COVID-19: Airborne threat possible, especially indoor

After months of debate on whether COVID-19 can spread via airborne route, mounting evidence suggests that this is indeed the case — particularly in enclosed areas.   

A recent study showed that tiny respiratory droplets lingered in the air for a longer period in poorly ventilated spaces. Better ventilation substantially reduced the time that the droplets stayed airborne. [Lancet Respir Med 2020;8:658-659]

This, according to the researchers, holds important implications on requirements for use of face masks to curb viral transmission.

“This finding is relevant because typically poorly ventilated and populated spaces, like public transport and nursing homes, have been reported as sites of viral transmission despite preventive physical distancing,” the researchers pointed out.

Small droplets produced during speech and coughing can range from about 10 μm down to submicrons in diameter. Previous studies have shown that these tiny droplets transmitted from infected individuals contain viral particles, which can stay viable and infectious in aerosols for as long as 3 hours, explained the researchers.

“Aerosols containing a small concentration of virus in poorly ventilated spaces, combined with low humidity and high temperature, might result in an infectious dose over time,” they noted.

To address this, the researchers analysed droplet production and distribution using a laser diffraction measurement.

They found that the droplets produced by a healthy volunteer when coughing were of two distinct types: large droplets, which ranged from 100–1000 μm in diameter; and small droplets of 1–10 μm in diameter — with the small droplets being more common.

On the other hand, only the small droplets were found during speech.

They observed that the large droplets fell onto the ground rapidly — with the visibly large ones (around 500 μm in diameter) falling down rapidly within 1 second due to gravity. By contrast, the small droplets (~5 μm) took 9 minutes to reach the ground, when emitted at a typical adult height of 160 cm. 

“These small droplets are of specific interest because they have been associated with aerosol transmission of the SARS-CoV-2,” the researchers stated.

Key role of ventilation

To further investigate the distribution of the small droplets, the researchers used a customized spray nozzle to disperse a controlled amount of small droplets (average 5 μm in diameter) homogeneously into the air, thus reproducing the effect of coughing.

The droplets were dispersed in three rooms with different extent of ventilation: mechanical ventilation coupled with an open entrance door and a window, mechanical ventilation only, or no ventilation at all.

The number of droplets in the air was quickly halved within 30 seconds in the best ventilated room, while this took about 5 minutes in the room with no ventilation. In the poorly ventilated room, the time it took for the droplets to halve fell in the intermediate of 1.4 minute.

“The persistence of small respiratory droplets in such poorly ventilated spaces could contribute to the spread of SARS-CoV-2. Our findings confirm that improving ventilation of public spaces will dilute and clear out potentially infectious aerosols,” the researchers said.

“To suppress the spread of SARS-CoV-2 we believe healthcare authorities should consider the recommendation to avoid poorly ventilated public spaces as much as possible,” they urged.

The findings also keep the importance of using face mask amongst the general public to prevent virus transmission on the front burner.

“Transmission by aerosols of the small droplets studied here can only be prevented by use of high-performance face masks,” the researchers highlighted. “[While] a conventional surgical mask only stops 30 percent of the small aerosol droplets studied here for inhaled breath; for exhaled breath the efficacy is much better.”