Exerpt from an ACHRNEWS article:
ALL ABOUT AIRBORNE TRANSMISSION
One of the big questions about coronavirus — especially from an HVAC perspective — is how significant of a role airborne transmission plays in its spread. As professor of architectural engineering at the Pennsylvania State University and founding director of its Indoor Environment Center, William Bahnfleth, ASHRAE presidential member, is well versed in what’s been published recently on that topic.
“The current consensus is that it's predominantly large droplet transfer,” he reported. This means droplets too large to remain airborne that are spread through coughing and sneezing within a fairly close range of other people. The Center for Disease Control (CDC) puts this range at about six feet.
“If I were to cough or sneeze in your direction unprotected, some of the virus-containing droplets coming out of my mouth or nose might enter your mouth, eyes, or nose and cause an infection,” he said.
Another mode of transmission involves intermediate surfaces. For example, an infected person might cough or sneeze into their hand and then leave infective material on a door knob that someone else might touch and then transfer it to their own mouth or eye.
“But there's also the potential for airborne transmission,” Bahnfleth said. “And if viruses that are viable are in those droplets that you're producing, some of them will be small enough that they will stay airborne for a long time. So, it's not impossible that infectious particles in the air could stay aloft long enough to be collected, say at the return grille of an HVAC system, go through a duct, and infect someone in a different space.”
“Because there are three distinct ways of transmitting an infection,” he added, “even perfect control of airborne pathogens would not eliminate all risk.”
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