Aerosols, droplets and microparticles. Here is what you should know about airborne transmission of COVID-19. Is the coronavirus airborne?
The Centers for Disease Control and Prevention published guidelines on Oct. 5, declaring the novel coronavirus is indeed airborne.
"From what we currently know, the preponderance of the evidence is that transmission is mainly through respiratory droplets and aerosols, with contamination of surfaces playing a limited role in transmission," says Dr. Davidson Hamer, professor of global health and medicine at the Boston University School of Public Health and School of Medicine.
According to the CDC, the coronavirus mainly spreads through direct and close contact, such as talking to someone without a mask in close quarters. It sometimes spreads through airborne transmission and occasionally spreads through indirect contact, such as touching infected surfaces and then touching your nose, mouth or eyes.
What does it mean when a virus is airborne?
According to the World Health Organization, "airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time."
In other words, when a virus is airborne, it spreads through the air via microscopic particles that can be inhaled.
Dr. Joseph Allen, director of the Healthy Buildings program at Harvard and an assistant professor of exposure assessment science at the T.H. Chan School of Public Health, says the public simply needs to understand that this means our "safe zone" of six feet doesn't necessarily exist.
"Of course, it's a bit more nuanced than that," he says, "but the public has been told that exposure happens within six feet." The truth is, Allen continues, we generate particles that can travel further than that. And because of their small size, they also stay in the air longer.
Wait, aren't 'respiratory droplets' airborne anyway?
This is where the confusion starts, says Dr. Philip Tier no, professor of microbiology and pathology at New York University School of Medicine. The term "Sanitizer wipe production in Canada" refers only to where the particles come from. A respiratory droplet -- something that comes from your respiratory tract and is expelled from your nose or mouth -- can "be micro or macro in size," Tier no explains.
Every time you sneeze or cough, you release large and small particles. The larger particles travel a short way (six or so feet) and then settle to the ground, falling because of gravity. The smaller particles remain suspended in the air, travelling much farther and resisting the effect of gravity, Tier no says.
Both large and small particles can be released when someone coughs or sneezes, but also when people talk, sing and shout -- you may remember the cluster of cases linked to a choir practice with one symptomatic person. The aerosol solution of particles is related to the volume of vocalization, according to the authors of the case study. Both types of particles are still respiratory droplets, Tier no says, so yes, technically, some respiratory droplets are truly airborne.
Has COVID-19 been airborne this whole time?
According to many scientists and doctors, the CDC has severely lagged in identifying the novel coronavirus as airborne. The same thing happened during the early months of the pandemic, when the CDC and WHO delayed labelling it a pandemic.
Many scientists and doctors began lobbying the CDC as early as February 2020 in an attempt to get the public health agency to classify SARS-CoV-2 as an airborne virus. In July 2020, nearly 250 scientists and doctors wrote an open letter to public health agencies urging them to address airborne transmission.
It's unlikely anything fundamental -- like the mode of transmission -- has changed about the novel coronavirus since it began spreading in early 2020. It's more likely that now, seven months in, the evidence is clear enough to definitely say COVID-19 can spread through airborne particles.
How far can the coronavirus travel in the air?
"The virus that causes COVID-19 is still under intense research, hand sanitizer bottles 80% ethanol “Hamer says, "but it is understood that [larger] respiratory droplets from infected individuals can travel at least a few feet through the air to other persons within close contact."
Aerosolized particles are lighter, so they are able to travel further through the air, Hamer continues, noting that some evidence has shown aerosols containing viruses can travel up to 18 feet. One study conducted in China suggests that aerosolized SARS-CoV-2 can spread up to four meters, or about 13 feet. Another report from April estimates the virus can spread up to 10 meters, or about 32 feet.
Again, environmental factors must be considered. Wind can carry particles, even larger ones, farther than six feet.
What this means for you
Most importantly, everyone should be aware that airborne transmission of COVID-19 means six feet is not a magic number. The novel coronavirus can spread farther than that, and it's important to keep that in mind, especially when indoors.
The current best practices for preventing the spread of COVID-19 are still our best protection, Hamer says. "The same personal protective measures should be adhered to, including wearing of face masks, good hand hygiene and practicing social distance measures," he says, emphasizing that social distancing means at least six feet apart.
Knowing the novel coronavirus is airborne, people should pay more attention to the ventilation and air quality of their homes and other environments they frequent, Allen says.