Clearing the air15 July 2021

Sanuvox Coilclean IL units basking cooling coils in UVC light

Poorly-ventilated indoor spaces have been highlighted as potential COVID infection hotspots by health authorities. With people heading back to offices as lockdown eases, what has been learned from hospitals that will ensure employee safety? By Tom Austin-Morgan

When COVID-19 erupted in March 2020, the first response by healthcare facilities was to intensify surface cleaning and ramp up sanitisation procedures. But as the pandemic continued, the science behind how COVID spreads was scrutinised, with a specific focus on HVAC systems. To manage a highly transmittable disease such as COVID-19, medical facilities have used multiple methods to increase air exchanges indoors.

One such approach is to convert a room (or an entire floor) into a ‘negative pressure space’. These feature mechanical ventilation, run by a building management system, which maintains lower air pressure within the room than that in the rooms or hallways outside it. Air naturally flows from higher pressure areas to lower pressure areas, thereby preventing contaminated air from the isolation room escaping outwards.

This high-tech solution has been utilised in some hospitals in the US. Computational fluid dynamics, particle-based modelling and real-life trials have proven the success of negative pressure spaces at controlling transmission between patients and staff.


A less high-tech and cheaper method is to install a HEPA filtration system into the HVAC system. This can affect air distribution, so it’s important that HVAC systems are designed with enough fan power at the air handling units (AHU) to support their use. Alternatively, HEPA filter racks can be installed that are only deployed in a pandemic situation, saving on operation and maintenance costs.

AHUs can be engineered to handle dual conditions for heating and cooling coils: minimum outside air intake for normal conditions, and 100% outside air for pandemic and emergency conditions.

High-intensity UV lights or other similar air treatment systems can also be used at the cooling coils to help keep them clear of biomass. They can also be installed in the ducts to kill virus particles in the airflow.

“This is not a new technology but, although it’s commonly used in humid countries in the Americas and Asia, it’s relatively uncommon in the UK,” says Bill Anderson, director of Puravent, a provider of UV air purification systems. “We were horribly unprepared for the COVID pandemic. Countries like Singapore learned lessons from SARS, for example, but the UK didn’t implement UVC treatment systems, despite the technology being proven and mature.”

However, the government’s SAGE (scientific advisory group for emergencies) committee says that the efficiency of UV in air purification is unproven, and there is a risk of oxidising agents being released into the air.

In response, Anderson says: “The efficacy of UV-C air and surface purification has a research history measured in decades. If only SAGE officials could appreciate that UV splits into different bands, which each have different characteristics. He continues: “They should know that mercury vapour lamps made with quartz casing emit a tight output of germicidal UV-C light at or near 254nm wavelength, which doesn’t create ozone. Broad-spectrum UV lamps, typically with glass casing, produce shorter wavelengths which create ozone. In small doses, ozone can be useful in certain applications; however, it is a dangerous gas and should be avoided in occupied spaces.”


One of the most challenging aspects of COVID-19 retrofit projects is getting the appropriate levels of medical-grade air and oxygen to support ventilators to treat the most critically-ill patients. For that reason, it’s important to identify future ventilator loads accurately. Installing larger pipes is one way to do this, allowing medical or surgical units to be easily upgraded to intensive- or critical-care units.

Keith Esdon, CEO, Bed-Air, a company that provides isolation units for individual hospital beds for immune compromised patients, argues that clearer standards are required to guide buyers for the NHS. Esdon, who also has 15 years working in air quality for a variety of companies, adds: “An NHS hospital group bought air cleaners that simply comprised a fan and UV lights – no filters – with a top speed of approximately 400m3/hour. They weren’t able to ask the manufacturers for evidence of testing because the website gave no data. Without filters, particles won’t be trapped. UV-C can kill microbiologically, but it needs a long time, proximity of exposure and intensity. The velocity would be around 1.2m/second, so air will pass by the UV in a third of a second, which won’t kill anything.”

In February, the British Council for Offices published a paper ( calling for UK offices to ensure adequate levels of ventilation ahead of a return to work. The paper highlights that while most offices that follow good practise, there are also many examples of poorly-ventilated offices that must be ‘addressed urgently’.

In March, the government published major new guidance, ‘Ventilation of indoor air spaces to stop the spread of coronavirus’ ( In addition to advising opening of windows and doors, it emphasises the importance of making sure mechanical ventilation works properly. Systems should be set to bring fresh air in, and not recirculate indoor air. It also recommends seeking advice from HVAC engineers to ensure ventilation systems meet current standards to make buildings COVID-safe.

Jenny Smith, head of marketing at ventilation supplier Vent-Axia, advises that businesses undergo a risk assessment to ensure their premises are adequately ventilated, check whether existing ventilation is installed and that it works. She adds: “Ventilating for longer and opting for ventilation with higher airflow volumes will also help reduce the risk.”

Also helpful is following HVAC maintenance guidelines, including checking that filters are clean and if they need replacement.

Bed-Air has recently diversified to provide portable air purification devices called Meeting-Air for higher-risk indoor spaces. Meeting-Air draws in air through its circular base and into a H14 HEPA filter, which provides 99.975% filtration, and ejects the purified air from the top of the device which then disperses around the room. It is based on the same technology used in the much larger Bed-Air product.


Government guidance recommends a minimum ventilation rate of 6-12 air changes per room per hour to prevent airborne transmission of pathogens in healthcare facilities. Anderson adds: “Six to twelve air changes is similar to what was normal when buildings were cleared of cigarette smoke. These powerful air cleaners disappeared when buildings went non-smoking, and targets were brought in to cover economy and heat conservation.”

As more people receive their vaccinations and return to shared spaces, the window of opportunity to fix poor indoor air quality is closing. Smith says: “Now is the time for businesses to ensure there is enough airflow to dilute the virus in the air and improve indoor air quality.”

Tom Austin-Morgan

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