The antimicrobial filters are shown to kill pathogens, such as Candida albicans, Escherichia coli, and MRSA in under 15 min and to destroy SARS-CoV-2 viral particles in under 30 s following contact with the filter.

In a study, published in the journal Scientific Reports, the antimicrobial treatment for air filters – coated with a chemical biocide called chlorhexidine digluconate (CHDG) – were rigorously tested and compared to commonly used standard ‘control’ filters in the laboratory, in industrial air condensing units, and in trial on-board trains operating on the UK’s railways.

Filters were tested for antiviral efficacy against SARS-CoV-2 (Wuhan strain). 104 infectious units (IU) were applied to the filter surfaces and incubated for periods up to 5 min. Following the incubation period, viral particles were lifted from the filter surface using cell culture media and incubated with Vero cells to determine the capacity of remaining virions to infect target cells. The treated filters demonstrated excellent virion inactivation rates with the number of infected cells being decreased from 80 ± 3% from the control filters to 9 ± 9% on the treated filters after 30 s exposure to the filters and reducing further to 0 infected cells after 1 min incubation on the treated filters. Some toxicity of the viral suspension on the Vero cells was also observed. The extract from the filters triggered no decrease in cell number or survival from any of the filters. The only samples which demonstrated a significant reduction in Vero cell numbers were those in which the SARS-CoV-2 virus was added directly to the cells.

The filters were installed for three months in matched pairs across carriages on the same train-line, before being removed and shipped for analysis with researchers counting colonies of bacteria remaining on them. The trial found no pathogens survived on the treated filter, even after three months on-board the train.

Further tests also found the treated filters are durable and are able to maintain their structure and filtration function over the lifetime of their use.

Dr. Felicity de Cogan

Royal Academy of Engineering Industry Fellow at the University of Birmingham

The COVID-19 pandemic has brought to the forefront of public consciousness the real need for new ways to control the spread of airborne respiratory pathogens. In crowded spaces, from offices to large indoor venues, shopping malls, and on public transport, there is an incredibly high potential for transmission of COVID-19 and other viruses such as flu. Most ventilation systems recycle air through the system, and the filters currently being used in these systems are not normally designed to prevent the spread of pathogens, only to block air particles. This means filters can actually act as a potential reservoir for harmful pathogens. We are excited that we have been able to develop a filter treatment which can kill bacteria, fungi and viruses – including SARS-CoV-2 - in seconds. This addresses a global un-met need and could help clean the air in enclosed spaces, helping to prevent the spread of respiratory disease.

Dr. de Cogan added that, while there have been other novel filters to ‘purify air’ – from high-efficiency particulate air filters used in aerospace cabins, to UV light, and silver nanoparticles added to filter mesh – these have fallen short as they either lack energy efficiency or speed in effectiveness and are not ideal for the majority of existing heating, ventilation and air conditioning systems which would require significant infrastructure upgrades to use them.

NitroPep Ltd is now further developing the filters to deliver them as a product on the market. The new technology is the latest stage of Dr. de Cogan’s ongoing research creating patented antimicrobial technologies, which has included the development of a coating for surfaces called NitroPep that is also effective against SARS-CoV-2.

The latest study comes after previous research carried out globally has found that the risk of developing COVID-19 increases with greater public transport use, while other former studies have shown higher rates of flu-like illnesses in people traveling on London’s underground, as well as a 6-fold increase in respiratory infection in people using a tram or bus.