Aerosols and Viral Transmission: “More Than We Thought”

“Bottom line is there is much more aerosol than we thought.”

That was Dr. Anthony Fauci, Director of National Institute of Allergy and Infectious Disease (NIAID), on September 10, 2020, during a presentation for Harvard Medical Grand Rounds. He was discussing the potential for infectious particles greater than 5 microns in size to stay in the air longer and travel farther distances than what has been conventionally accepted.

As described by Dr. Fauci in his presentation, “there was some misunderstanding between droplets and aerosolized particles” and that aerosol and particle physicists “have told us that we’ve really gotten it wrong over many, many years and that particles greater than 5 microns still stay in the air much, much longer than we thought. When we used to say, empirically, that greater than 5 microns it drops to the ground and 5 microns it might be aerosolized, as we know now that this is just not the case. Bottom line is there is much more aerosol than we thought.”

Why does this distinction matter? And why are scientists and researchers abuzz about it? In short, because the prevailing definition has been that the only particles that can travel in the air and be aerosolized are smaller than 5 microns. That particles larger than 5 microns can travel farther—farther than 6 feet—speaks to a greater probability of infections spreading via the air. Dr. Fauci and his addressing this distinction marks a possible change in what has been common thought (despite existing studies indicating otherwise) on the potential for aerosolization.

What’s a micron?

A micron, also known as micrometer, is a size one-thousandth of a millimeter or one twenty-five thousandth of an inch. As points of reference, a human red blood cell is approximately 7 microns in size, while a cross-section of human hair is around 75 microns, give or take 20 microns or so depending on the hair. If you read the packaging on your air filters, it will most likely also refer to the size of microns it can trap.

When animals and humans cough or sneeze, some of the expelled droplets, which contain viruses, may be larger in size and quickly drop to surfaces where they can be eliminated with normal surface cleaning procedures, but the smaller droplets can travel farther and stay in the air longer. The significance of what Dr. Fauci shared in his presentation is that they may not need to be as small, that is less than 5 microns, to travel farther and linger in the air longer as what was has been the accepted rule.

For more on how germs are spread and how PetAirapy’s UV disinfection technology can tackle those viruses and other pathogens traveling in the air, see our “The Science Behind UV Disinfection” page or contact us for more information.