Respiratory droplets exhaled through coughs and sneezes can span an area of up to 7 or 8 meters (23 to 26 feet), allowing for the possibility they contaminate surfaces.
As such, current social distancing recommendations meant to decrease transmission of SARS-CoV-2, the virus responsible for the respiratory disease COVID-19, may “underestimate” how far and for how long respiratory droplets may travel, according to a commentary written by Lydia Bourouiba, a professor at the Harvard-MIT Health Sciences and Technology, published in The Journal of the American Medical Association. Bourouiba argues that new requirements for personal protective equipment are needed.
“Although such social distancing strategies are critical in the current time of pandemic, it may seem surprising that the current understanding of the routes of host-to-host transmission in respiratory infectious diseases are predicated on a model of disease transmission developed in the 1930s that, by modern standards, seems overly simplified,” writes Bourouiba.
SARS-CoV-2 is transmitted through respiratory droplets that spread when an infected person coughs or sneezes. The Centers for Disease Control and Prevention (CDC) recommends a 2-meter (6-foot) distance be kept to avoid transmission, while the World Health Organization currently recommends healthcare personnel maintain a 1-meter (3-foot) distance from symptomatic patients. However, health experts note that airborne and droplet transmission are different and may require protective measures specific to each.
“Airborne transmission is different from droplet transmission as it refers to the presence of microbes within droplet nuclei, which are generally considered to be particles less than 5μm in diameter, can remain in the air for long periods of time and be transmitted to others over distances greater than 1 meter,” writes WHO in an update.
The size of a respiratory droplet and the environment in which it is sneezed out both influence how likely it is to contaminate a person or area. For example, large droplets settle faster and contaminate the area surrounding the person who expelled them, whereas smaller droplets are more likely to evaporate. Bourouiba says that droplet size regulations set forth by oversight agencies like WHO and the CDC are “arbitrary” and do not accurately reflect what actually occurs with respiratory emissions.
Peak exhalation speeds follow “short-range semi ballistic emission trajectories", reaching speeds between 10 and 30 meters per second (33 to 100 feet per second). Additionally, a “turbulent gas” cloud, or puff, can encase variously sized droplets within it.
“Under these conditions, the lifetime of a droplet could be considerably extended by a factor of up to 1000, from a fraction of a second to minutes,” said Bourouiba. In addition to environmental conditions, the rate of respiratory droplet evaporation is also dependent on the “degree of turbulence and speed” of the sneezed-out gas cloud.
Though the exhalation properties of SARS-CoV-2 have not yet been studied, varied sized droplets created within and outside of the respiratory tract may contain viruses responsible for respiratory infections.
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