Fluid dynamics work hints at whether spoken word can spread COVID-19

By Carolyn Beans

Image courtesy of Lydia Bourouiba (MIT).

A sneeze can project a droplet-containing gas cloud seven or eight meters, possibly spreading disease. But the spoken word is also far from innocuous. Image credit: Lydia Bourouiba (MIT).

As of April 3, the Centers for Disease Control and Prevention (CDC) is advising that everyone—sick or healthy—wear masks to prevent the spread of COVID-19. Recent research on how fluids travel from our respiratory tracts when we sneeze, speak, or breathe shows why the practice could be so important—even if it can’t yet explain exactly how coronavirus spreads so quickly or definitively determine whether masks or face coverings for all will significantly reduce disease spread.

When a person coughs or sneezes, large fluid droplets that may contain pathogens go flying. Respiratory infections can spread when another person comes into direct contact with these droplets or touches a surface contaminated by them.

In addition to these visible droplets, the largest of which are formed by saliva, we also expel much smaller droplets that can originate in the mucous coating of the lungs and vocal chords. These smaller droplets, often called aerosol particles, are invisible to the naked eye, and behave somewhat like dust particles suspended in a sunlit room, says William Ristenpart, a chemical engineer at the University of California, Davis. “These aerosol particles that are about one micrometer can stay in the air for a very long time—hours.”

Lydia Bourouiba, director of the Fluid Dynamics of Disease Transmission Laboratory at the Massachusetts Institute of Technology, uses optical methods like backlighting combined with high-speed videography—1,000 frames per second or more—to help quantify the physics of pathogen transport with exhalations like a cough or sneeze. Her team’s findings, reported in the Journal of Fluid Mechanics in 2014 and Experiments in Fluids in 2016, show that most respiratory droplets do not travel independently on their own trajectories. Instead, droplets in a continuum of sizes are trapped and carried forward within a moist, warm, turbulent cloud of gas. “This cloud makes the droplets concentrated, and carried forward further than they would be able to reach otherwise,” Bourouiba says, adding that this “potentially changes the physics of evaporation.”

The simple act of speaking also propels droplets into the air—though typically too small to be visible. Ristenpart and his team used an aerodynamic particle sizer to record the number and size distribution of droplets emitted as people spoke at a range of different volumes. Their research, described in Scientific Reports in 2019, revealed that as people raise their voices, they emit more droplets, but the size distribution of the droplets remains the same. This finding was true regardless of the language spoken.

Even breathing could release potentially infectious aerosols, says Donald Milton, an infectious disease aerobiologist at the University of Maryland School of Public Health. Milton and his team collected droplets from volunteers with flu symptoms by asking them to spend 30 minutes with their faces positioned in front of the large opening of a cone as it drew in air. The device captured the large droplets produced by sneezing and coughing as well as the aerosolized droplets produced by sneezing, coughing, breathing and talking on different surfaces. His team recorded the number of times each participant coughed and sneezed. They also asked the participants to say the alphabet three times, so some of the droplets the team collected were likely released during speech. “People didn’t have to cough to shed virus,” Milton explains. The work, published in PNAS in 2018, shows that the flu virus exists even in the tiny aerosolized droplets resulting from breath or speech alone. It is not yet confirmed, however, whether exposure to these aerosolized droplets could lead to flu infection.

Researchers are still exploring whether fine aerosolized droplets could spread coronavirus. The data are not yet conclusive, says Yvonne Maldonado, infectious disease epidemiologist at the Stanford University School of Medicine. One question is whether this virus can survive in a 5-micrometer particle for a long period of time in a healthcare environment, she says. “Because that would mean that if you walked into a room, you would just have to breathe to become infected.”

Maldonado thinks that scenario is unlikely. “Fortunately for us, there aren’t very many known aerosol viruses,” she says. Measles and tuberculosis are among the few exceptions.

Still, some researchers believe that transmission via tiny droplets warrants a closer look. “We know that asymptomatic people are transmitting COVID-19 and they’re not coughing,” says Milton. He points to a preprint study, not yet peer reviewed, by researchers at the University of Nebraska Medical Center that suggests that coronavirus was present in air samples collected from the rooms of individuals infected with coronavirus, regardless of whether the patients showed symptoms like coughing and sneezing.

The White House Office of Science and Technology Policy recently asked the National Academies Standing Committee on Emerging Infectious Diseases and 21st Century Health Threats to consider whether the SARS-CoV-2 virus could be spread by conversation, in addition to sneeze/cough-induced droplets. The team determined that current evidence supports the possibility that COVID-19 could spread through aerosolized droplets released via patients’ exhalations. They noted, however, that the latest research, including the Nebraska study, does not yet confirm whether the coronavirus identified in air samples is actually viable and capable of infecting those who breathe it in.

Because speech can release droplets, and many contagious individuals do not know that they are infected, some public health experts are suggesting that we should all wear a mouth covering when we need to go out in public. “Eliminating speech droplets could be a significant part of reducing community spread,” says Anne Rimoin, a professor of epidemiology at the University California, Los Angeles, Fielding School of Public Health. Medical masks, she cautions, should be reserved for medical personnel. “What we’re talking about is that a simple cloth mask or face covering can stop droplets from spreading.”

A homemade cloth mask wouldn’t stop the small aerosolized droplets, says Rimoin. And public health experts often warn against wearing them because they can become contaminated if not handled properly, notes Maldonado.

Still, despite these limitations, homemade masks could still do a great deal of good, says Rimoin. “We are so used to working in a system where perfection is our goal, but we are in the greatest public health emergency of our lifetime and, as such, we have to get creative and do the best we can and not let the great be the enemy of the good.

The CDC has now called on all persons to wear cloth face coverings, though the fear remains that this guidance could prompt a run on medical masks that are already in short supply for the healthcare workers who need them most. The CDC guidance does specify that the recommended face coverings are “not surgical masks or N-95 respirators” which are “critical supplies that must continue to be reserved for healthcare workers and other medical first responders.” They offer tips on how to make face coverings from cotton fabric, t-shirts, or bandanas and coffee filters at home. “The key is that these are two separate issues: Facial coverings for the general public to keep their droplets to themselves, and an urgent and critical need for PPE for healthcare workers,” says Rimoin. “It’s not reasonable to expect our healthcare workers to be out there without the protection that they need.”

Another possible reason to wear a mask is that six feet between individuals may not provide wide enough berth to limit spread via even the larger droplets. In a recent article published in the Journal of the American Medical Association, MIT’s Bourouiba notes that under the right temperature and humidity conditions, a sneeze can release a gas cloud that carries droplets within it as far as seven to eight meters.

Many questions remain about the fluid dynamics of how coronavirus is transmitted. In a recently published editorial in Aerosol Science and Technology, Ristenpart and colleagues suggest that face-to-face conversation is a plausible hypothesis. But they note many unknowns, including whether aerosols produced by speech actually contain the virus and, if so, how temperature and humidity affect its viability. “Clearly [coronavirus] is highly transmittable. Clearly it is in the respiratory tract,” says Ristenpart. “We need more experimentation.”