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Air filters on Addenbrooke’s wards removed nearly all traces of airborne Covid-19 virus, Cambridge University Hospitals study shows



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Research on the Covid wards of Addenbrooke’s Hospital has shown air filter machines are able to remove almost all traces of the airborne virus.

A study was carried out by doctors, scientists and engineers at the hospital and the University of Cambridge in January at the height of the second wave of the pandemic.

An intensive care Covid ward at CUH during the height of the pandemic second wave, January 2021. PIcture: CUH (53340115)
An intensive care Covid ward at CUH during the height of the pandemic second wave, January 2021. PIcture: CUH (53340115)

Air filtration machines were switched on for a week in Covid wards, then off for a week, then on for another week, and the air quality analysed.

With the machines on, almost all traces of airborne SARS-CoV-2 was removed.

Dr Vilas Navapurkar, who led the study and is a consultant in intensive care medicine at Cambridge University Hospitals NHS Foundation Trust (CUH), “Reducing airborne transmission of the coronavirus is extremely important for the safety of both patients and staff

“Effective PPE has made a huge difference, but anything we can do that could reduce the risk further is important.

“Because of the numbers of patients being admitted with Covid, hospitals have had to use wards not designed for managing respiratory infections.

An airfilter machine and Dr Vilas Navapurkar, who led the study into their effectiveness at Cambridge University Hospitals. Picture: CUH (53340113)
An airfilter machine and Dr Vilas Navapurkar, who led the study into their effectiveness at Cambridge University Hospitals. Picture: CUH (53340113)

“During an intensely busy time, we were able to pull together a team from across the hospital and university to test whether portable air filtration devices, which are relatively inexpensive, might remove airborne SARS-CoV-2 and make these wards safer.”

The team performed their study in a surge ward managing patients who required simple oxygen treatment or no respiratory support and in a surge intensive care unit (ICU) managing patients who required ventilation either through non-invasive mask ventilation or invasive respiratory support.

A number of high efficiency particulate air (HEPA) filters/UV sterilisers were deployed. They feature thousands of fibres knitted together to form a material that removes particles above a certain size.

From fixed positions, the filter the full volume of air in each room between five and 10 times per hour.

Prior to their use, the team found SARS-CoV-2 on all sampling days in the surge ward, but limited evidence of it on the ICU.

Once the machines were on in the surge ward, they found the virus on all sampling days on the first week and three of five days in the second.

In ICU, they found traces of the virus on one sampling day while the machine was active.

The air filters also significantly reduced levels of bacterial, fungal and other viral bioaerosols on the surge ward and the ICU ward, highlighting an added benefit.

Jane Olds, matron in adult critical care on a Covid ICU ward. Picture: CUH (53340117)
Jane Olds, matron in adult critical care on a Covid ICU ward. Picture: CUH (53340117)

First author on the study, Dr Andrew Conway Morris, from the Department of Medicine at the University of Cambridge, said: “We were really surprised by quite how effective air filters were at removing airborne SARS-CoV-2 on the wards.

“Although it was only a small study, it highlights their potential to improve the safety of wards, particularly in areas not designed for managing highly infectious diseases such as Covid-19.”

The team used PCR assays, similar to the ‘gold standard’ Covid tests to assess the air quality.

Prof Stephen Baker, from the Cambridge Institute of Therapeutic Immunology and Infectious Disease at the University of Cambridge, said: “Cleaner air will reduce the risk of airborne disease transmission, but it’s unlikely to be the case that just installing an air filter will be enough to guarantee the air is clean enough. Every room and every situation will be different. A key part of our work has been developing a robust way of measuring air quality.”

Dr Navapurkar added: “We’re all familiar with the idea of having standards for clean water and of hygiene standards for food. We need now to agree standards for what is acceptable air quality and how we meet and monitor those standards.”

The research was supported by Wellcome, the Medical Research Council and the National Institute for Health Research Cambridge Biomedical Research Centre.

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