Knowing the physicochemical properties of exhaled droplets and aerosol particles is a prerequisite for a detailed mechanistic understanding and effective prevention of the airborne transmission of infectious human diseases. This review provides a critical consideration and synthesis of scientific knowledge on the number concentrations, size distributions, composition, mixing state, and related properties of respiratory particles emitted upon breathing, speaking, singing, coughing, and sneezing. A parametrization of respiratory particle size distributions is derived and presented based on five log-normal modes related to different origins in the respiratory tract, which can be used to trace and localize the sources of infectious particles. This approach may support the medical treatment as well as the risk assessment for aerosol and droplet transmission of infectious diseases. It was applied to analyze which respiratory activities may drive the spread of specific pathogens, such as Mycobacterium tuberculosis, influenza viruses, and severe acute respiratory syndrome coronaviruses 2 (SARS-CoV-2). The results confirm the high relevance of vocalization for the transmission of SARS-CoV-2, as well as the usefulness of physical distancing, face masks, room ventilation, and air filtration as preventative measures against coronavirus disease 2019 and other airborne infectious diseases.

• Received 9 April 2021

DOI:https://doi.org/10.1103/RevModPhys.95.045001