Exposure to expired infectious aerosols in proximity and distance (20/36)

Talk by Yuguo Li

Department of Mechanical Engineering / School of Public Health, The University of Hong Kong

General Observations

  • Transmission at close range seems to dominate, which explains why social distancing worked
  • Distant transsmission reported occasionally, often leading to super spreading events
  • Mass face masks have worked due to (incomplete) filtration or jet blockage.
  • Most infection occurred indoors. We studied 2-3 airborne outbreaks due to insufficient ventilation.

Major transmission routes:

  1. The expired air stream from a person is a 3-dimensional volume, between 1 and 2 meters in length. With close contact you have large droplets, short-range airborne, and immediate surfaces. At a distance you have airborne, dust resuspension, and distant fomites.
  2. Large droplets can deposit on the lips, eyes, or nostril mucosa of a person, but this only represents about 10% of your face. The other 90% is the immediate body surface, which is recognized, but not well studied. A droplet can contact somewhere else, or transmit via a hand shake, and then self-inoculation can occur when a person touches their eyes, nose, or mouth.
  3. Short range airborned transmission with expired fine droplets and droplet nuclei that are directly inhaled has only recently been recognized.

Students were observed touching their own hair, face, neck, and shoulders 10% of the time, mostly with their non-dominant hand.


In the exhaled air stream, larger droplets will settle, but the leading vortex may play a role in keeping small particle airborne.


Turbulence is also important. The red dot shown in the middle on the right hand is idealized and turbulence will result in it being more scattered.


What are the (large) droplet sizes that actually deposit on your face? A large enough droplet will deposit, but a smaller one is more likely to flow around your face. The threshold for this is 50-100 microns, not the 5 or 10 microns, which is sometimes stated. This threshold is based on the droplet Stokes numbers, which determines its deposition efficiency on a sphere.


With these numbers, you can calculate the amount of short range deposition that occurs from short range airborne transmission compared to short range droplet transmission. Short range airborne transmission is not only mechanistically dominant, but it is also 10x more important than droplet transmission even at a very close distance of about 1 ft.


However, this conclusion assumes viral concentration is constant across different particle sizes. If the smaller particles have a higher viral concentration, they could turn out to be even more important.

Short range airborne transmission has been ignored for a long time. We know that when two people in close proximity exhale a tracer gas, the concentration is much higher at close distances. At a distance of 1 to 2 meters away, you reach a plateau representing the average concentration in the room.

If you had 2 people sitting back-to-back there would be no exposure. Not being face to face can change exposure levels, and people also frequently move their head in different directions. So the actual dispersion is in multiple directions as opposed to the narrow jet model.

There are two modes of airborne transmission. At a close range of 0.5-2m, the air speed is higher, and 50 micron particles can be airborne. These large particles can have higher viral viability. At a distance, the particles evaporate and viral viability can be lower.

The exposure triangle:

  1. Environment

    • Ventilation rate (dilution)
    • Air flow (transport)
    • Thermal stratification (lack of mixing)
    • Turbulence (mixing)
    • Room air distribution design
  2. Bioaerosols

    • Size distribution at origin
    • Size changes (evaporation)
    • Changes in the flow
    • Source (respiratory, medical, fecal)
    • Release jet behavior
    • Voice loudness
  3. People

    • Proximity to source
    • Inhalation
    • Surface touch
    • Self-touch behavior
    • Personal protective equipment
    • Relative location to the source

We can model the probability of infection as:

P = 1 - e^\frac{-Iqp\delta T}{Q}, where

I = number of infectors

q = quanta produced by one infector (quanta/min)

p = pulmonary ventilation rate of each susceptible (m^3/min)

\delta T = duration of exposure (min)

Q = room ventilation rate (m^3/min)

The room air conditions can also play a large role since stratified layers of air allow the respiratory plume to travel farther. [our note: the Reflow Labs Airsafe was designed with this in mind to break up these layers, rather than delivering clean air randomly]


For the fomite surface transmission route we have scale-free logistic growth where one hand can touch multiple surfaces and each surface can be touched by multiple hands. It is important to note that hand hygiene and surface hygiene need to be used together, rather than just hand hygiene.

Should our transmission route concepts be more transfer-focused?

  • (Droplet) spray transmission - Not the large droplete, but their momentum, their spray.
  • (Air) flow transmission - Still air cannot carry anything, but the flow of air carries droplets arround. Of course, people can go to a room of still air with airborne virus, and inhale, but inhalation is also airflow.
  • (Surface) touch transmission - Fomites will not hurt you as long as you do not touch them. It is not just objects, but our skin that can also be contaminated.

Concluding remarks: Respiratory diseases that are usually considered non-airborne can turn into long-range airborne diseases in a poorly ventilated spaces. Airborne transmission exists at a continuum of distances with short-range airborne transmission dominating exposure, but more evidence is needed regarding the details of human behaviour in close range. Long-range airborne transmission can be significant with insufficient ventilation, and putting sufficient ventilation into place should reduce the social distancing that is needed. Contribution from immediate surfaces remain unknown. Fecal aerosols may transmit SARS-CoV-2 and drainage pipes can be efficient carriers.

Previous: Which size particles are infectious and for how long? CQ2 Panel (16/36)

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Yuguo Li is Chair Professor of Building Environment at Department of Mechanical Engineering and Honorary Professor of School of Public Health at the University of Hong Kong. His main research interests are on indoor air quality, city climate, and environment studies of infection. He currently serves as Editor-in-chief of Indoor Air. Since 2003, he has been studying the mechanisms of transmission by airborne and fomite routes for respiratory and enteric infection. Dr. Li received his PhD from the Royal Institute of Technology (Sweden) in Fluid Mechanics.

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