Respiratory virus shedding in exhaled breath and efficacy of face masks

This study was undertaken in an outpatient clinic of a private hospital in Hong Kong between March 2013 and May 2016, and has just recently been published in Nature Medicine. 

A total of 111 patients infected with human coronavirus (n=17), influenza virus (n=43) and/or rhinovirus (n=54) (three participants had coinfections) formed the focus of the study. They were asked to provide exhaled breath for 30 minutes.

Half the participants were randomly selected to wear a surgical face mask, which they were asked to attach themselves. Participants were instructed to breathe normally during collection and the number of coughs was recorded.

The study includes some really nice results and illustrative figures including virus loads in nasal and throat swabs, together with droplet (>5 µm) and aerosol particles (<5 µm). In particular, the authors argue that their results demonstrate the efficacy of surgical masks to reduce coronavirus detection and viral copies in large respiratory droplets and aerosols. This key result, which has been widely publicised, supports the use of surgical face masks by ill people to reduce onward transmission of COVID-19.

This has led to some changes in advice regarding the use of masks in public – but there are many practical considerations that need to be highlighted. Here’s a helpful Q & A on the use of masks in the public setting.

This is a helpful contribution regarding the value of infected people wearing masks in order to reduce virus shedding and protect others. The study does not however address whether wearing a mask will help protect you from acquiring an infection – this is a separate question altogether.


Leung, N.H.L., Chu, D.K.W., Shiu, E.Y.C. et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat Med (2020).

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