Understanding Fluid Dynamics In Buildings and How They Impact Infection Risk

It is now globally established that the most common transmission of viruses such as Covid and the flu is through the air.

The size of a Covid or Flu infectious aerosol is around 100 nanometres or around 0.01 microns – in other words very, very small.

Because of their miniscule size and the rules of fluid dynamics, these aerosols will follow, and be transported on the air flow within a building or space.

In this article we’re going to explore how HVAC systems can significantly impact the transmission of airborne contamination, and what measures can be taken to reduce infection spread in the workplace.

Now, the majority of corporate, healthcare and hospitality spaces have mechanical ventilation with supply and return air systems to create the air flows.

One of the main problems with many mechanical ventilation systems is that they have traditionally been designed for human comfort in terms of temperature of the air in a building, rather than for good ventilation or filtration.  These systems are designed to mix the air in a space efficiently,  but it is unfortunately true that many buildings have HVAC’s that only change the air in a room, less than 2 times per hour which is very low. 

Also, more recently there has been a drive also towards Energy efficient systems, which recycle the air in a space, meaning no constant supply of fresh air, so that carbon dioxide rises and airborne viruses can continually circulate in the same space. 

Recent studies based on Covid contact tracing have shown how in a restaurant setting that one infectious person was proven to have infected people as far as 50m away due to how these aerosols not only linger in the air but will follow the air flow pathways of the ventilation system.

With mechanical ventilation, the air flow pathways are crucial, and it’s very important to understand them. 

For example in some hospital settings – often the supply of air will be in a ward, or in a patient’s room, but the return air duct could be located in the corridor.  This means infectious aerosols can be transported from where they are being generated and back towards the return air duct, infecting any person that happens to cross their path.

In this example – you can massively reduce the likelihood of airborne viruses from escaping a zone by implementing in room air purification with HEPA filters.  These devices trap the infectious aerosols before they escape the room, and actually work very well alongside existing ventilation systems. 

This has been proven in many hospitals across the globe, including in the well publicised trial at the NHS Addenbrooks.

Another example can be seen in the hotel industry.  Where most bedrooms have the supply of air coming into the room, but the return or exhaust air duct in the bathroom.  In principle this should work well to contain airborne infection to within a room.  However, many exhaust systems in hotels are on one loop, which means you don’t actually have a fan in each room but one central fan that connects to several exhausts.  So this means if one person decides to turn up their ventilation to high, but another person has their ventilation on low, or even off, this means that the air in this room will have to find it’s way to an exhaust that has capacity as an imbalance has been created due to the rule of Physical Continuity.  It doesn’t matter if there’s doors in the way, the air will get out, and so will viral aerosols!   It’s also important to note that if an exhaust isn’t working or is blocked, this same principle will happen.

This has been proven in comprehensive studies over hotel quarantine when infectious guests were passing on the virus to security guards in corridors or even to other rooms within the same level. 

So what can be done?  Well, firstly, as we’ve said – you need to understand the air pathways in your building.  Make sure you know where the supply and return air ducts are.  Getting in a fluid dynamics specialist or airflow consultant is never a bad idea. 

Find a way to increase fresh air supply, as this will help lower carbon dioxide levels.

The difficult part is the filtration.  Most HVAC’s do not have sufficient filtration to trap Covid or Flu particles.  It’s also very expensive to upgrade HVAC filters, and if you upgrade the filters, you can actually significantly reduce the air flow in a building. 

That’s why many organizations globally are choosing to implement in-room air purifiers with HEPA filters.  Portable or fixed solutions linked into the building management system, these devices can hugely increase the number of air changes per hour in a given space, and constantly filter and trap Covid and Flu particles before they are spread far.

It is a far more cost-efficient way to improve indoor air quality than to re-invest in a completely new HVAC system.

In room air filtration has numerous other benefits in addition to massively reducing infection risk.  Because the filter traps the fine particulate matter that causes respiratory illnesses, or worsens symptoms for those who suffer from allergies or illnesses such as Asthma, we have had many clients noting the improved well being and performance of staff and patients, following implementation.

Thanks for reading this article  If you’d like to find out more, feel free to get in touch with one of our advisors for a free no obligation consultation to find out the best ways to prevent infection and improve your indoor air quality.