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Chest clinic
Images in thorax
Face coverings and mask to minimise droplet dispersion and aerosolisation: a video case study
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  1. Prateek Bahl1,
  2. Shovon Bhattacharjee2,
  3. Charitha de Silva1,
  4. Abrar Ahmad Chughtai3,
  5. Con Doolan1,
  6. C Raina MacIntyre2
  1. 1 School of Mechanical and Manufacturing Engineering, UNSW, Sydney, NSW, Australia
  2. 2 Biosecurity Program, Kirby Institute, UNSW, Sydney, NSW, Australia
  3. 3 School of Public Health and Community Medicine, UNSW, Sydney, NSW, Australia
  1. Correspondence to Mr Prateek Bahl, School of Mechanical and Manufacturing Engineering, UNSW, Sydney, NSW 2052, Australia; prateek.bahl{at}protonmail.com; Professor C Raina MacIntyre, Biosecurity Program, The Kirby Institute, UNSW, Sydney, NSW 2052, Australia; rainam{at}protonmail.com

https://doi.org/10.1136/thoraxjnl-2020-215748

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    To evaluate the effectiveness of the Centers for Disease Control and Prevention (CDC) recommended one- and two-layer cloth face covering against a three-ply surgical mask, we challenged the cloth covering against speaking, coughing and sneezing. The one-layer covering was made using ‘quick cut T-shirt face covering (no-sew method)’ and the two-layer covering was prepared using the sew method prescribed by CDC.1 To provide visual evidence of the efficacy of face coverings we used a tailored LED lighting system (GS Vitec MultiLED PT) along with a high-speed camera (nac MEMRECAM HX-7s) to capture the light scattered by droplets and aerosols expelled during speaking, coughing and sneezing while wearing different types of masks (figure 1 and online supplementary video). The video for speaking was captured at 850 frames/s (fps) and for coughing and sneezing a frame rate of 1000 fps was used due to the higher expulsion speeds. Frames of the captured videos were pre-processed by applying a calibration and a two-axis stabilisation.2 We used a healthy volunteer without any respiratory infection and informed consent was obtained before conducting the experiments. The cough performed was voluntary and to induce a sneeze the subject used a tissue paper to stimulate the mucus membrane of the nasal cavity. The cloth coverings were made from 175 GSM cotton fabric with a thread count of 170 TPI and the surgical mask used was a three-ply Bao Thach face mask.

    Supplementary video

    Figure 1
    Figure 1

    Face coverings and face mask to minimise droplet dispersion and aerosolisation in three different scenarios. Detailed high-speed visualisations of all the scenarios are provided in the online supplementary video .

    Discussion

    From the captured video it can be observed that, for speaking, a single-layer cloth face covering reduced the droplet spread but a double-layer covering performed better. Even a single-layer face covering is better than no face covering. However, a double-layer cloth face covering was significantly better at reducing the droplet spread caused by coughing and sneezing. A surgical mask was the best among all the tested scenarios in preventing droplet spread from any respiratory emission. These visualisations show the value of using face masks and the difference between types of masks. Several other factors determine the efficacy of cloth masks such as type of material, the number of layers, the arrangement of different layers and frequency of washing. However, based on the visualisations presented, in case of shortages of surgical masks, a cloth face covering with at least two layers is preferable to a single-layer one. Guidelines on home-made cloth masks should stipulate multiple layers (at least 3). One study showed that a 12-layered cotton mask was as effective as a surgical mask, but a single-layered cloth mask was not protective against beta-coronaviruses.3 There is a need for more evidence to inform safer cloth mask design, and countries should ensure adequate manufacturing or procurement of surgical masks.

    References

    1. ↵
      1. Centers for Disease Control and Prevention (CDC)
      . Use of cloth face coverings to help slow the spread of COVID-19. Available: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html [Accessed 22 May 2020].
    2. ↵
      2. Bahl P ,
      3. de Silva CM ,
      4. Chughtai AA , et al
      . An experimental framework to capture the flow dynamics of droplets expelled by a sneeze. Exp Fluids 2020;61.doi:10.1007/s00348-020-03008-3
    3. ↵
      2. Chu DK ,
      3. Akl EA ,
      4. Duda S , et al
      . Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. Lancet 2020;395:1973–87.doi:10.1016/S0140-6736(20)31142-9 pmid:http://www.ncbi.nlm.nih.gov/pubmed/32497510
      OpenUrlCrossRefPubMed

    Footnotes

    • Contributors The manuscript was written through contributions of all the authors. All authors have approved the final version of the manuscript. PB conceptualised and designed the study, designed and conducted the experiments, analysed the data and drafted the manuscript. SB conducted the experiments and drafted the manuscript. CdS designed and supervised the experiments and revised the manuscript. AAC revised the manuscript. CD conceptualized and designed the study and revised the manuscript. CRM conceptualised and designed the study and revised the manuscript.

    • Funding This research was supported by NHMRC Centre for Research Excellence (Grant Number 1107393), Integrated Systems for Epidemic Response. CRM is supported by a NHMRC Principal Research Fellowship, grant number 1 137 582.

    • Competing interests None declared.

    • Patient consent for publication Obtained.

    • Provenance and peer review Not commissioned; internally peer reviewed.

    • Data availability statement All data relevant to the study are included in the article or uploaded as supplementary information