HVAC mitigation strategies for reducing the risk of airborne disease transmission in schools are a hot topic. Governments aim to safely return students into the classroom for the final term of the year.
The Victorian government has committed $190M to make upgrades and improvements to schools to slow the spread of COVID-19 and keep students safe, and already we are being asked by schools how these protect our kids and if there is more that they could be doing. Using the Victorian Governments Press Release for a ‘safe return to school’, I hope to demystify how these Heating, Ventilation and Air Conditioning (HVAC) measures are employed to improve spaces from the perspective of a mechanical engineer.
Disease mitigation strategies
A useful tool for managing risks is the hierarchy of control, which provides a systematic approach with six steps from most to least effective: elimination, substitution, engineering controls isolation, administrative controls, and personal protective equipment.
It is now widely accepted that SARS-CoV-2 (the virus that causes COVID-19) can be transmitted via the airborne route and minimised (or accelerated) via building HVAC systems. Therefore, engineering controls that utilize smart HVAC strategies to supplement additional controls offer a greater level of protection against airborne transmission. ASHRAE guidelines have literature on various effective mitigations on airborne infectious diseases in building HVAC systems.
We can target airborne transmission using ASHRAE’s five key strategies once we understand the best approach to your buildings and spaces. The government’s announcement budgeting for outdoor classrooms and purifiers and opening windows are doors each fit into key mechanical systems mitigation strategies, as explained below.
Increasing fresh airflow by opening windows and doors
Engineering Principle: Dilution
Dilution is the most straightforward mitigation strategy and involves changing the air within buildings with fresh air to dilute the virus concentrations within the space to acceptable levels. There is currently no research to inform the safe virus exposure concentration. The consensus amongst HVAC and health professionals alike is to increase the fresh air in the room as much as possible/practical.
Increasing outside air through an HVAC system requires additional cooling and heating energy, expanding mechanical services plants, and further pressurising the space (due to the extra air) requiring further relief/exhaust provisions. Careful HVAC system design is necessary to minimise energy consumption while balancing the additional air pressures in the space.
How it works
Increasing fresh air in space involves introducing greater outside air quantities through the HVAC system and or provision of openable windows to enable natural ventilation when the ambient weather permits (e.g. calm winds, no rain, and cool temperatures).
Previous advice from the government and HVAC experts to open windows and doors is a simple and effective way of reducing the risk of COVID-19 virions spreading.
Purchasing air purification (HEPA Filter) devices to remove potentially infectious particles
Engineering Principle: Capture
Experimental studies have shown that humans can generate respiratory microdroplets during normal breathing, speech, and coughing with sizes ranging from 1.6 microns to 150 microns. The smaller the microdroplets, the greater their ability to stay airborne; they can linger in the air for minutes or even hours. Science suggests that the coronavirus particles can cling to human-generated microdroplets leading to the risks of airborne transmission of the disease.
Correctly specified air filtration by the HVAC system provides an effective way of continually capturing and removing the COVID-19 virions from the air. Typical minimum air filtration required in the ventilation of buildings are of G4 rating. G4 rating or equivalent MERV 7 to 8 ( Minimum Efficiency Reporting Values, this rating is used predominantly in the US), when tested using Australian Standard AS1324.1, will capture on average 58% of 1-micron particles. Increasing filtration efficiency to F6 (or equivalent MERV 10 to 11) will enable filters to capture 82% of 1-micron particles. However, higher filtration efficiency comes with an energy penalty, which only increases as finer particles are filtered.
Understanding the sizes of the COVID-19 virions and the human-generated microdroplets that the virus adheres to is key to the filtration strategy. This will ensure optimal air filter selection to minimise energy consumption impact and provide an acceptable reduction in transmission risks to the occupants of the building.
Why it works
The Victorian government have announced that they are purchasing 51,000 Samsung purifiers, which can filter to .3 micron so that it can catch the microdroplets, and the coronavirus virions will be filtered from spaces, making them safer to occupy. We have extensive knowledge of particle filtration applications in schools and hospitals and can assist in the design, audit, and specification.
Outdoor classrooms through shade sail grants
Engineering Principle: Elimination
Elimination relates to extracting and expelling the internal air containing microdroplets contaminated with COVID-19 virions to the atmosphere without recirculating back into the room. The most effective elimination mitigation strategy in HVAC systems design includes 100% fresh air or ‘one-pass systems.
One-pass systems have higher energy consumption than recirculated systems having the minimum required fresh air. A carefully implemented energy-efficient strategy is essential in reducing the carbon emission footprint and mechanical services plant size.
Why it works
Outdoor classrooms are an excellent solution to get our kids back to school now. They are 100% fresh air spaces with an almost zero energy footprint. As we look to winter in 2022, we will have to consider how we adopt the elimination method in our HVAC systems for indoor spaces.
How do the other mitigation strategies fit in return to school?
The government has stated that they will be undertaking infrastructure audits and ventilation assessments to identify any further action which can make a school safer. Integral Group has been working with an Occupational Hygienist better to understand the risks of each schools’ spaces. You can learn more about that here.
Limit air movement between rooms
Engineering Principle: Containment
Air containment mitigations strategy is widely used in healthcare spaces. It limits the air from one room to be transferred into another room through the installation of airlocks.
Typical HVAC designs in buildings often use the relief air of an air-conditioned room to be transferred into the corridor/lobby spaces to provide some cooling/heating of these transient spaces to reduce energy consumption from using a dedicated air conditioning system. This represents the risks of exposing microdroplets from the source space to other areas within the building.
To achieve air containment and minimise airborne transmission risks, buildings require careful pressure regime design of relevant rooms. For example, having the potential source room in negative pressure relative to an adjacent space will ensure airborne microdroplets do not spill out to the adjoining room. Building self-contained rooms can have complex HVAC system components and controls, leading to high capital and maintenance costs.
Why it probably wouldn’t work in schools
New buildings built to contemporary sustainability standards have low air leakage. Using a containment mitigation strategy with either filtration or disinfection could be a possible solution in creating COVID-19 resistance spaces. Implementing this strategy wouldn’t work for a government rollout to all state schools. It needs to be assessed on a case-by-case basis and is subject to higher initial and maintenance costs than the mitigation strategies already discussed.
Virus Deactivation via UV light
Engineering Principle: Disinfection
The use of UV light has been scientifically proven, and its use in the disinfection of surfaces, room air and airstreams dates back to the early 1900s. According to ASHRAE guidelines, introducing ultraviolet (UV) light to the air stream containing the microdroplets long enough can assist in deactivating the virus, rendering them non-infectious to building occupants.
Why schools and government should consider it
Retrofitting UV disinfection lights onto new or existing air handling systems might not be as costly or difficult as first thought. Use of UV lights to deactivate virus-laden air in an HVAC system is established, including having UV lights at the cooling/heating coils or within ductwork/air mixing plenums. They offer low energy consumption compared to other strategies, and no harsh chemicals are used.
The challenges of using UV disinfection in HVAC systems include high space requirements, proper safety requirements, and access to the lighting equipment.
I, along with many of my colleagues are keen to get our kids back into the classroom. And the investments from the Victorian State Government are a great start in making that happen. Our experienced mechanical engineering team are working with occupational hygienists and continually researching and upskilling on the impacts of mechanical and HVAC systems in reducing the spread of COVID-19.
If you are interested in learning more about mechanical services mitigation strategies for your school or other facilities, please reach out to Thau Do here.