pre-symptomatic spread; shedding in the upper or entire respiratory tract( larger / more particles with more virus); larger particles( more virus); deposition in the lower respiratory tract( less virus needed); low environmental stress, humidity and temperature; high contagiousness, superspreader; early phase of disease; high susceptibility if the exposed individual; and close distance or direct contact. Factors disfavoring infection include low virulence of the pathogen; symptomatic spread; shedding in the lower respiratory tract only( alveoli); small particles; high environmental stress, humidity, temperature, UV, salinity, pH and airflow; low contagiousness; late phase of disease; low susceptibility of the exposed individual; and far distance.”
Zingg broached the concept of a continuum of distances.“ Interestingly, experiments show that while the dichotomy of droplets versus aerosols and the transmission roots mutually excluding each other as we thought in the past, is not supported by experiments, but there is now what we call a continuum, and there is a distinction between short distance, let’ s say conversation distance, and long distance. There is the projectile transmission of particles, so in a way, distance still matters. So, this idea that we have aerosols and distance does not matter, and we have droplets where distance matters, this dichotomy is basically wrong, and we see that it’ s much more complex. This does not mean that the idea of proximity playing a role is completely off the table. The other thing of interest is how we measure transmission. Most commonly, when we go to the literature, we measure clinical signs like influenza-like illness or infection, like with lab tests in both symptomatic and asymptomatic persons. But there are some caveats here; a physical endpoint does not predict exposure. A lab endpoint does not predict infection. A clinical endpoint does not explain physical transmission. And an epidemiological endpoint only allows assumptions of transmission. In addition, there is a dynamic parameter; the problem is that physicists rely more on experiments and clinicians rely more on clinical studies, so the approach is very different. We have to bring the two together.”
Zingg said the key question is, how do we mitigate transmission?“ We have pharmaceutical interventions and we have non-pharmaceutical interventions; the point is that non-pharma interventions basically target the physical component of transmission, thus physics cannot be ignored. So, we have to take them into account somehow, even if physics cannot explain, by themselves, infection. And usually we aim for mitigation; there is no absolute suppression of transmission. So, there’ s mitigation. There is a balance of measures we must take into account, but we can say the droplet / aerosol dichotomy to explain transmission and direct informed prevention is physically wrong, and we have to get over that. If there is a continuum and proximity still plays a role, why keep the old terminology? The document says at some point that the new term of‘ direct deposition’ is akin to the existing droplet transmission mode. So, in a way, there is an open door, so to speak. On the other hand, droplets and aerosols do not have the same meaning in different scientific disciplines. Respiratory particles and‘ transmission through the air’ now offer, in my opinion, a broader concept; however, the disciplines must better align their methodologies and concepts, how they measure things, how they
design an experiment, and how the effects are measured. I think it is justified to revisit terminology to allow the different scientific disciplines to collaborate constructively in the future, for innovation in the field of respiratory infections.”
To review, the WHO document uses the following common descriptors to characterize the transmission of pathogens through the air( under typical circumstances):
· Individuals infected with a respiratory pathogen can generate and expel infectious particles containing the pathogen, through their mouth or nose by breathing, talking, singing, spitting, coughing or sneezing. These particles should be described with the term“ infectious respiratory particles”( IRPs).
· IRPs exist on a continuous spectrum of sizes, and no single cut off points should be applied to distinguish smaller from larger particles. This facilitates moving away from the dichotomy of previously used terms“ aerosols”( generally smaller particles) and“ droplets”( generally larger particles).
“ Infectious respiratory particles( IRPs) are infectious particles being emitted by an infected person by breathing, talking, singing, spitting, coughing and sneezing,” Zingg explained.“ IRPs exist in a wide range of sizes( submicrons to millimeters) as a continuum( no cut-off point can be applied). IRPs are exhaled as a‘ puff cloud’( independently from an air current) before being dispersed and diluted further by background air movement. Travel of IRPs through the air is influenced by many environmental factors.”
According to the WHO document, the descriptor“ through the air” can be used in a general way to characterize an infectious disease where the main mode of transmission involves the pathogen travelling through the air or being suspended in the air. Under the umbrella of“ through the air transmission,” two descriptors can be used:
1. Airborne transmission or inhalation, for cases when IRPs are expelled into the air and inhaled by another person. Airborne transmission or inhalation can occur at a short or long distance from the infectious person and distance depends on various factors( airflow, humidity, temperature, ventilation, etc.). IRPs can theoretically enter the body at any point along the human respiratory tract, but preferred sites of entry may be pathogen-specific.
2. Direct deposition, for cases when IRPs are expelled into the air from an infectious person, and are then directly deposited on the exposed mouth, nose or eyes of another person nearby, then entering the human respiratory system and potentially causing infection.
Direct and indirect contact transmission is mentioned in the WHO document. Zingg mentioned that Pathogens that can be transmitted to another human via contact transmission( direct contact) and not via transmission through the air( such as via hands) or indirectly via touching secondary objects( fomites), or that enter the human body through routes( open wounds, sharps or needlestick injuries) or pathogens with an environmental reservoir with a predilection for the lungs are NOT covered in the WHO document.
There are some caveats and limitations, Zingg noted.“ There must be a clear understanding that when describing transmission of pathogens, this must work backwards from factors affecting infection risk,
Transmission in the strictest sense describes the physical part of how something gets from A to B. In the medical field, transmission has physical and biological components; physics describes only some aspects of pathogen transmission.”— Dr. Walter Zingg
nov-dec 2025 • www. healthcarehygienemagazine. com •
15