Movement and transmission of human exhaled droplets/droplet nuclei

Abstract

Most respiratory infectious diseases may be spread by human exhaled droplets and droplet nuclei. The once-in-a-century coronavirus disease 2019 (COVID-19) pandemic has shown that the airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cannot be ignored. Understanding the movement and transmission of human exhaled droplets and droplet nuclei is of great significance for exploring the basic transmission mechanisms of SARS-CoV-2 and for developing effective engineering control strategies for infectious diseases. In this paper, we summarize research results on transmission of human exhaled droplets and droplet nuclei, in combination with real-time studies focused on COVID-19. First, droplets exhaled by normal speech, coughing and sneezing range from 10–50, 73–100, and 80–340 µm in size, respectively, with small-sized droplets outnumbering large ones. Second, particle size has a significant impact on the movement and spread of droplets. Large droplets rapidly settle in the air and travel a short horizontal distance, with a small-scale infection range of the source, which is the premise of droplet transmission. Small droplets completely evaporate into droplet nuclei before settling to the ground and then travel a long distance with the airflow, which is the premise of airborne transmission (aerosol transmission). However, what constitutes “large droplets”, and how far can they move on earth? Answering these two questions is crucial for establishing the theory for the mechanism of droplet transmission. Therefore, we clarify the definition of the “critical size” and “critical distance” to distinguish between droplet transmission and airborne transmission, which vary with the composition and size of droplets, environmental factors (temperature, humidity, air velocity, etc.), and the form of air distribution. Different research results may have different critical size or critical distance. Subsequently, we discuss droplet evaporation and its influencing factors. The evaporation of droplets is significantly decided by the relative humidity (RH) of the ambient air and the initial size of the droplets. Droplet nuclei, contributing significantly to airborne transmission, usually have a particle size of less than 1 µm. Their diameters are about 0.25 to 0.5 times that of the initial droplets. Additionally, the droplets within critical size could completely evaporate into droplet nuclei in a low-RH air environment, or settle within a critical distance to cause droplet transmission in a high-RH environment. As a result, we consider the critical size of droplets to be about 30–110 µm, and the critical distance for droplet transmission and airborne transmission to be about 2.5 m. Finally, we collect some latest research on COVID-19. The size of the droplet nuclei containing SARS-CoV-2 is mainly distributed in two ranges: 0.25–0.5 and 0.5–1 µm. Taking the high momentum turbulence effect of sneezing or coughing into account, the virus can be transmitted up to 7–8 m away. These results are expected to be applied for understanding the opportunistic airborne transmission of COVID-19 and for the disease control and prevention in engineering and public places.