In today’s blog, we will be looking at controlling the spread of COVID -19 in mechanically ventilated buildings. More and more focus has been paid to airborne transmission as the pandemic has unfolded, and as evidence for it mounts, understanding it and taking steps to mitigate it has become a crucial part of any multi-occupant building’s COVID-secure strategy.
We’ll start with a quick recap on the routes of transmission that allow COVID-19 to spread from person to person, and the corresponding mitigation measures recommended by the relevant authorities. The first method of transmission is called the faecal-oral transmission route. Just as unpleasant as it sounds, this involves aerosols generated by flushing a used toilet carrying the virus from one person to another. Signs encouraging shutting toilet lids before flushing and using single-occupancy toilets where possible are the key mitigation measures, while toilet ventilation strategy should also be checked to ensure it runs 24/7.
Next, we have droplet transmission, which is the most common route of transmission, and it’s the one that’s got the most attention in the media and government policy. It can be broadly categorised into 3 groups, based on what happens to the droplets after they are released by the infected person: Indirect transmission involving a surface or “fomite”, where droplets land on a surface and are picked up by another person, then into their body when that person touches their face. The most effective way to mitigate indirect transmission is encouraging regular hand-washing, and disinfection common touchpoints regularly, in addition to the standard cleaning schedule.
Direct transmission where the virus is spread from direct contact between two people (ie a handshake) and is again best mitigated through encouraging regular hand washing, as well as behavioural changes such as fist/elbow bumps instead of handshakes.
For airborne droplet transmission, when droplets released through coughing, talking, singing etc land directly on another person, social distancing and mask-wearing are the recommended mitigation measures. Keeping 2 meters apart means most droplets won’t have the range to reach from person to person, and masks work to both reduce the number of droplets released from the infected person, and prevent droplets from infecting the other occupant.
And finally, we have the airborne transmission via aerosols or droplet nuclei. It differs from airborne droplet transmission in that while droplets tend to fall to the ground quickly within two meters, aerosols can remain airborne for hours, and circulate in poorly ventilated spaces over distances greatly exceeding two meters. At the start of the pandemic, there wasn’t much focus on airborne transmission, as it wasn’t thought to be significant and public health bodies didn’t want to distract from preventing droplet transmission. However as the pandemic has progressed airborne transmission has received more focus, and more evidence has come to light showing it can be a significant route of transmission.
Preventing airborne transmission is tricky, as aerosols are much smaller in comparison to droplets (<5 micrometres as opposed to 10+ micrometres). Therefore standard surgical masks are not sufficient, and while a fitted N95 mask is effective, these are in global short supply and should be reserved for healthcare settings. As a result, it falls to ventilation strategy to ensure that aerosols are not able to build up in a space to a concentration high enough to cause significant risk of infection.
To understand the importance of ventilation in preventing airborne transmission, let’s look at what the relevant authorities are saying. In September this year, the environmental modelling group (EMG) as part of the government’s scientific advisory group for emergencies (SAGE) said the following as part of their guidance on ventilation in the COVID-19 pandemic:
“Ventilation should be integral to the COVID-19 risk mitigation strategy for all multi-occupant public buildings and workplaces. This should include identification of how a space is ventilated and articulation of the strategy that is adopted to ensure the ventilation is adequate”
This is backed up in the CIBSE COVID-19 guidelines released last month:
And finally, the Global Heat Health Information Network left no room for ambiguity in their statement in collaboration with the WHO:
So it’s clear that the relevant authorities now recognise airborne transmission as a significant factor in the spread of COVID-19, and the onus is now on building owners and operators to ensure it is taken into consideration when making a building COVID-secure.
It’s also worth us taking a look at the risk factors that can exacerbate the risk of airborne transmission in multi-occupant buildings. Obviously, a lot of risk factors for COVID are personal, but some risk factors are controllable through occupant behaviour. The biggest of these is called the viral inoculum, which means the initial dose of the virus that a person is exposed to. While it hasn’t yet been shown to be the case COVID-19, due to obvious difficulties in collecting the data, it has been shown for influenza and SARS-1, and there is anecdotal evidence for it being the same for SARS-2. Therefore, the EMG advises that particular attention should be paid to environments favourable to the accumulation of airborne virus particles, as this accumulation increases the likelihood of a large viral inoculum. They list these as:
Hopefully, that’s given you a bit of a better understanding of the situation we’re in with regards to ventilation and COVID -19. We’ve heard how the authorities are recommending ventilation forms part of any multi-occupant building’s COVID mitigation strategy, but what should be done about it?
Number one is to undertake a comprehensive ventilation and controls survey to ensure your building complies with current COVID guidelines. This involves checking the overall ventilation strategy for the building against the current guidelines, reviewing controls strategy to avoid things like recirculation, and installing CO2 sensors in spaces to ensure they are receiving proper ventilation. This is number one because it’s easy and inexpensive, and it demonstrates to occupants that due diligence is being undertaken and that guidelines are being followed to ensure their safety.
Number two is educate building occupants. The EMG emphasise this in their statement by saying “The effectiveness of ventilation in many environments is strongly influenced by user behaviour (high confidence). Clear messaging is needed about the reasons why good ventilation is important and how to effectively operate ventilation systems or achieve good natural ventilation.”
Finally, those looking for the most comprehensive solution to indoor air quality should explore technological options to complement existing infection control measures. We’re going to take a look at 3 different air purification methods than can provide enhanced protection against bioaerosols and other forms of indoor air pollution.
The first of these is ultraviolet light or UVC. Ultraviolet Light works as a disinfectant by using high energy photons (light particles) that can pass through the cell membrane of micro-organisms and damage the genetic code, inactivating or killing them. It is commonly installed as UV bulbs in the air conditioning ductwork or the AHU itself. In terms of advantages, UVC is completely safe. It differs from UVA and UVB in the fact that it cannot penetrate the top layer of skin and therefore doesn’t cause cancer, but it’s still effective against most microorganisms. It’s worth mentioning that it is more effective on some than others, for example, coronaviruses are very susceptible to UV light, but fungal spores are not. It’s also relatively simple to set up and maintain.
However, UVC is ineffective against volatile organic compounds without a catalyst. VOC is a catch-all term for various airborne organic particles that can cause a wide range of issues, from cancer to bad odours. UVC light on its own isn’t particularly effective against VOCs unless it is combined with a catalyst such as titanium dioxide. A catalyst lowers the amount of energy required to break down the VOC’s to below what can be supplied by the UVC photons. This catalyst will require regular replacing, and there are concerns about toxic by-products such as ozone and secondary VOC’s that should be monitored for. This further increases the complexity of the system. Finally, UVC provides no additional benefit to the removal of particulates, and that is left down to the usual filtration methods.
Next, we look at ozone disinfection. Ozone is effectively O3 to oxygen’s O2, and it can diffuse through the cell walls of micro-organisms and damage the genetic material through redox reactions. It’s commonly used to disinfect unoccupied rooms in hospitals. While ozone is effective at removing VOC’s as well as bioaerosols, ozone is toxic to humans and is generally unsuitable for purifying the air in an occupied space. In certain situations where the ductwork is long enough, and with careful monitoring, it can be suitable, however, generally, it is not recommended.
Finally, we have air ionisation. It’s a solution we’re able to deliver thanks to our partnership with ionair®, who have been delivering air purification solutions through air ionisation since 1993. The top diagram shows how it works: a strong electric field removes electrons from various molecules in the air, creating ions which are electrically charged. The Ions have two effects, redox reactions that inactivate germs and VOCs, similarly to ozone disinfection, and agglomeration. This means that when fine particulates are given an electric charge by ionisation, they “clump together” kind of like iron filings in a magnetic field. This makes them fall out of the airstream onto surfaces and makes them easier to remove by filtration.
Also shown is a schematic for a typical ionair® solution. As you can see it shares some similarities with a BMS, with a central smart controller taking inputs from sensors, like ozone sensor, air quality sensor, to control outputs to an ionisation module. This ensures the strength of the magnetic field is optimised to the humidity and flow conditions to provide the best possible purification without generating a dangerous level of ozone. All sensor readings are also stored and logged for performance analytics, and the whole system can be integrated with a BMS via a variety of comms interfaces. It’s definitely a more involved solution, but it’s also the most comprehensive solution for indoor air quality.
So in summary, Air ionization is effective against VOCs, microorganisms and particulates, As well as being safe for use in occupied spaces, as long as ozone content is monitored. However, a properly designed air Ionization system requires expert commissioning and monitoring. That’s the trade-off for a fully comprehensive air quality solution.