Well-fitting N95 respirators are much more effective at blocking airborne viruses than surgical masks – HONG KONG BUZZ


Eminent Professor of Epidemiology Benjamin J. Cowling Photo: HKU

Anyone who listens to Backchat often knows Professor Benjamin J. Cowling. The Professor of Epidemiology at the University of Hong Kong is the go-to person for RTHK producers for pandemic talks. The professor comes across as prominent and up to date with the data. He speaks with an ascending cadence reminiscent of an antipode, which he is not.

BUZZ took a look at Ben’s scholarly papers, over 500 published. Most relevant now for readers struggling to understand a pandemic and aware that we are entering a dangerous influenza season is “Airborne Influenza Transmission: Implications for Control in Health Care and Community Settings”. It was written before Covid-19 turned the world upside down.

N95 respirators, when properly fitted, are much more effective at blocking airborne viruses than surgical masks. Citing studies by “Noti” and his colleagues, Ben writes that about two-thirds of viral particles were prevented from entering the mouth by surgical masks or N95 respirators that were not properly fitted. Up to 99% of viruses were excluded by properly fitted N95 respirators. Unfortunately, the article indicates that in the general community, proper fit testing of respirators is generally not feasible. It has also been noted that transocular infection can be significant, in which case eye protection is also necessary.

Large droplets travel only short distances before settling, but small aerosol particles quickly evaporate to form droplet nuclei and can remain airborne for long periods of time. The influenza virus can be spread by direct or indirect contact between individuals. Hand hygiene is thought to have some effect in reducing the transmission of influenza. The influenza virus can escape from the infectious person’s airways, survive the path from the infection to the infected, enter the infected person’s airways, invade host cells, and initiate infection. Some studies report that the influenza virus can be detected in large and small particles from exhaled air and coughing. The aerosolized virus may remain viable while passing through a room, depending on the air flow. (BUZZ previously reported an HKU study which showed that particles of the Covid virus can travel through the air up to 30 feet in a poorly ventilated room.)

Professor Ben Cowling is Head of the Division of Epidemiology and Biostatistics at the HKU School of Public Health. Above is a journalistic summary of his editorial in Clinical Infectious Diseases. You can find the entire article on https://academic.oup.com/cid/article/54/11/1578/322002


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