  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 article provides a critical review 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. We derive and present a
parameterization of respiratory particle size distributions based on five
lognormal 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
SARS-CoV-2 viruses. 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 preventive
measures against COVID-19 and other airborne infectious diseases.

Medical Physics