In early July, the World Health Organization acknowledged that the coronavirus might be airborne, or more specifically, spread by aerosols as opposed to just larger droplets. That move was the product of ongoing studies of droplet behavior and reports of people who gathered indoors and ended up sick.
That work has proven invaluable in our understanding of the coronavirus.
We know that the primary route of transmission appears to be through respiratory droplets (which can remain airborne). There are still lots of things to learn about the dynamics of those droplets themselves – how long they last, what conditions allow them to persist, and how long the virus itself may survive in the environment, even once the person breathing them out has left.
This week, two new studies tackled bits and pieces of this bigger story. A study released Tuesday in Physics of Fluids used a model to demonstrate that humidity and temperature both play key roles in how long droplets survive. A study released Tuesday in Nature Communications on guinea pigs suggests that tiny particles of aerosolized dust may help spread influenza, opening a line of inquiry for how SARS-CoV-2 can also spread.
The best way to truly prove that an element is really spreading the coronavirus involves a combination of aerobiology (how droplets behave, whether the virus can survive in certain environments) and epidemiology, which provides real-world evidence that a virus has spread. That’s how we arrived at the idea that loud talking might spread the coronavirus: Studies showed that talking could release thousands of small droplets that remain airborne. Case studies, like the rapid spread of Covid-19 through a choir in Washington, gave us proof that such spreads could happen.
When we look at newer studies on droplet or aerosol behavior, we're getting the first part of the picture, but missing the second. That said, these two studies point to two more ways they're digging deeper into how the coronavirus lingers and where it thrives.
Humidity and temperature – In the Physics of Fluids paper, a team at the University of Missouri created a mathematical model to predict the behavior of droplets in certain temperature and humidity conditions.
Held at a fixed temperature of about 72 degrees, the team found that intensely humid conditions (up to 95 percent) could extend the airborne lifetime of a 50-micron droplet by 23 times, suggesting that droplets are "extremely sensitive" to humidity.
That said, temperature also plays a key role. When it came to a 100-micron droplet (the larger end of droplet size), the authors found that in very humid air, colder temperatures actually shortened the droplet's airborne lifetime. In dry air, they found that the airborne lifetime of the droplet increased as temperatures dropped.
So far, the study suggests that the interaction between humidity and temperature may play a role in how coronavirus-laden droplets survive, though the study's lead author Bin Bin Wang, an assistant professor of civil and environmental engineering at the University of Missouri tells Inverse that it's too early to apply these findings the pandemic just yet.
"Please keep this in mind, we don't know how viruses are correlated to the droplets, and the dry-out of a droplet would affect the fate of the virus. We need to be quite careful in interpreting our findings," Wang cautions
That said, the relationship between humidity and Covid-19 is becoming clearer, at least when it comes to outdoor conditions. A study on humidity and coronavirus cases in Australia published August 18 in Transboundary and Emerging Diseases reports that decreases in humidity were linked to a greater spread of the coronavirus.
For example, they found that a decrease in humidity of about one percent (drier air) was linked to about a 7.7 percent increase in the number of coronavirus cases reported. That effect was most pronounced when humidity was above 79 in the early stage of the pandemic as cases grew and above 75 percent in the later stage as cases dropped off.
Karen Kormuth, an assistant professor of biology at Bethany College, who was not involved with the study says that the humidity questions are particularly relevant for indoor transmission of the coronavirus, where risks are higher. She points out that looking at humidity at around 95 percent, which increased droplet lifetime the most in the Physics of Fluids study, is "really at the higher end of the spectrum."
When she has measured the humidity of her own lab it tends to be lower in the winter, clocking in around the teens or twenties, and as high as about 50 percent in the summer.
"What I think we need to be thinking about more is the humidity inside a controlled indoor environment," she says.
On that front, she says we still have a "limited understanding" of how real droplets that are exhaled from human respiratory tracts behave.
Aerosolized dust – Studies focusing on droplet transmission hinge on the fact that the person spewing the droplets is still in the room. The study released in Nature Communications suggests that there's another avenue to explore: aerosolized fomites, or tiny dust particles. These may also contain samples of virus even without the presence of a breathing being.
Scientists at Univerity of California, Davis "painted" immune guinea pigs with the flu virus and put them into a dusty environment with a susceptible partner, taking care to reduce the risk of respiratory spread. Three of 12 susceptible guinea pigs ended up with influenza, which the team suggested came from the 1,000s of dust particles that the guinea pigs turned up while scampering about their cages.
In a follow-up experiment, they also found that a crumpled tissue, if rubbed for eight minutes, could produce 1,000s of small particles that contained flu virus.
At this point, the fomite-based transmission of coronavirus is not as much of a concern as droplet-based transmission. However, the study's lead author William Ristenpart, a professor at UC Davis, explains that aerosolized fomites are far different than traditional ones. Open questions remain as to whether Covid-19 can become aerosolized from other sources like tissues, contaminated clothing, or a dusty floor.
"We are not saying that people are barking up the wrong tree by focusing on respiratory droplets," says Ristenpart. "Our point is that there is a whole other tree that needs to be considered, for example, the possibility of aerosolized fomite transmission."
Ed Nardell is a professor in the environmental health department at Harvard's TH Chan School of public health. Nardell, who was not involved in the study, tells Inverse that this is "a very good aerobiology study."
At the same time it "doesn't nail it as evidence that this is happening in real life," Nardell says.
The CDC still advises that the cleaning of "visibly dirty surfaces" is still a best practice measure for the prevention of Covid-19. However, so far we've yet to come across examples of people getting sick through aerosolized dust particles.
"It hasn't been clearly established prior to our work that aerosolized fomites can transmit respiratory viruses, so it really hasn't been on the radar yet for Covid research," Ristenpart adds.
Nardell says this work is still useful to us, even though some scientists have argued that fomite-based transmission doesn't seem to be as risky as contact with droplets. "For months people were Cloroxing their grocery bags because they could have been a source of transmission and all of the sudden we're told not to worry about that," he says.
"It isn't a distraction," he continues. "It deserves to be worked out."
Airflow in enclosed environments – What remains true is that it's worth avoiding poorly ventilated indoor spaces. If you must be inside, good ventilation can make a huge difference, and wearing a mask is essential if you're with people outside of your pod.
Take a recent case study published in JAMA Network Open on how Covid-19 spread on an international flight between Germany and Israel in early March: There were 102 passengers on the plane, and 24 of those passengers were part of the same tourist group, which had come into contact with a Covid-19 positive hotel manager a week before the flight.
Seven people in that group eventually came down with Covid-19 due to prior exposure — only two additional passengers in this case study appeared to have gotten the virus during the 4 hour and 40-minute flight. The authors admit that, while it's possible there were cases they may have missed, the airflow in the cabin from the ceiling to the rear likely limited transmission. Had passengers been wearing masks, the authors speculate that risk may have plummeted even further.
Kormuth says that there are still some simple precautions we can all take as the science on droplet behavior and what kinds of vehicles host coronavirus continue to develop.
"Just increasing ventilation, as much as possible, even if it's just as simple as having a personal air purifier, turning on a fan to kind of mix up the air, or opening a window – that's really what's critical to reducing the risk," she says.