For the past few years, up until the pandemic hit, Bill Ristenpart, a chemical engineer at UC Davis (and next-level coffee geek), had been bringing a team of researchers and crates of expensive instruments across the country each summer to New York City and into the lab of Nicole Bouvier. An infectious disease physician and researcher at Mount Sinai Hospital, Bouvier studies respiratory viruses, influenza A in particular. Ristenpart’s specialty is fluid dynamics. In the case of flu, that means measuring how physical properties like temperature, humidity, and wind speed change the flight of the respiratory bloblets that fly out of human and rodent noses and mouths. Together, with dozens of guinea pigs and nearly $2 million from the National Institutes of Health, they hoped to figure out a century-old mystery: Why is there a flu season?
Here’s all the WIRED coverage in one place, from how to keep your children entertained to how this outbreak is affecting the economy.
That, they still don’t know. Instead, their work has turned up compelling evidence that some respiratory viruses, at least in lab animals, don’t always travel through liquid droplets, the way scientists have long assumed. Infected guinea pigs don’t just breathe or sneeze out bits of influenza. They can actually launch infectious particles into the air from their fur, paws, and cages.
Remember “fomites,” those germ deposits on surfaces that led to so much hand-washing and hand-wringing over face-touching during the early days of the pandemic? Well, sometimes, rather than settling on large objects like tables and cell phones, germs stick to the surfaces of solids that are so tiny you can’t even see them, like microscopic fibers, dead skin cells, and dust. Those minuscule solids can later get kicked up into the air. When they do, Bouvier and Ristenpart call them “aerosolized fomites.” And according to their research, these germy particles can make other animals sick. In fact, in their latest study, aerosolized fomites appeared to be the primary way their guinea pigs passed around the flu.
“Our experiments very clearly show that when guinea pigs move around they stir up dust. And if that dust is contaminated with virus, then it can transmit that virus through the air to another animal in a separate cage,” says Ristenpart. Their work also raises the possibility that this fourth route of transmission—aerosolized fomites—could potentially matter for human health as well, he says. Especially during a global outbreak of a new respiratory virus. “When you rub your face or brush your shirt or crumple a piece of tissue paper, you’re aerosolizing micron-scale particulates,” says Ristenpart. “And if that surface had been previously contacted by virus-containing mucus, then you’re also aerosolizing virus that other people can inhale.”
The UC Davis/Mount Sinai team published these assumption-shaking findings Tuesday in the journal Nature Communications. Though the experiments were conducted pre-pandemic, and with an influenza virus, their results now land in the middle of a heated dispute about how the novel coronavirus SARS-CoV-2 is transmitted. At the heart of the controversy is disagreement over the size, behavior, and relative importance of the droplets that infected people emit from their respiratory tracts—specifically, whether those expiratory particles can travel long distances and stay airborne for long periods of time. Now, this study adds a new wrinkle. What about viral particles released into the air through other routes—kicked up from the ground, shaken out from a bedspread, crinkled off a dirty tissue? How much do people need to care about that?
The answer—for now at least—is probably more than a little, says Richard Corsi, dean of engineering and computer science at Portland State University, who was not involved in the study. Now an administrator, Corsi spent decades studying the quality of indoor air. He observed that people are constantly modifying their environments with their movements, both by shedding skin and fiber from clothing and unsettling clouds of particles from the floor. Some scientists have even been able to measure the unique microbes that live in these personal aerosol clouds. So he’s not surprised that viruses might be able to hitch a ride in the same way that other microbes do. “I think this paper highly suggests that we shouldn’t assume away the pathway of resuspension of fomites from surfaces,” says Corsi. “It doesn’t mean that it’s the most important transmission pathway. But it’s a pathway.”
That’s true for influenza, and it could also be true for SARS-CoV-2. Corsi points to a study conducted by Chinese researchers in February and March inside two Wuhan hospitals during the height of the initial Covid-19 outbreak. They collected aerosol samples from a variety of locations within the hospital and found that levels of airborne coronavirus were highest in rooms where medical staff changed out of their protective apparel. The authors hypothesized that the source of these viral aerosols—which were very small, smaller than a single micron—was from the disruption of removing contaminated clothing.
That was a reasonable assumption, says Jeffrey Siegel, a mechanical engineer and sustainable buildings researcher at the University of Toronto, who was not involved in either study. “Putting on and taking off clothing generates a lot of particulates,” he says. But crucially, the Wuhan study only measured the amount of viral RNA in the air and on surfaces. It couldn’t say whether those viral particles were capable of making people sick. But the UC Davis/Mount Sinai paper—while it’s specific to the flu virus and not SARS-CoV-2—suggests that infectivity can be preserved through the process of resuspending contaminated particles, says Siegel. “And if that is the case, it opens up a whole new range of things that we should be concerned about,” he says.
We’ll get to those in a minute. But first, guinea pig burritos.
That’s what the UC Davis/Mount Sinai researchers took to calling a contraption they rigged up to measure the respiratory droplets coming out of a guinea pig’s mouth and nose. Their process involved anesthetizing a few of the animals and wrapping up the conked-out caviidae in airtight aluminum foil sleeves into which a small aperture for each one’s nose and mouth had been cut. That opening led into a particle-sizing instrument that could detect tiny solids from 20 microns down to half a micron.
The researchers went to such measures because early on, when they started their experiments by shoving flu germs up the noses of guinea pigs and monitoring how long it took for an animal in an adjacent cage to get sick, they noticed something strange. Rather than a steady stream of particles flowing from the infectious guinea pig to the healthy one, there were big spikes of particle production—thousands of particles per second. When they installed a camera above the cage, they noticed that every time the guinea pig moved around, the particle sizer captured one of these poofs. “That suggested what was making the animals sick wasn’t respiratory in origin, but was dander or dust,” says Bouvier.
Like toddlers, guinea pigs often rub their noses and then touch their surroundings. When the researchers swabbed down the cages and the animals’ ears, paws, and fur, the virus was everywhere. In order to isolate just the droplets coming of the animals’ noses and mouths, they needed to eliminate any movement. But you try making a guinea pig sit still. Hence, the burritos.
With the animals passed out in their aluminum sleeves, the researchers observed far fewer particles being emitted—just one every few seconds. And for a brief while, they were excited that they had captured the true amount of respiratory droplets. Then they ran what was supposed to be a negative control. That’s a sanitary way of saying they euthanized a few guinea pigs, wrapped them up in an identical fashion, and ran the experiment again. Then they stared in disbelief as the particle sizer recorded just as many particles flowing from the cage. They repeated the process again, this time with an empty sleeve. It detected nothing. “The particles were definitely coming from the animal even when it wasn’t breathing,” says Bouvier. Taken together, the experiments revealed it was the contaminated dust that was really transmitting the virus, and it could be spontaneously suspended from the animal’s body even when it wasn’t alive. “That was just very surprising,” says Bouvier.
The researchers followed that up with another experiment. This time, they infected a guinea pig and allowed it to recover, so it was immune to that strain of influenza A. Then they dipped a small brush in liquid virus and painted a coat of flu germs onto the animal’s fur and let it dry. Days later, a vulnerable guinea pig in a cage downstream got sick. The only explanation the researchers could find was that the virus on the fur of the first one got into the air and then was inhaled by the second.
And that means maybe Siegel is right about us having a new range of things to be concerned about. At the very least, it suggests a need for more study of aerosolized fomites, not just for flu and Covid-19, but for all sorts of respiratory viruses. “We’re not saying everyone’s barking up the wrong tree by focusing on respiratory particles,” says Ristenpart. “We’re saying there’s a whole other tree that needs to be investigated.”
Aerosol scientists have previously considered the possibility of influenza virus being transmitted via resuspension, or aerosolized fomites. Many that WIRED spoke to believe it’s plausible that SARS-CoV-2 could behave in much the same way. But they stressed that just because it can happen doesn’t mean it does, or that it does very often. “We do not know how important this route of transmission is compared to aerosols that are directly emitted,” Linsey Marr, an expert in airborne viruses at Virginia Tech, wrote in an email. (Marr was also a reviewer on the Nature Communications study.)
In part, that’s because SARS-CoV-2 is such a new pathogen. Among its many unknowns is the amount of coronavirus required to infect someone. More investigation into that will be necessary to assess whether dust particles can transmit enough virus to make someone sick. But studying how important resuspended viral particles are for spreading a disease like Covid-19 or influenza in the real world is also incredibly difficult. Because scientists can’t really track individual particles into a person’s body to know which ones cause an infection, it’s next to impossible to tell if the viral particle that makes someone sick came from a respiratory droplet moving at them like a bullet, or from a respiratory aerosol floating far away in the air, or from a fomite that was once on a surface and got resuspended and inhaled, or a fomite that was somehow directly transferred to a person’s nose or mouth (usually from someone touching it with a hand or finger).
That’s why the guinea pig experiments are so important, says Corsi. “It’s a well-done, systematic study that provides physical evidence from real animals, that says it’s possible for resuspended particles to contain virus, travel through the air, and be inhaled by another animal, making them sick,” he says.
Siegel put it a bit more strongly. When he finished reading the Nature Communications paper, he told his wife that this was the first time he’d thought to himself: “Oh, we’ve got a problem here.”
For months, he’s been advising businesses and industry groups about improving ventilation and sanitation measures for the Covid-19 era. Among his recommendations are being extra cautious when changing air filters—wearing protective gear, doing it outside, flushing out the space immediately afterward. Assume used filters carry potential for infection, he tells people. The same goes for floors in high-risk areas. Siegel has urged dentists and oral surgeons to install mats outside of their operating rooms that pull particles off the bottom of people’s shoes, so that any viruses that settle on the floor don’t get tracked out and resuspended later. “I’d said these things feeling like maybe I was being overly cautious, secretly hoping that infectivity had no chance of being preserved,” says Siegel. “But this article provides further evidence that being more careful is warranted. Even if we don’t yet have anything definitive, there are still simple things we can do in the meantime just to be on the safe side.”
Those things include making sure that people who clean buildings like offices and schools are equipped with proper protective wear, including N95 masks, says Siegel. Often, employers don’t ask cleaning staff to wear masks if they’re working alone at night. But vacuuming, dusting, and sweeping all kick up tons of particles that could potentially be contaminated if an infectious person was recently in that space.
In a four-year study of seven high schools in Texas, Corsi and his collaborators continuously monitored the air inside 46 classrooms for days at a time. They observed several massive spikes in solid particles throughout the day, corresponding to when the students entered and exited the classrooms. There was also one almost equally large burst of particles at 8 o’clock in the evenings—when the custodial crew came around to clean. For anyone who looks at the Nature Communications paper and says, “Well, but we’re not guinea pigs! We don’t have fur!” Corsi says to consider the Texas classrooms, which showed that people just walking around a space also moves significant amounts of solid particles into the air.
Read all of our coronavirus coverage here.
And humans are just as good at contaminating their environments as rodents. “There’s a ton of stuff on the floor that gets resuspended by people’s feet,” says Corsi. “We don’t know yet if there are significant amounts of virus getting aerosolized this way, but it’s definitely something we should be thinking about.”
Extra ventilation during cleaning could help cut down on the potential dangers of aerosolized fomites. So could disinfecting floors before sweeping them. These don’t have to be expensive retrofits, Siegel says. There’s some evidence that DIY air purifiers will work just as well as in some scenarios as an industrial upgrade. Corsi and Siegel also suggest getting rid of carpet, which is harder to clean and resuspends far more particles than linoleum, tile, or concrete floors.
While it’s possible that Covid-19 could be spread through this potential fourth transmission route of aerosolized fomites, potentially creating additional occupational hazards for those in lines of work that put them in contact with a lot of particulate matter, most people don’t have to do anything differently. You definitely don’t have to panic about killer germ dust.
“This information adds to our understanding about where virus in the air might be coming from, but it doesn’t change how we should approach it,” Marr wrote in an email to WIRED. “The same things that we’ve been doing—wearing masks, keeping our distance, avoiding crowds, and ensuring good ventilation—will also help reduce the risk of transmission from breathing in virus that gets into the air this way.” So mask up and keep on carrying on (from a distance).
More From WIRED on Covid-19
- San Francisco was uniquely prepared for Covid-19
- Bill Gates on Covid: Most US tests are “completely garbage”
- Scientists put masks to the test—with a cell phone and a laser
- Tips for taking a quarantine getaway road trip
- Hybrid schooling may be the most dangerous option of all
- Read all of our coronavirus coverage here