In 2003, when SARS had infected thousands, killed hundreds, and caused a worldwide scare, Lidia Morawska was studying the effects of inhaling fine particles of pollution. But then the World Health Organization asked Morawska, a physicist at the Queensland University of Technology, to join a team in Hong Kong trying to understand how the coronavirus that causes SARS was spreading.
Morawska decided to take an unconventional approach. Rather than look at how people inhale contagious matter from one other, she dug into the reverse process: exhalation.
“I found three papers investigating anything to do with exhalation of particles from human respiratory activities. There was basically next to nothing,” she says. “This amazed me because this is such an important area, such a critical area.”
Almost two decades later, the rapid spread of the novel SARS-CoV-2 virus has reignited interest in research into how our lungs launch infectious material into the air, namely the tiniest respiratory droplets called aerosols. Understanding how aerosols form in the body is crucial to figuring out why this virus spreads so readily and what’s fueling so-called superspreading events, where a small number of disease carriers end up infecting many individuals. Such incidents are a hallmark of COVID-19.
Since Morawska began her investigations, scientists have learned a lot about airborne respiratory fluids and, in particular, what might make someone a superspreader, or superemitter. Certain attributes, such as the shape of one’s body, and certain behaviors, such as loud talking or breathing fast, appear to have a major role in spreading the disease.
”They're not sneezing. They're not coughing. They're just breathing and talking,” says Donald Milton, an aerosol transmission expert from the University of Maryland. “They might be shouting. They might be singing. Karaoke bars have been a big source of superspreading events. We saw one at a spin cycle club up in Hamilton, Ontario, where people are breathing hard.”
However, figuring out who are the most prodigious producers of aerosols has proven difficult—with many biological and physical factors affecting aerosol generation that are tough to parse out or even measure.
Say it, don’t spray it
To aerosol scientists like Morawska who are more focused on the physics, an aerosol is any particle, wet or dry, that can be suspended in the air for minutes to hours. Aerosols are usually less than 100 micrometers in size, or about the width of a human hair. The human respiratory tract produces a wide variety of aerosols, from tiny droplets only a few micrometers across to globules around 100 micrometers, and even gobs bigger than aerosols that are visible with the naked eye and are more typically described as respiratory droplets.
“The smallest aerosols are generated in the deeper part of the respiratory tract,” Morawska says. These are especially important for disease transmission because they can stay aloft longer and travel farther compared to the big gobs that fall quickly.
These smallest aerosols are created within the bronchioles, the thin, branching airways deep within our lungs. By carefully measuring the aerosols produced by people when they breathe in different ways, Morawska and colleague Graham Richard Johnson proposed in a seminal 2009 paper that the respiratory fluid lining these tubes creates films that burst like soap bubbles when the bronchioles contract and expand. This is now considered to be the main mechanism that creates aerosols deep in the lungs.
Something similar happens higher up in the respiratory tract, in the sound-producing larynx.
“The vocal folds are opening and shutting too fast really for the naked eye to see,” says William Ristenpart, a chemical engineer at the University of California, Davis, who studies disease transmission. Much like the bronchioles, these folds pull apart respiratory fluid when they are slamming together during speech and singing, creating tiny droplets. Imagine vigorously washing your hands, and the soapy film rupturing as you pull them apart.
This process happens very fast, a hundred or so times a second, and the droplets it creates hitch a ride on exhaled air—which brings us to the oral cavity. The largest droplets of the respiratory tract are generated in the mouth, with its flapping lips and its saliva-laden acrobatics of speech, and these are the ones you probably know best.
“In talking especially, sometimes you can feel little droplets flying out,” Ristenpart says. “That’s where ‘Say it, don’t spray it,’ comes from.”
While the nose is also a path for aerosols to escape, the main route is through the mouth. All the aerosols and droplets are trapped in an explosive puff of gas, which governs their movement and spread for the first few seconds.
“The gas cloud in fact keeps the drops that were emitted concentrated as it moves forward in a room, carrying them within it,” says Lydia Bourouiba, a fluid dynamics scientist at MIT.
More droplets, coming right up!
Though the general mechanics that create respiratory aerosols are the same among people, a large amount of variation exists between how much spray individuals actually produce. Look at a crowd of people standing by a bus stop on a cold day, and you’ll notice everyone’s breath fog looks different in terms of size.
This shouldn’t be surprising, considering the complexity of the respiratory tract. Morawska uses the analogy of a perfume bottle’s more uniform mist: “Unlike in the perfume bottle, where there’s only one tube, there’s many different passages in the respiratory tract—passages of different widths and different lengths.”
To quantify this complexity even for a single person would be cumbersome, but scientists can still spot those who excel at making aerosols. In a 2019 study, Ristenpart and his colleagues showed that the louder someone speaks, the more aerosols they emit. However, the scientists also found that some participants in their study produced an order of magnitude more aerosols than others–even when speaking at the same volume. These people have become known as superemitters.
“Clearly there's got to be some type of underlying physiological reason that causes people who are speaking at about the same amplitude and the same pitch to emit wildly different numbers of particles,” Ristenpart says. One possibility, he says, is that the thickness of the fluid and how it reacts to deformation can vary between people. Previous research has shown that inhaling salt water mist that’s less viscous than mucus-packed respiratory fluid made individuals produce fewer aerosol particles overall. On the flip side, folks with fluid that is naturally higher viscosity may be producing more aerosols.
Complicating matters is that a respiratory infection can cause changes in the respiratory fluids. For example, the viscosity of the respiratory lining increases during bronchial infections such as bacterial pneumonia and severe influenza due to the loss of water and increased production of cellular proteins. Chronic conditions like asthma and cystic fibrosis can also thicken up the fluids.
Answering the many questions that remain is challenging due to the nature of aerosols themselves. For example, the particles are sensitive to environmental conditions, and the bigger ones with more liquid can dry out quickly, leaving behind mostly tiny, more concentrated particles that skew readings. The temperature, humidity, and air flow within scientific instruments can also change the aerosols one is trying to measure.
These nuances call to mind the peculiarities of quantum mechanics, where conducting a measurement on a subatomic particle influences the result. Though these aerosols are much larger, measuring their ephemeral nature is similarly challenging.
Morawska acknowledges this challenge with a chuckle. “Being able to measure and give an answer which represents what's actually happening is extremely difficult,” she says.
These difficulties, in part, have stymied the study of disease transmission by aerosols for decades. “Even now, in 2020, how influenza spreads is controversial,” says Ristenpart, who recently published a study showing that flu viruses may piggyback on dust particles.
But this field of science is having a moment in the spotlight now due to COVID-19. Aerosols have helped reveal why the SARS-CoV-2 coronavirus is even more transmissible through the air than the original SARS of 2003. Many experts now agree that better ventilation of indoor environments and masking can help curb this aerosol-borne disease. That’s why Morawska, Milton, and many of their colleagues in aerosol science called in July for more focus on airborne transmission of SARS-CoV-2 via aerosols, which the CDC and WHO are now beginning to emphasize.
Keeping a sustained focus on this research is another matter, even though superspreading has captivated scientific and public minds since the era of Typhoid Mary nearly a century ago. Just like Morawska, Bourouiba also shifted her fluid dynamics research focus to epidemiology after the SARS outbreak in 2003. She has seen interest in aerosol research spike during respiratory disease outbreaks like SARS, MERS, and the H1N1 flu virus, but then it fades away. Changing that is imperative, she says.
“If this pattern from the decision-makers and the funders continues to be so shortsighted,” Bourouiba says, “we will always just have Band-Aid sort of approaches for tackling these questions.”