COVID-infected cough droplets could travel 20 feet outdoors, infect people even after evaporating

SINGAPORE — Here’s another reason to keep that face mask handy even if you’re outdoors. New research shows that cough droplets containing SARS-CoV-2, the virus that causes COVID-19, can travel beyond six meters — and still infect people once they have evaporated. Scientists made the discovery while tracking the flight path of such particles hoping to get a better grasp on the spread of coronavirus.

Findings reveal large, medium and small cough droplets can spread in different ways depending on the outdoor conditions. Larger droplets, which fall to the ground quickly due to gravity, can travel one meter even without wind. Smaller particles, which are lighter and travel further, can still infect a person with coronavirus even after they have evaporated into the air.

Cough droplets can travel up to six meters
Recirculating flows, namely wakes, are observed both in the front of the cougher (left) and at the back of the listener (right). A droplet may be entrained and trapped in the wake, significantly altering its trajectory and fate. (Image credit: A*STAR Institute of High Performance Computing)

“An evaporating droplet retains the nonvolatile viral content, so the viral loading is effectively increased,” says study author Dr Hongying Li, from the Agency for Science, Technology and Research (A*STAR), in a media release. “This means that evaporated droplets that become aerosols are more susceptible to be inhaled deep into the lung, which causes infection lower down the respiratory tract, than larger un-evaporated droplets.”

A typical cough emits thousands of droplets across a wide size range.

In their new study, scientists looked at droplet dispersion using air flow simulation tools. They found large droplets settled on the ground quickly due to gravity but could be projected one meter by the cough jet even without wind. Medium-sized droplets could evaporate into smaller droplets, which are lighter and more easily borne by the wind, and these traveled further.

A slight breeze can carry COVID-infected droplets well beyond six feet

Findings reveal a single 100-micrometer cough droplet can travel up to 6.6 meters (about 21.65 feet) under wind speed of two meters per second. And the droplets could travel even further under dry air conditions due to droplet evaporation.

The results highlight once more the importance of social distancing as well as wearing a mask to prevent the spread of coronavirus.

“In addition to wearing a mask, we found social distancing to be generally effective, as droplet deposition is shown to be reduced on a person who is at least one meter from the cough,” says study author Dr. Fong Yew Leong, also from A*STAR Singapore. “The ongoing COVID-19 pandemic has led many researchers to study airborne droplet transmission in different conditions and environments. The latest studies are starting to incorporate important aspects of fluid physics to deepen our understanding of viral transmission.”

The scientists used computational tools to solve complicated mathematical formulations representing air flow and the airborne cough droplets. This applied to cough droplets around human bodies at various wind speeds and when impacted by other environmental factors.

Weather conditions make a significant difference

Researchers highlight their findings are greatly dependent on the environmental conditions, such as wind speed, humidity levels, and ambient air temperature.

They are also based on assumptions made from existing scientific literature on the viability of the COVID-19 virus.

While the team focused on outdoor airborne transmission in a tropical context, they plan to apply their findings to assess risk in indoor and outdoor settings where crowds gather, such as conference halls or amphitheaters.

Their research could also be applied to designing environments that optimise comfort and safety, such as hospital rooms that account for indoor airflow and airborne pathogen transmission.

Findings are published in the journal Physics of Fluids.

SWNS reporter Laura Sharman contributed to this report.