Role of aerosols in transmission of Covid-19

As the global Covid-19 pandemic is battering the world, virologists and epidemiologists are relentlessly studying to understand how the deadly virus gets transmitted. Does Covid-19 transmit via expiratory particles? What role do aerosols play in transmission of Covid-19?

Aerosol is a collection of liquid or solid particles suspended in air. Anthropogenic aerosols which impact the human health and environment are in the size range varying from 0.001 micrometer to 25 micrometer.

Airborne transmission of Covid-19 may occur by two distinct modes. During a sneeze or a cough, droplet sprays or aerosols of respiratory tract fluid carrying the virus can impact directly on a nearby individual. Also as these aerosols are in the size range of less than 5 micrometer, they can remain airborne albeit for a few hours contaminating the local region. So any one entering that zone runs the risk of inhaling those aerosols. Even before the advent of Covid-19, many studies have been carried out on the behaviour of aerosols and its role in the assessment of inhalation risk. Some of the outcomes of these studies are interesting and noteworthy.

It is well known that asymptomatic individuals carrying the virus also cause infection. What is the mechanism involved? They do not cough or sneeze to any appreciable extent. Do they generate aerosols?

Previous studies have shown that both ordinary breathing and speech emit large quantities of sub-micrometer aerosols which are invisible. These aerosols when accompanied by the virus can infect a by stander.

There are two more interesting facts: Ordinary speech generates ten times more aerosols than normal breathing. Louder the speech, more the release of aerosols. It has also been reported that certain individuals by the manner they vocalise generate much larger quantities of aerosols. A ten-minute conversation with such an infected, asymptomatic speaker can release a cloud of about 6,000 aerosol particles. Therefore, it is worth advising the Covid-19 affected individuals, both symptomatic and asymptomatic to reduce their talk and more importantly they should wear their masks all the time. Clearly the assessment of transmission potential of Covid-19 virus through expiration is an interdisciplinary task involving virologists and aerosol scientists.

In combating the pandemic, facial masks have become ubiquitous weapons. How do these masks act as filters to collect aerosol particles? It is interesting to note that aerosol filters do not act like sieves.

Can you imagine that at a certain size range, smaller the particle size, easier the capture by the filter! To understand this, one has to understand the filtration mechanisms. There are three main processes by which the aerosol is captured by the filter: inertial impaction, interception and diffusion.

Larger particles having too much inertia due to size or mass cannot follow the air stream as they are diverted around a filter fiber and get captured. In the interception regime, as particles pass close to a filter fiber, they may be physically intercepted by the fiber. When the particles are very small, they are constantly bombarded by air molecules. Due to Brownian motion they deviate from the main stream.

In this diffusion regime, smaller the particles, more the deviation and more the probability of capture by the fiber. In all cases, once a particle comes in contact with a filter fiber, it is removed from the air stream and strongly held by molecular attractive forces. It is very difficult for such particles to be removed once they are collected. So, by simply tapping the mask you cannot remove the particles from the filter. By washing the filter, many of the bio-aerosols and soluble aerosols can be removed.

Clearly, Covid-19 pandemic has triggered more intense research studies worldwide on behaviour of bio-aerosols in the atmosphere as well as in human respiratory system.

(The author is former Director, Health & Safety, Department of Atomic Energy)