Busting myths on airborne transmission of SARS-CoV-2

With support from scientific studies and principles, researchers dispel several myths about the transmission of SARS-CoV-2.

The rapid spread of COVID-19 caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to significant research on the mechanisms of how the virus spreads, which is usually by airborne transmission.

However, there are several common beliefs associated with airborne transmission. In a new review article published in the Journal of Hospital Infection, a team of international researchers explores six commonly associated and generally accepted myths about viral transmission, using scientific studies to refute them.

Myth 1: "Aerosols are droplets with a diameter of 5 μm or less"

This originated from an old incorrect definition and was also recently used by the World Health Organization (WHO). Talking, coughing, and sneezing can generate respiratory droplets, whose sizes can be less than 1 μm to more than 100 μm. A large range of sizes of the droplets can remain suspended in the air. A definitive size cut-off for the particles cannot be established because their ability to remain suspended in air depends not only on their size but also on the speed at which they are expelled and how the air around them behaves.

Depending on the airflow, even particles greater than 5 μm can travel more than 1 or 2 meters, the distance within which the aerosols are thought to fall to the ground. So even large droplets can behave like traditional aerosols. Thus, a more appropriate threshold to differentiate between an aerosol, that which remains suspended in air, and a droplet, that which falls to the ground, would be 100 μm.

Myth 2: "All particles larger than 5 µm fall within 1-2 m of the source"

Because there are always slow air currents in the air indoors, exhaled particles, 5–10 μm in size, do not fall within 1–2 m of the source. The particle size must be around 50–100 μm to fall in this range. Sneezing or coughing can cause them to travel further. Small particles can travel further in the column of warm air from a person's body heat, people's movement, and convective airflows. These particles can thus travel more than 2 meters. In still air, a 50-μm particle will fall down in about 20 seconds from a height of 1.5 m. There will be turbulent airflows in busy hospitals, which will cause aerosols to remain suspended for longer and travel greater distances.

Myth 3: "If it's short-range, then it can't be airborne"

Airborne transmission is generally believed to be long-range, or more than about 1–2 meters. Infectious agents are more likely transmitted over short ranges. If you can smell the garlic or alcohol in another's breath, which usually happens at short distances, viruses can also be inhaled at this distance. Studies on the flu virus have shown the virus can be transmitted over conversational distances. Long-range transmission depends on airflow patterns, ventilation, amount of virions produced, and other factors. So long-range transmission can occur, even though the risk may be small.

Myth 4: "If the basic reproductive number, R0, isn't as large as for measles, then it can't be airborne"

The reproduction number is the number of people one person infects. The route of transmission is not relevant to this definition. The accuracy of R0 depends on the ability to identify all the secondary cases. For airborne viral diseases like chickenpox and measles, identifying cases can be done simply visually, leading to accurate R0 numbers. However, for COVID-19, R0 is more challenging to estimate, as so many cases are asymptomatic. Transmission may also occur before patients become symptomatic, and not all cases are equally contagious. There are also other organisms like hantavirus and Bacillus anthracis, which causes anthrax, which is not transmitted from person-to-person and has R0 = 0, but is considered airborne diseases.

Myth 5. "If it's airborne, then surgical masks (or cloth face coverings) won't work." "The virus is only 100 nm (0.1 μm) in size, so filters and masks won't work."

Studies have shown that surgical and cloth masks can limit exhaled particles and protect wearers from others' inhaling particles. Surgical masks can reduce virus transmission 67–75%. Homemade cloth masks can also reduce exposure to incoming particles by 50–75%. High-efficiency particle air (HEPA) filters do not act as simple sieves to filter out particles. They work by a combination of impact and interception, diffusion, and electrostatic forces. Exhaled viruses are always trapped in saliva or mucus droplets, which are larger than 0.5 μm, and these are what are trapped by masks and filters, not the virus alone.

Myth 6: "Unless it grows in tissue culture, it's not infectious"

Culturing viruses is not easy, as more than one virus is needed for initiating a successful culture. Sampling techniques suck viruses from air into a bubbling liquid virus culture medium. The high forces of these methods may damage the virus, which will prevent it from growing in a culture. Natural human inhalation and exhalation is much more gentle, causing no harm to viruses. Thus, not detecting viruses in air samples does not mean the absence of the live virus. Viral RNA is a better method of detecting live viruses.

Thus, the myths can be quickly dispelled based on physical, virological, and epidemiological principles of aerosol spread, accurately identifying secondary infections and using suitable infection control methods. Transmission prevention strategies should include not only personal protective equipment but also adequate ventilation and air filtration.

Journal reference:
Lakshmi Supriya

Written by

Lakshmi Supriya

Lakshmi Supriya got her BSc in Industrial Chemistry from IIT Kharagpur (India) and a Ph.D. in Polymer Science and Engineering from Virginia Tech (USA).

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