The ultimate ventilation guide to reducing COVID-19 transmission in commercial buildings
November 17, 2021 By Anthony Broatch
Why is this important?
We are likely to see new variants of COVID-19 in the future that may require further vaccination and restrictions. Taking steps to reduce the spread of the COVID-19 virus and other viruses is critical to ensure our society continues to operate without new or further restrictions.
How does pathogens like COVID-19 spread?
The New Zealand Ministry of Health states “The virus that causes COVID-19 is mostly spread in particles that escape from an infected person’s mouth or nose when they breathe, speak, cough, sneeze or sing.” These airborne particles can remain in the air for hours unless adequate ventilation is provided. “catching COVID-19 from surfaces is the least common, but it is still important to clean surfaces to reduce the risk.” [1, 4, 5, 6].
The use of face masks reduces the number of airborne particles released from a person however only by 90% during speaking and 74% during coughing on average for surgical masks, respectively, compared to wearing no mask based on a study from scientific reports [2]. These figures can be reduced using cotton masks.
As COVID-19 is mostly spread via the air, controlling the spread of COVID-19 by well-designed ventilation systems is critical to effectively reducing the spread of the virus. No HVAC system will be 100% effective at controlling the spread of pathogens however certain systems will reduce the likelihood.
Most common commercial and residential building ventilation systems (high walls, ducted units, cassettes, and central ducted systems) re-circulate air and also rely on heavily mixing air in a space to ensure an even temperature distribution for comfort. Due to the large amount of mixing these types of systems are excellent at evenly distributing pathogens throughout a space and typically allow concentrations of pathogens to build up unless sufficient outdoor or filtered air is provided.
Figure 1: Traditional ventilation system
What are the best ways to ensure my ventilation systems reduces the spread of pathogens like COVID-19?
Overview
There are various ventilation system options to reduce the spread of pathogens each with varying degrees of effectiveness. Combining multiple types of approaches will provide more effective than one approach alone.
Each approach is listed below with details on effectiveness.
Displacement ventilation
A recent study by Tomasz Lipinski has shown that displacement ventilation systems are the most effective at reducing the spread of pathogens. In this type of system, a large amount of air is introduced at a low level and a low velocity in the space to reduce mixing and is exhausted at a high level in the space above the occupied zone. Using this type of system, most of the airborne particles released by occupants rise above their heads and are exhausted or returned to the central system which can then be filtered out. The figure below shows how a displacement ventilation system reduces the spread of airborne pathogens such as Covid-19. This can be viewed in contrast to the traditional ventilation system above.
Figure 2: Displacement ventilation system
The supply grills should supply air at a low velocity to reduce turbulence that causes air mixing of airborne particles. These types of systems should incorporate outdoor air to dilute pathogen concentrations or filters to remove pathogens from any recirculated air.
This type of system is possible to retrofit however may require new bulkheads for low-level supplies and system changes to ensure comfort.
High levels of outdoor air
Adding outdoor air dilutes the number of airborne particles in a space that might contain harmful pathogens. The more outdoor air the higher the dilution rate. It is recommended to have an outdoor air change rate of at least four to six times per hour which is much higher than the current New Zealand Building Code requirements [4, 5]. This can be reduced if you are removing airborne particles using a filter.
Outdoor air can be introduced by a mechanical system or opening windows. When providing outdoor air you should be mindful of the path that air takes to leave the building as airborne particles will spread along that path. An example of this is supplying outdoor air into one room such as an office and exhausting it from another such as a kitchen or bathroom
Introducing high levels of outdoor air has additional benefits such as improved concentration of occupants, preventing mould, and removing odours and other contaminants.
It should be noted that increasing outdoor air volumes also increases the amount of energy required to heat and cool that air to the desired room conditions. Due to this when introducing large quantities of outdoor air installing a heat recovery unit will reduce power consumption.
Opening windows
Natural ventilation by opening windows and doors is a great way to provide a large amount of air movement in a space to dilute any pathogen containing airborne particles.
There are a few downsides to this type of solution listed below:
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If used alone there is no temperature control of the space
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If combined with an existing HVAC system it will likely reduce the system’s ability to effectively heat and cool a space resulting in comfort issues
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It will cause drafts at certain times
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In winter cold outdoor air might cause comfort issues
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The air supply is unfiltered
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Airflow can be turbulent which may mix airborne particles in a space more than a displacement ventilation solution.
The amount of airflow is also unreliable as it relies on external wind resulting in occasions with less than ideal airflow. The best natural ventilation also occurs when windows/doors can be opened on opposing sides of the building which might not be possible in a closed office or meeting room. Natural ventilation still typically provides higher levels of outdoor air than a typical mechanical system design to the New Zealand code and is recommended as a good option for reducing pathogen transmission by the New Zealand goverment.
Figure 3: Opening windows
Filtering Recirculated Air
It is recommended that ventilation systems that recirculate air in business should incorporate hospital-grade Merv 13 filters. These filters have a minimum efficiency of 70% at removing small particles. [4, 5]. Higher efficiency filters could be used such as HEPA filters which have a min efficiency of 99.95% to reduce risk.
It should be noted that, as a general rule, higher efficiency filters with the same surface area require a larger amount of pressure to pass air through. This results in larger fan size/cost and higher power usage.
When fitting new filters to existing systems the existing fans should be assessed to ensure they are sufficiently sized to overcome the new filter pressure. If this is not done the units may not be able to produce sufficient airflow to ventilate the room.
High air change rates
The amount of air changes of outdoor or filtered air in a space is proportionate to the rate of pathogen removal. Having a high air change rate on a system with recirculated air will only remove pathogens if the filter grade is high enough and/or if there is outdoor being introduced into the system. Increasing the air change rates is effective with any system type but most effective with a displacement type ventilation system as with this system, not only are the particles diluted, but they are also removed from the occupied zone above occupants heads with a lower amount of air mixing.
The air changes rates of outdoor air or filtered recirculated air should be at least four to six times per hour [4, 5].
Portable HEPA air purifiers
These systems recirculate and filter the air in a room. With these systems, the removal of air born particles is not instant and particles may exist in the room for long periods before eventually being filtered out by the unit. The amount of air mixing they cause also depends heavily on the placement. Due to this, they have the potential to inadvertently increase the mixing of airborne particles spreading them more quickly throughout a space. Consideration also needs to be given to the placement of the unit to prevent blowing contaminated air from one person to another.
As with the above when selecting this type of system the air changes rate of filtered recirculated air should be at least four to six times per hour [4, 5].
Panasonic nanoeX series air-conditioning systems
Panasonic has released an innovative series of air-conditioning systems called the nanoeX series which, based on internal studies, help inhibit airborne viruses and other pollutants. The degree to which these units reduce the risk of transmission of COVID-19 has not been established however they may be a good option if you are only looking at installing split systems over other options on the market. This is not a paid endorsement for Panasonic.
Incorporating similar technology into a displacement ventilation system would be an ideal solution to further reduce the risk of transmission.
Ultraviolet (UV) filter banks
UV light destroys the genetic information in the DNA of viruses preventing them from reproducing effectively inactivating the virus.
The effectiveness of UV light depends on the wavelength, intensity of the radiation, and time the virus is exposed to the radiation.
Air moving quickly through ductwork will not have sufficient UV light exposure time to effectively destroy the virus that causes COVID-19 [7]. UV systems are more effective at preventing mould growth on cooling coils that can have continuous exposure to UV light. A more effective approach is to use UV light to disinfect the air at a high level in a room continuously as well as using UV light to disinfect surfaces when a space is unoccupied.
Pressure regimes
Pressure regimes should be considered when designing systems to prevent or control the spread of pathogens between spaces. A pressure regime is the pressure in one space relative to another and dictates the direction that air flows between spaces. The below image shows a pressure regime for a typical negative pressure isolation suite to illustration how pressure influences air movement.
Figure 4: Negative pressure Isolation room pressure regime
In your building, you may not necessarily want air moving between spaces as that could result in the spread of pathogens. An ideal situation building as opposed to them being contained within the space they originated. In this scenario, you would want to have a neutral pressure difference between the rooms served.
Connected HVAC systems
Connecting HVAC systems with common returns can also cause the spread of pathogens between spaces. This occurs when a HVC system sucks up air born particles from one space and distributes them through the HVAC system to another space. Connected HVAC systems without filtration to remove harmful airborne particles should be avoided to reduce the spread of pathogens.
There is also a risk with connected HVAC systems that when the fan is not running air can transfer through the ductwork between spaces.
System run on after hours
Systems used to prevent disease transmission should ideally run on for a period after use to ensure the space is adequately flushed of any pathogens.
Preventative maintenance
Preventative maintenance of equipment should be carried out periodically to keep it running and prevent any unplanned downtime from unexpected equipment failure. Preventative maintenance of fans and filters is the most important for preventing pathogen transmission. This is because failure of fans or blocked filters will result in inadequate airflow in a space allowing pathogens to build up.
What should I consider before making changes?
Altering an existing system without properly evaluating the impact could cause increased running costs and negatively impact comfort. Long term practicality of the solution should also be considered.
Considering energy efficiency is especially important considering the government climate change policies that will likely come into effect soon.
The systems above are effective against most pathogens, not just COVID-19, so overall they will improve the occupant and building health in the long term. Where high levels of outdoor air are introduced through mechanical system heat recovery should be considered to reduce running costs.
Conclusion
There are many options available to reduce the spread of pathogens like COVID-19. Each option reduces the likelihood of transmission to different degrees so the solution you choose has to consider the level of risk you are comfortable with as well as your budget.
Displacement ventilation systems that reduce air mixing, have sufficient airflow to quickly remove any pathogens from the occupied zone, and are fitted with effective filtration will provide your best means of reducing the risk of pathogen transmission.
If you are interested in finding out what options you have to install a new ventilation system or improve your current ventilation system to reduce the spread of pathogens like COVID-19 along with the associated costs get in touch today.
About the author
Anthony Broatch
Chartered Senior Mechanical & Hydraulic Engineer
BE(Mech), CEng, MCIBSE, MEngNZ
Reshape Consulting Ltd
I have had been involved in a large number of healthcare projects across New Zealand, the largest being the mead clinical centre in Waikato, a five-storey 32,000m2 clinical centre completed in 2014. My healthcare knowledge combined with my love of learning and enthusiasm for my work keeps me up to date with the latest industry practices and provides me with a solid base of understanding of HVAC systems that can be designed to control pathogens and reduce infection.
References:
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New Zealand Ministry of health. About COVID-19. Available from: https://www.health.govt.nz/our-work/diseases-and-conditions/covid-19-novel-coronavirus/covid-19-health-advice-public/about-covid-19
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Scientific Reports. Efficacy of masks and face coverings in controlling outward aerosol particle emission from expiratory activities. 24 September 2020. Available from: https://www.nature.com/articles/s41598-020-72798-7
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University of Otago. Throwing Open the Windows: The Need for Ventilation Improvements as Part of Covid-19 Outbreak Control in Aotearoa. 30 August 2021. Available from: https://blogs.otago.ac.nz/pubhealthexpert/throwing-open-the-windows-the-need-for-ventilation-improvements-as-part-of-covid-19-outbreak-control-in-aotearoa/
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Allen JG, Ibrahim AM. Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission. JAMA. 2021; 325(20): 2112-3. DOI: 10.1001/jama.2021.5053.
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The Lancet COVID-19 Commission Task Force on Safe Work, Safe School, and Safe Travel. Six Priority Areas. 2021. Available from: https://static1.squarespace.com/static/5ef3652ab722df11fcb2ba5d/t/60a3d6713c9af62b4c2037ff/1621350002802/Safe+Work%2C+Safe+School%2C+Safe+Travel+%28Feb+2021%29.pdf
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Wang CC, Prather KA, Sznitman J, et al. Airborne transmission of respiratory viruses. Science. 2021; 373(6558): eabd9149. DOI: 10.1126/science.abd9149.
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Light relief: Tackling the coronavirus with ultraviolet. July 2020. Available from: lighthttps://www.cibsejournal.com/technical/light-relief/
