consumption and humidity control in humid climates.
  • Improve central air and other HVAC filtration to MERV-13 or highest level achievable.
  • Keep systems running longer hours (24/7 if possible).
  • Add portable room air cleaners with HEPA or high-MERV filters.
  • Add duct or air-handling-unit mounted ultraviolet germicidal irradiation (UVGI) devices.
  • Maintain temperature and humidity as applicable to the infectious aerosol of concern.
  • By-pass energy recovery ventilation systems that leak potential contaminated exhaust air back into the outdoor air stream.

Forensic Investigation of Sick Building Claims

The mechanical engineers at Robson Forensic are highly qualified professionals with extensive experience in the HVAC and building systems. They are able to evaluate design, installation, operation and maintenance of systems to determine if they are capable of preventing the transmission of pathogens. Our team of engineers, industrial hygienists, and facilities engineers are able to provide a holistic approach in evaluating readiness of the built environment for reoccupation in a post-COVID 19 world.

For more information, submit an inquiry or contact us at 800.813.6736.

Featured Expert

Rene Basulto, Mechanical Engineer & Building Systems Expert

Rene Basulto, P.E., MSEM, CGC, C.F.E.I.

Mechanical Engineer & Building Systems Expert
Rene is a mechanical engineer with 40 years of experience in the design, construction and maintenance of building systems, including HVAC, plumbing, industrial systems, and fire protection systems. He… read more.


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