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expert reaction to video case study on homemade face coverings making comments about how many layers they need to curb COVID-19 spread

A case study, published in Thorax, made comments on homemade face coverings and how many layers they need to curb COVID-19 spread.


Dr Bryan Bzdek, Research Fellow at Bristol Aerosol Research Centre, University of Bristol, said:

“This study examines the efficacy of different face masks, comparing homemade one-layer and two-layer masks to a surgical mask and a control where the subject wore no mask.

“Only one subject was studied and no quantitative results were presented, so this study provides only a qualitative examination, limiting its broader usefulness.  For example, the droplet size distribution is not discussed, nor any discussion of the fraction of droplets that are filtered out by the masks.  Consequently, one cannot draw any definitive conclusions from the paper.  There tends to be a fairly wide variability across people in terms of droplet emission, which is missed by examining just one person.

“The results suggest that face masks can help to reduce droplet transmission, though this is not quantitatively demonstrated.  The authors’ experimental approach can only examine large droplets, which are most likely to sediment within 1-2 m of the emitter.  The approach cannot examine much smaller aerosol droplets, which are unlikely to be filtered out by these masks, as these smaller aerosol droplets will follow the path of exhaled breath.

“Although the images indicate that face masks do reduce droplet emission, it is clearly not eliminated, emphasising that face masks are not a replacement for physical distancing measures.  We should still maintain physical distance whenever possible, even when wearing a face mask.”



‘Face coverings and mask to minimise droplet dispersion and aerosolisation: a video case study’ by Prateek Bahl et al. will be published in Thorax at 23:30 UK time on Thursday 23 July 2020, which is also when the embargo will lift.

DOI: 10.1136/thoraxjnl-2020-215748


Declared interests

None received.


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