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Biophysics

Air purifiers could spread viruses in confined spaces

22 Feb 2021 Isabelle Dumé
Modelling droplets inside a lift
Modelling the spatial distribution of saliva droplets inside a lift reveals that an air purifier may actually increase droplet spread, by inducing flow circulation that can add to the transport of contaminated droplets in the cabin. (Courtesy: Talib Dbouk and Dimitris Drikakis)

Air purifiers in lifts (elevators) and other confined spaces could actually increase the spread of airborne virus droplets, according to new calculations by two researchers in Cyprus. This surprising result means that health and safety authorities should revise current guidelines accordingly by considering how these droplets circulate and disperse, they say.

When people infected with the Cov-SARS-2 virus, responsible for the current COVID-19 pandemic, sneeze, talk or simply breathe, they expel thousands of contaminated saliva droplets. These droplets are now recognized as being the main way in which the virus spreads to other individuals and people are being advised to wear masks to curb contamination. Regularly airing indoor spaces to reduce the concentration of viral particles is also important.

Since the quality of air in a small space can quickly deteriorate even when there is just one person in that space (mainly because of the increased CO2 levels produced as we breathe out), lift manufacturers routinely include ventilator systems. These can, however, increase the rate at which air – possibly laden with bacteria and viruses – circulates. While adding an air purifier to the lift in addition to the ventilator should help in theory, the way in which the purifier affects air circulation, and thus transmission of airborne viruses is unclear.

Computational study

Air purifiers work by sucking in stale air and exhausting cleansed air, but this adds to the overall circulation of air – an aspect that previous studies, or indeed air purifier manufacturers themselves, have not considered.

To investigate this issue, Dimitris Drikakis and Talib Dbouk of the University of Nicosia analysed how air circulates in a three-dimensional volume of 27.28 m3 (x=1.24; y=1; z=2.2 m), which is equivalent to the space in a lift designed for five people. They equipped their virtual lift with a standard ventilation system and then installed an air purifier at a height of 1.9 m (z=1.9 m) in the lift. The purifier has an air intake (outlet) and an air exhaust (inlet) and circulates air at roughly 60 m3/h.

Using a (Eulerian–Lagrangian) multiphase model that they recently developed, the researchers simulated what happens when someone in the lift coughs lightly. They did this by ejecting 1000 “contaminated” saliva droplets (each with a mass of 20 mg) at a speed of 1 m/s from the person’s mouth, which they placed at two different positions (x=0; y= 0.41; z=1.6) and (x=0; y=0.67; z=1.6) in the lift. They then modelled the spatial distribution of saliva droplets inside the lift. They performed their computations at 20 °C, 50% relative humidity and atmospheric pressure.

The researchers found that the air purifier actually increases the spread of saliva droplets in the cabin. This is because the air intake integrated inside the purifier equipment induces flow circulation that can add to the transport of these droplets, explains Drikakis. They also found that the risk of airborne virus transmission is lowest for low ventilation rates. This is due to reduced flow mixing inside the lift, says Dbouk.

Revise guidelines?

Unsurprisingly, the risk of contamination increases with the number of infected individuals in the lift, add the researchers. Restricting the number of people allowed in such a small place could therefore minimize virus spread as would designing better air purifier and ventilation systems that reduce the flow of circulating air.

The results of the study, which is detailed in Physics of Fluids, could be applied to any confined space, they explain. These include small rooms, underground car parks, shops, aircraft cabins, submarines and spacecraft.

“The broader implications of the present findings are that health and safety authorities must revise guidelines accordingly by considering the flow circulation and droplet dispersion effects arising from air purifiers and sanitizers in confined spaces,” say Drikakis and Dbouk.

“For our part, we now plan to investigate the airborne transmission mechanisms for COVID-19 and other diseases,” they tell Physics World. “Indeed, a recent study we published regarding fluid mechanics and epidemiology shows how temperature, humidity and wind effects can predict the second wave of the pandemic.”

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