Landmark study finds coronavirus easily transmitted outdoors

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JANUARY 8, 2021 — Editor’s note: This study, reported in UTSA Today’s most-read article of 2020, adds to the growing body of work that is fast-tracking UTSA as a great public research university. [Originally published September 29, 2020.]

Recently published research spearheaded by UTSA finds that outdoor areas may not be as safe as first thought to protect against the coronavirus. The study was conducted by Kiran Bhaganagar, an associate professor in the Department of Mechanical Engineering. To develop her findings, Bhaganagar utilized data available from a United States COVID-19 hot spot—New York City.

According to Bhaganagar, this is the first study to measure spread of coronavirus in outdoor conditions. There is enough evidence that aerosols spread the virus indoors. To conduct her research, Bhaganagar obtained detailed meteorological fields to create highly accurate models that show likely spread of the virus under various weather patterns in New York City from March through April 2020.

“This work is further evidence that outdoor air cannot dilute the virus particles.”

Using computer modeling, she created a real-time, high-fidelity simulation of a virus-filled cough as it was released into the atmosphere from an infected person. As the person coughs or sneezes, respiratory droplets that contain infectious particles are released into the air. The simulations found that combinations of certain weather conditions favor the spread of the virus.

Bhaganagar discovered windy conditions did little to spread the coronavirus droplets; in fact, that weather condition actually caused the aerosol droplets to disperse faster.

Instead, the testing showed that the combination of warmer or moderately cold air temperatures, with low wind speed and weak turbulence, increases the amount of time the virus can be airborne before getting dispersed in the air—up to 30 minutes in many cases. In low wind conditions, Bhaganagar discovered coronavirus aerosol particles can spread from 1 to 2 kilometers, or a little over a mile.

Simulating Viral Spread

This computer animation shows how the right weather conditions likely spread coronavirus droplets across the Hudson River and into New Jersey.

Depending on the time of the day and depending on the wind direction, the direction of spread was either toward Massachusetts, toward New Jersey or toward southern New York.

Based on Bhaganagar’s cumulative information, she said it’s highly likely that outdoor conditions also contributed to the wildfire-like spread of the coronavirus in the NYC metro area during spring 2020.

“This work is further evidence that outdoor air cannot dilute the virus particles, and there is strong evidence the spatial spread across states is linked to airborne transmission,” said  Bhaganagar.

Based on the distance coronavirus aerosol can spread outdoors, Bhaganagar says her study suggests that outdoors 6 feet may not be adequate social distance to protect from the virus. She added the use of masks and other means of virus protection in outdoor areas are additional precautions to consider against contracting COVID-19.

Bhaganagar’s entire study was published in Environmental Research, Science Direct, on September 17. UTSA Ph.D. student Sudheer Bhimireddy assisted on the project.

— Bruce Forey