Ventilation Mitigates Short-Range Transmission

There’s no basis for the common assumption that ventilation only helps with long-range transmission

Joey Fox

Published in

It’s Airborne

6 min readApr 10, 2024

Depictions of short-range and long-range airborne transmission. Source

Short-Range vs. Long-Range Transmission

The risk of infection from an airborne pathogen is related to the dose — the concentration of the pathogen you inhale over time. Higher concentrations lead to a higher dose and a higher risk of infection.

Airborne transmission can generally be categorized into short-range and long-range transmission. Short-range refers to standing within 1–2 m of a person while long-range refers to being further away.

Transmission at short-range is high risk because the initial exhaled air contains a higher concentration of infectious particles before it mixes into the space.

For both short-range and long-range transmission, the fundamental approach to mitigating airborne disease transmission remains consistent: minimize the risk of infection by decreasing the concentration of inhaled viruses.

For long-range transmission, first sufficient clean airflow needs to be supplied to the space, either through ventilation, filtration or UV disinfection. Then proper air distribution throughout the room needs to ensure that the clean air properly mixes with the dirty air and dilutes the pathogen concentration. This ensures that people are ultimately inhaling the clean air that is supplied to the space.

When discussing short-range transmission, we can categorize the air into two primary components: the concentrated cloud of air initially exhaled (I’ll call it the breath plume) and air within the room.

To mitigate short-range transmission, the same principles for mitigating long-range transmission can be applied:

ensure the breath plume mixes with air from the room
ensure the air in the room is clean

Ensuring Better Mixing

In short-range transmission, the breath plume is a small volume of space in the room that has a higher concentration of pollutants. People intuitively know how to deal with this issue.

What do people do when there’s a bad smell in their nose caused by a higher concentration of pollutants? They try to dilute it by mixing it with the surrounding air which has a lower concentration of those pollutants.

Another way to think of it is to imagine being outside on a windy day or being beside a smoker with a fan right next to you. The smoke plume will get dispersed rapidly, so there won’t be a higher concentration of pollutants.

Higher air speeds dilute the initial plume rapidly and consequently mitigate short-range transmission.

Creating Higher Air Speeds

The actual air speeds allowed in a space are based on ASHRAE Standard 55 — Thermal Environmental Conditions for Human Occupancy. The allowed speeds are shown in figure 5–4.

Allowed air speeds for spaces at different temperatures. clo refers to clothing level. 1 clo is typical winter clothing and 0.5 clo is typical summer clothing. The dark grey area is allowed airspeeds with occupant control. For example, ceiling fans with a speed controller. The light grey area is allowed airspeeds where occupants have no control of the air speed.

The general design for space temperature is between 21 °C to 24 °C and for air speed at 0.2 m/s or lower. Air speed that is too high feels drafty while air speed that is too low feels stuffy. To ensure better airflow throughout the room, spaces should be designed to provide maximum air speeds that do not create discomfort.

People generally do not perceive low air speeds, but even when not perceptible, increasing air speed can assist in mitigating airborne disease transmission. The exhaled plume will get diluted faster and the inhaled dose would be reduced.

As can be seen in the Figure 5–4 above, air speeds can be significantly increased if occupants have control over the air speed (the dark grey area). A good method to allow for this would be to include ceiling fans in the space with speed controls on the wall and proper training for the occupants.

Ceiling fans can increase airspeed in a room. Image by mrsiraphol on Freepik

Pedestal fans can also be used, but care needs to be taken to ensure problems are not created with direct currents. One option is to have it pointing slightly upwards. The CDC has a good resource on using fans (see FAQ 11).

While there is research demonstrating reduced short-range transmission outdoors with higher wind speeds, additional research is required to determine the effectiveness of increasing indoor air speeds in mitigating indoor transmission.

For my personal risk assessment, if I can feel a draft in a space, I assess the space as being at reduced risk for short-range transmission. For long-range transmission, I assess based on many other factors discussed here, especially CO2 concentrations.

Good IAQ Reduces Risk of Short-Range Transmission

Aside from increased air speeds mixing with and diluting the initial plume, good air quality reduces the risk of short-range transmission as discussed in this publication.

The premise is as follows: the exhaled plume mixes with the air in the room. As the plume travels further from the source, a larger portion becomes comprised of the room’s air.

The question is — how much does the concentration of virus in the room affect the virus concentration in the final plume? The answer is that it is significant.

Ensuring the air in the room is cleaner will ensure that the air in the plume is cleaner. This graph indicates how the risk varies based on distance and ventilation rate in the room:

https://pubmed.ncbi.nlm.nih.gov/34704625/

It is clear that mitigation against long-range transmission with adequate air cleaning also helps reduce the risk of short-range transmission.

Outdoor Transmission

One of the defining characteristics about airborne disease transmission is that it primarily occurs indoors. One long standing explanation for this was called the open air factor which stated that the chemical makeup of outdoor air lead to a faster inactivation of pathogens which reduced the risk of airborne transmission. There is still no direct evidence that this causes the reduced risk of infection outdoors.

Rather, all the factors previously discussed here assist in mitigating outdoor transmission. Because there are no ceilings or walls, air is not trapped and rebreathed, so there is very low risk of long-range transmission. Consequently, as the plume mixes with outdoor air, the virus concentration in the plume becomes low.

Furthermore, typical wind speeds are generally much higher outdoors than indoors, which causes the initial plume to be diluted quicker and reduce the risk of short-range transmission. Even with low wind speeds, cooler outdoor temperatures can cause warmer exhaled air to rise quickly out of the breathing zone.

Prevalence of Long-Range Transmission

Many public health authorities focused on the fact that most transmission occurred at short-range. An implication, as refuted in this post, is that ventilation would not be an effective tool against short-range transmission, so should not be a primary mitigation focus.

However, public health authorities primarily contact traced people that were at short-range and missed most cases of transmission. Studies have shown that “the long-range airborne transmission route is dominant”.

Conclusion

Providing good indoor air quality can mitigate both short-range and long-range airborne transmission. Understanding what factors contribute to short-range transmission can assist in designing spaces to mitigate short-range transmission and allow people to perform better risk assessments of spaces.