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Use of UV

Amongst all the reams of hastily compiled guidelines issued by various sectors about adapting ventilation systems to enable the return to work, in-duct UVC air purification gets the occasional mention. Whilst this is encouraging, in-duct UVC should not merely be seen as a quick fix to help make workplaces safer from Covid-19. It is much more than that. In-duct UVC is a long term investment in the workplace environment that pays back even in non-covid times by delivering lower absenteeism costs. In that context the supply of purified air, free of viable SARS-CoV-2 is a short term bonus.

The recent history of UVC in HVAC systems is interesting. Sick building syndrome was much discussed in recent decades with one of the contributing factors identified as centralised building ventilation systems, and in particular the microbial contamination that can accumulate in cooling coils and drip pans. This was a particular issue in the more southerly states of the USA where higher ambient humidity and temperatures are typical. Although the cooling coils would be protected by air filters, if the source of the contamination was the cooling coils then the filter stages being fitted upstream of the source, were not in a position to offer any benefit. UVC was widely marketed as a solution to this problem and by basking cooling coils in UVC light microbial growth could be prevented.

A number of academic studies were carried out to quantify the effectiveness of this approach and there seemed to be broad consensus that UVC coil cleaning was both effective at keeping coils free of microbial growth and that there was a significant reduction in workplace absenteeism. One such double blind study by Montreal Chest Institute (part of McGill University in Montreal) which was published by the Lancet, described how basking of cooling coils in UVC reduced bacteria on irradiated surfaces by 99% and for the building occupants the overall sickness rate was reduced by 20% and respiratory symptoms by 40%.

Whilst the effectiveness of UVC coil cleaners is well understood, it should be remembered that this effect derives from continuous exposure of surfaces, preventing microbial growth and the subsequent entrainment of viable microbial particles from those surfaces to the airflow. It does not follow that a UVC coil cleaner type fitting will also be equally effective at treating airborne bioaerosols. That 99% figure does not apply to treating airborne micro organisms.



As employers eagerly investigate ways to make building ventilation systems safer, the purification of ducted airflow using UVC has become topical. There is a danger that an eager market may believe that UVC is the ‘answer’. It is not as simple as that. A more realistic perspective is that UVC at the sufficient dose to suit the application can be part of the answer in some instances. UVC can treat airflow but will do nothing for potential sources of infection in the occupied spaces be it occupants or contaminated surfaces.

For deactivating airborne biological aerosols the equipment needs to deliver a far higher dose of UVC energy. The dynamics of a treating moving air require in-duct UVC equipment where the UVC intensity and the duration of exposure are designed to deliver a dose sufficient to deactivate the target micro organism to a high efficiency. For this reason UVC treatment of the airflow in the ducting is preferable to fitting equipment within the constricted space in the air handling unit.

The parameters of the potential in-duct UVC application are important because without knowing the airflow, the duct size, ideally the air temperature range and the ducting material, selecting an in-duct UVC product is little more than a guess and any performance claim is therefore fanciful. Specifying in-duct UVC equipment is not dissimilar to specifying thermal coils in that there is some number crunching to be done first.

Understanding the parameters and having identified the target micro organism, normally but not necessarily SARS-CoV-2, a suitable UVC in-duct appliance can be specified. One of the key reasons behind this approach is to be able to model what the actual efficiency of the fitting will be in that specific application. Testing in-situ of an in-duct UVC fitting for its effectiveness against SARS-CoV-2 is difficult and expensive so inevitably confidence in the effectiveness of a UVC solution comes via the mathematics behind the modelling program.

There is a huge body of data that has been accumulated over the years where, for a staggering number of micro organisms it is known detail the size of UVC dose required to deactivate each type. It means that a UVC fitting specified to treat SARS-Cov-2 to say 99.9% will also treat other airborne micro organisms to a greater or lesser degree depending on the basic dose needed and how the micro organism ranks compared to others. UVC does not discriminate between types – it deactivates all micro organisms to some degree.



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