Explained: How COVID-19 Causing SARS-CoV-2 Virus Transmits Through Air

Coronavirus transmission through touching contaminated surfaces is less of an issue, but largely airborne transmission means COVID-19 is even more potent.

The SARS-CoV-2 virus, which causes COVID-19, largely spreads via airborne transmission (direct and close contact by inhaling respiratory droplets expelled by an infected person), a study published in The Lancet medical journal said on April 15, 2021. Fomite transmission, through indirect contact such as touching contaminated objects or surfaces, was thus less of a concern. The nature of the recent findings has led some confusion over whether the virus transmits differently from the way it did earlier, and if there are implications for staying safe amid India's surging second wave.

Many also have questions over whether large or small respiratory droplets cause infection, and how long infectious droplets remain suspended in the air. To get answers, we spoke with Dr Manhar Dhanak, department chair, professor and director of SeaTech in the Department of Ocean and Mechanical Engineering at Florida Atlantic University, US, who has conducted several studies on the transmissibility of SARS-CoV-2 droplets and the kinds of masks that offer effective protection.

Edited excerpts:

Govindraj Ethiraj: What more have we learnt recently about how the SARS-CoV-2 virus is transmitted? Are there things we didn't know about earlier and therefore need to be more concerned about now?

Dr Manhar Dhanak: Initially, it was thought that transmission of SARS-CoV-2 was mainly through touch, i.e. if you touch some surfaces [touched by an infected person]. We were also able to show early that transmission can be airborne, through very small, aerosol-size droplets. These tiny droplets can travel very far before eventually falling to the ground, thus can carry the infection further away from an infected person. When a person coughs, they emit all sizes of droplets. The larger ones are heavier and drop very quickly to the ground, but these lighter ones linger in the air for a longer time and are the primary cause of transmission through the air. Because they are so small, even after somebody coughs and then moves away from that place, the droplets can still linger in the air for a while.

We have been studying the effect of masks on droplets. The latest development, that transmission through touching surfaces appears to be less of an issue than airborne transmission, makes SARS-CoV-2 more potent. But if you take care of it through wearing masks, keeping social distance, etc., that helps significantly.

GE: Is there an analogy you could use to explain how infection can happen with lighter droplets remaining suspended in the air?

MD: The surface area of smaller droplets is much larger, compared to their weight. Suppose a piece of paper is flying in the air. If it's small, it's going to remain in the air longer, because of air resistance. So small droplets stay in the air longer before falling to the ground. They eventually do fall, but this is also affected by breeze or any such disturbance in the air.

GE: The next question then is, is there any known time for how long these small infectious droplets could remain suspended in the air?

MD: In our studies, we found droplets can remain suspended in the air for 8-10 minutes if the air is still and there's no motion. But most of the time, the air isn't that still even within a room, if you have ventilation or air conditioning. Outside, there are breezes that will impact the motion. These are the factors that govern how droplets move and how long they stay suspended in the air. In a closed room, they can remain in the air for a significant amount of time. So if you're going to be in a closed environment where there isn't enough ventilation, try to be there for as short a time as you can. Staying three minutes in a closed room is much better than 30 minutes because the chances of infection increase if there is an infected person in the room. Over time, the droplets in the air accumulate.

The viral load is another factor, i.e. when somebody coughs, how much do you ingest or inhale? If the person who coughs is wearing a mask, there is still going to be a chance of droplets escaping, but the mask significantly reduces the viral load that is now in the air.

GE: It is possible, therefore, that if someone coughs in a still room, thus emitting these droplets, and then leaves the room, you could enter it 10 minutes later and still be exposed because the air is still?

MD: That is correct if there is no or very poor ventilation in a room.

GE: You also spoke about the impact of breeze and air-conditioners, but couldn't these also blow droplets in your direction?

MD: That is true. We did do some studies in that regard. If you are downwind of a cough, i.e. if the wind is blowing behind a person you are facing who then coughs, it will hasten the transmission of droplets towards you. So see which way the wind is blowing when you are walking around outside, and be mindful of that.

GE: Any new thoughts on what kind of masks to use, or how to use these for protection from COVID-19, from the results of your studies over the last year?

MD: The United States Food and Drug Administration has come up with a standard for masks. They look at three things to determine what constitutes a good mask. One is how much filtration the material provides, i.e. what particles and droplets etc. manage to pass through a mask you are wearing. Then, how well does the mask fit your face, because particles and droplets can still escape into the air from the edges of a mask one is wearing.

The third standard is breathability: You should be able to breathe well through the mask. Typically cloth masks work well in terms of breathability, as air particles are much smaller even than the droplets that carry the SARS-CoV-2 virus. Based on these factors, the FDA [the US Food and Drug Administration] approves different masks. We are also currently conducting tests that address all these three areas, using a method that we have developed in the past using a mannequin.

It's important that when people wear a mask, they cover their noses as well as their mouths. Just covering the mouth and not the nose is not acceptable.

GE: What about N95 masks, which are almost considered a gold standard?

MD: N95 masks are more available now but the availability could still be better. Typically, these masks are used by medical workers on the frontlines. N95 masks do filter well and do prevent droplets from escaping through the mask, which is the important thing. But what's also important is that they have to fit well on your face.

GE: The present confusion seems to have started because of the use of the word 'airborne'. Was 'airborne' always the right word to use in the context of SARS-CoV-2/ COVID-19 transmission, or is there some distinction between an airborne disease versus aerosol particle transmission?

MD: It wasn't well understood in the very beginning but COVID-19 is an airborne disease, which implies that it could be transmitted through the air between two people in a room, even with them staying separate and not touching at all.

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