Moist air can extend the life of virus-laden aerosol droplets

The novel coronavirus that causes COVID-19 is believed to be spread through natural respiratory activities, such as breathing, speaking and coughing, but little is known about how the virus is carried through the air.

Report of scientists from the University of Missouri, in Fluid physics, on a study on how air flow and fluid flow affect exhaled droplets that may contain the virus. Their model includes a more precise description of the air turbulence that affects the path of an exhaled droplet.

The calculations with their model reveal, among other things, a large and surprising effect of humid air. The results show that high humidity can extend the lifespan in air of medium-sized droplets by up to 23 times.

The droplets exhaled in normal human breath come in a range of sizes, from about a tenth of a micron to 1000 microns. For comparison, a human hair is about 70 microns in diameter, while a typical coronavirus particle is less than a tenth of a micron. The most common exhaled droplets are about 50 to 100 microns in diameter.

The droplets exhaled by an infectious individual contain virus particles as well as other substances, such as water, fat, protein, and salt. Research has considered not only the transport of droplets in the air but also their interaction with the surrounding environment, in particular by evaporation.

Investigators used an improved description of air turbulence to account for natural fluctuations in air currents around the ejected droplet. They were able to compare their results with other modeling studies and experimental data on particles similar in size to expired droplets. The model showed good agreement with the data for corn pollen, which has a diameter of 87 microns, approximately the same size as most expired droplets.

Humidity affects the fate of exhaled droplets, as dry air can accelerate natural evaporation. In air with 100% relative humidity, simulations show larger droplets with a diameter of 100 microns falling to the ground about 6 feet from the exhalation source. Smaller droplets 50 microns in diameter can travel further, up to 5 meters, or about 16 feet, in very humid air.

Less humid air can slow the spread. At 50% relative humidity, none of the 50 micron droplets exceeded 3.5 meters.

Investigators also looked at a pulsating jet pattern to mimic coughing.

“If the viral load associated with the droplets is proportional to the volume, almost 70% of the virus would settle on the floor during a cough,” said author Binbin Wang. “Maintaining physical distance would significantly address the spread of this disease by reducing droplet deposition on people and reducing the likelihood of inhaling aerosols near the source of infection.”