Can physical distancing curb transmission of airborne aerosols?

Researchers from the University of Pennsylvania have found that the widely-recommended indoor distances of two metres might not be sufficient to curb transmission of airborne aerosols. 

In rooms with displacement ventilation, where fresh air continuously flows from the ground and pushes old air to an exhaust vent near the ceiling, aerosols travel farther and faster. 

This is the type of ventilation system found in most residential dwellings, and it can result in a seven-fold increase in viral aerosol concentrations in the human breathing zone compared to mixed-mode ventilation systems. 

Many commercial buildings use mixed-mode systems, which incorporate outside air to dilute the indoor air and lead to better air integration — and tempered aerosol concentrations, in line with the research. 

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“Our study results reveal that virus-laden particles from an infected person’s talking — without a mask — can quickly jaunt another person’s breathing zone within one minute, even with a distance of two metres,” said Donghyun Rim, professor of engineering science. 

“This pattern is particularly noticeable in rooms with insufficient airflow,” he added.

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The findings show that distance alone is insufficient to prevent human exposure to inhaled aerosols, and that other control techniques, such as masking and appropriate ventilation, will be used instead. 

The findings are published in the journal Sustainable Cities and Society and are available online. 

The researchers looked at three things: the amount and pace of air that is vented through an area, the indoor airflow pattern associated with various ventilation strategies, and the aerosol emission mode of breathing versus talking. 

“This is an example of one of the unexpected outcomes: 

The risk of contracting an airborne virus at home is likely to be significantly higher than in an office. 

“However, in home settings, running mechanical fans and standalone air purifiers can help lower the risk of illness.”  

Increasing ventilation and air mixing rates, according to Rim, can effectively minimise the transmission distance and potential build-up of inhaled aerosols, but ventilation and distance are not the same thing.