N95 respirators properly fitted far more effective at blocking airborne viruses than surgical masks

^{Eminent epidemiology professor Benjamin J. Cowling Photo: HKU}

Everyone who often listens to Backchat knows about Professor Benjamin J. Cowling. The professor of epidemiology at the University of Hong Kong is the RTHK producers go-to person for interviewing on the pandemic. The professor comes across as eminent and up to date with data. He speaks with an upward lilt suggestive of an antipodean, which he isn’t.

BUZZ took a look at Ben’s professorial papers, more than 500 published. Most relevant now to readers struggling to understand a pandemic and aware that we are entering the dangerous flu season is “Airborne Transmission of Influenza: Implications for Control in Healthcare and Community Settings”. It was written before Covid-19 upended the world.

N95 respirators when properly fitted are far more effective at blocking airborne viruses than surgical masks. Quoting studies by “Noti” and colleagues, Ben writes about two thirds of virus particles were blocked from entering the mouth by surgical masks or by N95 respirators that were not properly fitted. Up to 99 per cent of viruses were shut out by N95 respirators that were correctly fitted. Unfortunately the article says in the general community proper fit testing of respirators is usually not feasible. It was also noted that transocular infection may be important, in which case eye protection is necessary too.

Larger droplets only travel short distances before settling, but small aerosol particles rapidly evaporate to form droplet nuclei and can remain airborne for long periods. Influenza virus may spread through direct or indirect contact between individuals. Hand hygiene is thought to have some effect in reducing flu transmission. Flu virus can escape the respiratory tract of the infector, survive the journey from infector to infectee, enter the respiratory tract of the infectee, invade host cells and initiate infection. Some studies report flu virus can be detected in both large and small particles from exhaled breaths and coughs. Aerosolised virus can remain viable while travelling across a room, depending on the airflow rate. (BUZZ earlier reported an HKU study that showed Covid virus particles can travel airborne as much as 30ft in a poorly ventilated room.)

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