As knowledge of how COVID-19 is transmitted changes, so too does the advice on how to avoid contracting it.  While in the beginning, the emphasis was on keeping hands and surfaces clean, doctors now recommend social distancing and mask-wearing.

The problem is that all of these precautions rely on the assumption that COVID is passed primarily through large droplets. Unfortunately, there is mounting evidence that airborne transmission may also be a significant factor.

What’s the difference between large droplet and airborne transmission?

At the beginning of the outbreak, most scientists believed that the virus was spread by large droplets of spit or mucus that people expelled when they coughed or sneezed. Those droplets, up to roughly a millimeter across, would quickly fall from the air onto surfaces, typically not more than six feet away. The advice to wash hands and surfaces and maintain a six-foot distance was based on this theory of transmission.

Airborne transmission would mean that the coronavirus is also being transmitted by aerosols. Aerosols are tiny bits of particles that can be generated by people talking, breathing, or singing. They’re much smaller than large droplets and have the ability to travel further and linger longer in the air.

There is increasing evidence that COVID is being passed both through large droplets and aerosols.

What is the evidence that COVID is airborne?

Donald Milton, a professor of environmental health at the University of Maryland who studies how viruses are transmitted, has helped lead a group of 239 scientists to research the potential airborne transmission of COVID. The scientists explained that multiple studies have shown that viruses released during breathing, talking, and coughing are small enough to remain in the air and infect people farther than 6 feet away.

“At typical indoor air velocities, a 5 nanometre droplet will travel tens of meters, much greater than the scale of a typical room, while settling from a height of 1.5 metres (about five feet) to the floor.” Milton said.

What does that mean for reopening the economy?

While it still isn’t clear how important droplet size is to transmission, the numbers seem to show that visiting indoor businesses such as restaurants and bars puts you at higher risk for contracting COVID. Milton and his colleagues agree that crowded bars where people are standing close and need to shout to hear one another provide the ideal setting for COVID to spread.

“A lot of people crowded close together indoors where it is poorly ventilated — that is what drives the pandemic,” Milton explained.

Is there any way to reduce the rate of airborne infection?

Milton and his colleagues wrote an open letter to request more education about airborne transmission.  In the letter, they listed several tactics public buildings and businesses could use to minimize the risk of infection:

  • Provide sufficient and effective ventilation (supply clean outdoor air, minimize recirculating air) particularly in public buildings, workplace environments, schools, hospitals and age care homes.
  • Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights. (These would be placed high up in the ceiling to avoid damage to people’s eyes and skin)
  • Avoid overcrowding, particularly in public transport and public buildings.

Milton also suggested simple things such as keeping windows and doors open in buildings and using outside air in cars to improve ventilation.

In order to test if indoor air is being properly refreshed, Milton recommended using carbon dioxide monitors. If the indoor air has no more than 1,000 parts per million carbon dioxide content, air circulation is generally adequate.

What can individuals do to protect themselves?

Perhaps the most helpful thing we as individuals can do is to avoid indoor, high-risk situations as much as possible. If you do need to be around other people, wearing masks helps block aerosols at their source before they spread out into the air. Recent studies have also shown that the combination of high-quality air purifiers and humidifiers can significantly reduce the transmission rate in your home or other indoor environments.