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Air filters on wards remove almost all airborne Covid virus

Research carried out on Covid wards at Addenbrooke's Hospital suggests that air filter machines removed almost all traces of airborne Covid virus.

The study was led by a team of doctors, scientists and engineers at Addenbrooke’s and the University of Cambridge in January, at the height of the second wave of the pandemic.

Air filtration machines were placed in Covid wards and the air quality analysed with the machines switched on for a week, off for a week, and then on for another week. The team found that with the machines on, it removed almost all traces of airborne SARS-CoV-2.

An airfilter machine and Dr Vilas Navapurkar, who led the study

Reducing airborne transmission of the coronavirus is extremely important for the safety of both patients and staff

Dr Vilas Navapurkar, consultant in intensive care medicine at CUH

Dr Vilas Navapurkar led the study and is a consultant in intensive care medicine at Cambridge University Hospitals NHS Trust (CUH).

He said: “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. 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.”

Covid ICU
An intensive care Covid ward at CUH during the height of the pandemic second wave, January 2021

The team performed their study in two Covid wards in Addenbrooke’s.

One area was a surge ward managing patients who required simple oxygen treatment or no respiratory support; the second was a surge intensive care unit (ICU) managing patients who required ventilation either through non-invasive mask ventilation or invasive respiratory support.

The team installed a number of High Efficiency Particulate Air (HEPA) air filter/UV sterilisers. These filters are made up of thousands of fibres knitted together to form a material that removes particles above a certain size. The machines were placed in fixed positions and operated continuously for seven days, filtering the full volume of air in each room between five and ten times per hour.

In the surge ward, during the first week prior to the air filter being activated, the researchers were able to detect SARS-CoV-2 on all sampling days. Once the air filter was switched on and run continuously, the team were unable to detect SARS-CoV-2 on any of the five testing days. They then switched off the machine and repeated the sampling – once again, they were able to detect SARS-CoV-2 on three of the five sampling days.

On the ICU, the team found limited evidence of airborne SARS-CoV-2 in the weeks when the machine was switched off and traces of the virus on one sampling day when the machine was active.

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

ICU matron Jane Olds
Jane Olds, matron in adult critical care on a Covid ICU ward

We were really surprised by quite how effective air filters were at removing airborne SARS-CoV-2 on the wards

Dr Andrew Conway Morris, from the Department of Medicine at the University of Cambridge

First author on the study, Dr Andrew Conway Morris, said:

“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.”

Crucially, the research team developed a robust technique for assessing the quality of air, which involved placing air samplers at various points in the room and then testing the samples using PCR assays similar those used in the ‘gold standard’ Covid tests.

Professor 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.

Conway Morris, A, et al. The removal of airborne SARS-CoV-2 and other microbial bioaerosols by air filtration on COVID-19 surge units. Clin Inf Dis; 30 Oct 2021; DOI: 10.1093/cid/ciab933