The abundance of speech-generated droplets and the role of masks in curbing airborne transmission of disease

Laser light scattering images have shown that droplets emitted during speech are abundant¹, can linger in the air for extended periods², and can contribute to airborne transmission of disease³. How long a droplet lingers in the air is inversely proportional to the square of its size, which shrinks by roughly a factor of three due to evaporation of its water content. Airborne transmission involves particle deposition along a circuitous path leading from the upper respiratory tract (URT) to alveoli in the lower respiratory tract (LRT)⁴, with deep penetration limited to aerosol particles smaller than around 5 microns. Since the number of virions that can be harbored in a particle is proportional to its volume, infections are far less likely to occur in the LRT than the URT. However, the consequences of infection originating in the LRT are typically far more severe and can lead to hospitalization and death. Since ordinary face masks capture larger, wet droplets more efficiently than smaller, dry particles, universal wearing of masks while in densely populated public spaces can reduce significantly the volume of aerosolized speech droplets emitted by loquacious asymptomatic carriers, and hence the risk of airborne transmission of this virus. Ongoing investigations of the number and size distribution of droplets emitted during speech aim to assess their fate as well as the relative effectiveness of mitigation strategies.