Face coverings and respiratory tract droplet dispersion

Abstract

Respiratory droplets are the primary transmission route for SARS-CoV-2, a principle which drives social distancing guidelines. Evidence suggests that virus transmission can be reduced by face coverings, but robust evidence for how mask usage might affect safe distancing parameters is lacking. Accordingly, we set out to quantify the effects of face coverings on respiratory tract droplet deposition. We tested an anatomically realistic manikin head which ejected fluorescent droplets of water and human volunteers, in speaking and coughing conditions without a face covering, or with a surgical mask or a single-layer cotton face covering. We quantified the number of droplets in flight using laser sheet illumination and UV-light for those that had landed at table height at up to 2 m. For human volunteers, expiratory droplets were caught on a microscope slide 5 cm from the mouth. Whether manikin or human, wearing a face covering decreased the number of projected droplets by less than 1000-fold. We estimated that a person standing 2 m from someone coughing without a mask is exposed to over 10 000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single-layer mask. Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies. However, the methodologies we employed mostly detect larger (non-aerosol) sized droplets. If the aerosol transmission is later determined to be a significant driver of infection, then our findings may overestimate the effectiveness of face coverings.

Keywords: COVID-19; face covering; handmade mask; respiratory droplets; social distancing; surgical mask.

Figure 1.

Laser imaging of respiratory droplets in flight. (a) Schematic diagram of the experimental set-up. Boxes indicate imaging windows. (b) Examples of images captured at position A (directly in front of the mouth) for speaking (i, ii, iii) and coughing (iv, v, vi), without mask (i, iv), with the surgical mask (ii, v) and with the handmade mask (iii, vi). (c,d) Droplet deposition rate over the table centreline versus the horizontal distance from the manikin's mouth in (c) speaking and (d) coughing conditions. Data were estimated from the count of imaged droplets crossing the lower edge of the field of view per unit time and are reported as the mean ±1 s.e.m. of six independent replicates. (c) Both mask types statistically significantly reduced the droplet deposition rate at 0.21 m from the mouth of the source (p = 0.016 in both cases). Past 0.21 m, too few droplets were detected also without mask to observe statistically significant differences among the tested conditions. (d) No significant differences were detected between mask types, but both mask types statistically significantly reduced the droplet deposition rate at every tested position (p = 0.019).

Figure 2.

Surface deposition of droplets. Droplet deposition rate versus horizontal distance from the manikin's mouth in (a,c) speaking or (b,d) coughing conditions, measured by (a,b) UV illumination of paper sheets or (c,d) microscopic imaging of glass slides placed on the table centre line. Data are the mean ±1 s.e.m. of six independent repeats. Statistical analyses indicated that for (a), both mask types significantly reduced the droplet deposition rate at up to 47.5 cm from the source (p = 0.047). For (b), droplet deposition rate with and without masks were not significantly different (p = 0.055). For (c), both mask types significantly reduced droplet deposition rate up to 0.5 m from the source (p = 0.012). Past that point, droplets were undetectable in all cases. For (d), both mask types significantly reduced droplet deposition rate at all distances from the source (p = 0.016). No statistically significant difference was observed between the two mask types in any test.

Figure 3.

Droplet deposition from human volunteers. Droplet deposition rate in human volunteers for the no-mask and surgical mask cases under (a) speech and (b) coughing conditions. Data represent the number of droplets imaged in widefield microscopy on a glass slides placed vertically at a distance of 5 cm from the human volunteer performing the task. To estimate the deposition rate, droplet counts were divided by the imaged area and duration of the speech or coughing task. Measurements are plotted as a box between the 25th and 75th percentiles with a mark indicating the median. The whiskers extend over the full data range. Statistical tests showed p = 2.3 × 10⁻⁶ for both cases, thus rejecting the null hypothesis that the two samples were drawn from the same distribution. Therefore, the use of surgical masks statistically significantly reduced droplet deposition rate.