Speaking calmly and at a normal volume produces liquid droplets so small they can remain suspended in the air long enough to enter the airways of other people, potentially exposing them to viruses including the one that causes Covid-19, according to a new study led by scientists at the National Institutes of Health.
“Aerosols from infected persons may therefore pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation,” Harvard University biologist Matthew Meselson wrote in a commentary accompanying the paper, which used a laser to visualize airborne droplets created when volunteers uttered the words “stay healthy.”
The study was published on Wednesday in the New England Journal of Medicine.
The question of whether the coronavirus can be “aerosolized” has stirred controversy for weeks, with a study last month reporting that the virus may be able to stay suspended in air under special circumstances, such as when infected people undergo intubation and other procedures. But the new study suggests that the SARS-CoV-2 coronavirus can be transmitted from one person to another simply by speaking.
Large particles such as those expelled in a sneeze or cough “remain airborne only briefly before settling because of gravity,” Meselson wrote. But “breathing and talking also produce smaller and much more numerous particles” that are “too small to settle.” These aerosols are therefore carried by air currents as mild as those generated by people walking around a room, drafts from open windows and doors, and vents that create air flows.
The large droplets that settle on surfaces can cause infection if people touch them and then touch their face. In that case, the droplets and any virus they carry are deposited in the upper respiratory tract, where with any luck they’re removed in nasal secretions or swallowed before they cause an infection.
The aerosols, in contrast, are so small (a few microns, or a couple ten-thousandths of an inch) that they penetrate deep into the lungs, infecting cells.
For their study, scientists led by Philip Anfinrud and Adriaan Bax of the National Institutes of Health asked volunteers to say “stay healthy” into the open end of a cardboard box whose inside was painted black. They used a green laser to create a sheet of light three inches from the open end that, after the person spoke, captured any droplets that reached it. An iPhone 11 Pro video camera recorded the arriving droplets, which produced flashes as they passed through the laser light sheet, allowing the scientists to estimate their size.
“Numerous [aerosol] droplets … were generated,” the scientists reported. The “th” sound of the word healthy produced the most droplets, and speaking loudly but still in a conversational voice produced more droplets (347) than speaking softly (227).
When people said “stay healthy” through a slightly damp wash cloth, however, the number of droplets reaching the box fell to zero. The scientists did not measure how far the droplets could carry, and remain suspended in the air, under different environmental conditions, and no viruses were used in the experiment. But the earlier NEJM study suggests that droplets containing the coronavirus can become aerosolized.
Previous studies, unrelated to Covid-19, have found that droplets emitted during speech are smaller than those from coughing or sneezing and therefore more likely to hang in the air. But they do not do so forever. In still air, a particle with a diameter of 10 microns (roughly one-seventh the width of a human hair) remains aloft for roughly 9 minutes.
With aerosols now back on the table as a potential source of Covid-19 infection, Meselson wrote, it “suggests the advisability of wearing a suitable mask whenever it is thought that infected persons may be nearby and of providing adequate ventilation of enclosed spaces where such persons are known to be or may recently have been.”
As for who “such persons” might be, that’s often impossible to know: A study in Nature Medicine, also published on Wednesday, found that people with Covid-19 are infectious two to three days before they show symptoms.