Dwell time: Do you know why it’s important?

Ahead of UK coronavirus restrictions lifting, many facilities managers are gearing up to return their building to ‘normal’ occupancy. However, this process comes with new safety concerns; primarily, how can we suppress and control the spread of COVID-19. Considering that there will inevitably be a section of the population that will remain vulnerable, this is a critical concern.

On top of this, there are also concerns around opening buildings that have been out of action for extended periods. Night club owners and managers, for example, will have to consider renovating HVAC systems that have been out of use. This is another key concern, as disused systems can be breeding grounds for bacteria, including Legionnaires disease.

However, as has been consistent throughout the pandemic, the one thing that is certain is uncertainty. The government keep moving the goalposts in regard to reopening, leaving many buildings managers left feeling frustrated. Perhaps the best thing you can do at this juncture is take control of what you can and give your HVAC system the upgrade it needs.

Investing in a UVC air purifier

UVC air purifiers are the component that HVAC systems need post-pandemic. This is particularly the case in high-occupancy or high-footfall public places, like bars, workplaces, or shopping centres. The reason is that UVC light has a unique effect on the molecular structure of viruses and bacteria. Unlike ‘traditional’ purifiers that only act on molecules like pollution, dust and mould, UVC stops disease in its tracks.

This is because the frequency of UVC light alters the molecular structure of microorganisms’ RNA. This deactivates the organisms’ ability to replicate themselves, and thus, survive. This neutralises the virus or bacteria, rendering them harmless. This is true for all viruses and bacteria, including SARS-CoV- 2.

Sounds good right? However, there are some important variables to be aware of when selecting your unit. Many air purifiers will come with UVC light claims, but their efficacy will depend on other variables. One of the most important is dwell time, which we’ll look at next.

Why dwell time is crucial

Certainly, UVC light purifiers are key to cleaner, healthier spaces. However, these claims need to be supported by other aspects of the unit’s operation – the most important being dwell time. Dwell time is, in short, the amount of time that the air is exposed to UVC light. To explain why dwell time is important, we’ll use the virus that causes COVID-19 as a model. 

To ‘kill’ or destroy coronavirus’ ability to reproduce, it needs to be exposed to UVC for a minimum of 0.675 seconds. Any shorter and the virus is still in action. To further complicate the issue, the UVC chamber also needs to have a 440mm space in order to ensure full exposure. This calculation assumes the lamp is fully operative. As a UVC lamp ages, the radiation will need longer to purify the air.

Striking the balance

So, you might ask, why not just maximise the amount of time the air is exposed to the UVC lamp? In reality, the equilibrium is more delicate; certainly, inactivating viruses is important, but time is of the essence, especially in high occupancy spaces. Deactivating microorganisms is important, but this needs to be done quickly enough to ensure the air in the space is safe.  

The key is to find the sweet spot where the air is exposed for long enough to destroy the virus but not so long that it doesn’t provide the space with enough clean air quickly enough (CADR). Rejuvenair units are designed to hit this sweet spot – with a handy app to monitor the health of the unit components. If you’re looking to upgrade your ventilation ahead of unlocking, Rejuvenair is the choice you can rely on.