Aerosol spread of Covid-19?

2020 - the first case of COVID-19 was reported in India. Since then a lot of research has been conducted on the nature of the virus, its variants and how it is spread. In the early stage, the mode of transmission of the Corona virus was believed to be through fomite or surface contact. Soon enough studies declared that though fomite transmission is possible, the bigger culprit is aerosol or droplet transmission.

What is fomite transmission?

Say a person sneezes in your vicinity. Droplets are expelled from the person’s mouth.

Heavier droplets fall closer to the person who expels them, increasing risk of infection at close contact. When these droplets fall on surfaces, they become fomites.

Surface or fomite transmission occurs when a virus with the capacity to infect is present on a surface that is touched, and then the hand touches the nose, mouth or eyes. Precautionary measures to prevent this are disinfecting surfaces and washing hands.

In March 2020, several employees in a call center in South Korea contracted Covid-19, indicating a pattern of aerosol spread based on their close seating arrangement. In the rest of the building only 3 other people were Covid positive which could be due to close interaction with those infected patients for extended time periods. It has been argued that had there been significant fomite transmission, many more people in the building would have tested positive, given that there were other shared spaces like elevators and lobbies.

What is aerosol transmission?

Aerosols can be exhaled when simply breathing, when coughing or sneezing, even speaking and singing. Expulsions during coughing, singing or even talking loudly reportedly increase the distance that these aerosols travel.

Several events - singing and others organised during this time, were later considered as superspreading events and provided clarity on transmission indoors and in enclosed spaces like halls, prisons and hospitals. This is mainly because of an exchange of aerosols among people due to lack of air circulation and filtration in such spaces.

Reportedly, a family of five, including an asymptomatic patient, ate in a well air-conditioned restaurant in Guangzhou, China on January 24th, 2020. Within two weeks, nine customers who dined at the restaurant that day were found Covid positive. While four were relatives of the index patient, five others caught the virus who were seated on two different tables.

So, what is the difference?

In the airborne terminology, the sizes of particles differ.

Humans produce respiratory droplets that range from 0.1 micrometre to 1,000 micrometres in size. Larger droplets can be anywhere between 5 and 1,000 micrometres in size, while what is generally known as aerosols are less than 5 micrometres. The SARS-CoV-2 virus is approximately 0.1 micrometre in size.

Expelled respiratory droplets that are heavier and immediately fall to the ground within a distance of 1-2 meters of the source increase the risk of infection at close contact.

Airborne transmission occurs when suspended respiratory particles like cough droplets are dispersed across the area by air currents.

Aerosol transmission of a virus typically involves the virus either floating free in the air, or latching on to other aerosols such as dust or pollutants, but mainly phlegm or water, which can stick with the virus particles before evaporating and floating to larger distances.

Several case studies have been published that highlight much higher transmission of COVID-19 through aerosols than compared to that through fomites.

This is also the reason why in June 2020, a study published in the peer-reviewed Environmental Protection concluded that people in polluted areas are more severely affected by Covid-19.

So what do we do?

While it is a great practice to sanitize surfaces and wash hands regularly, it is also important to wear masks and avoid crowded indoor spaces that aren’t well ventilated. The absence of ventilation causes the particles to stay suspended in the air for longer, thereby increasing chances of infection. With the fear of new variants of Covid and other similar flu always on the verge, keeping your indoor air clean is just as important as keeping the surfaces clean.