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July 11, 2020

There are some questions you’d rather not ask. Because the answer might make things worse. “Is COVID airborne,” is that kind of question. A yes answer might lead to more precautions, more places to avoid, more rules. Still, the news is giving this issue a lot of coverage this week, so we have to ask: Why now? And what difference does it make?

Up until now prevention has been focused on the same droplet precautions that apply to most respiratory viruses. We know the familiar trio: distance, handwashing, masks. Droplets carry viruses in small wet drops, five or more micrometers in diameter. They are propelled forward when we talk energetically, cough, sing, laugh, cheer, or yell. (Do you remember kids saying, “say it, don’t spray it!”?) Droplets have a little weight, so they usually drop out of the air within, say, six feet.

Counting red blood cells, 1976

Aerosols are tiny droplets, four microns or less in diameter. From my years in blood banking I naturally compare these sizes with a human red blood cell, which about 7 micrometers (aka microns) in diameter. Maybe you’ve seen blood cells under a microscope. Viruses are much tinier. You would need an electron microscope to see them. So they could hitch a ride in the aerosol droplets that we emit when we talk or breathe. And aerosol droplets just float around in the air, buoyed up by normal air currents. In a word, airborne. When we’re around other people we breathe each other’s aerosols. How long the virus remains contagious in an aerosol remains under investigation.

Last week a group of 239 scientific investigators sent a communication to the WHO to convey their concern that airborne transmission of COVID has been overlooked. The report, entitled, “It is Time to Address Airborne Transmission of COVID-19 (link below) was authored by Lidia Morawska, a physicist whose specialty is airborne particulate matter and Donald K. Milton of the University of Maryland who focuses on “infectious bioaerosols, exhaled breath analysis, and . . . respiratory epidemiology.” (links below). The evidence for airborne transmission rests on both analysis of known spreading events (restaurants, choral singing, cruise ships, et. al.) plus laboratory research, such as the 17 studies referenced in the report.

The WHO had mentioned the possibility of aerosol transmission in medical procedures and has now added a cautious paragraph stating that more studies are needed to see whether transmission in closed indoor settings may be due to aerosol transmission.

In any case, new knowledge about aerosol transmission doesn’t invalidate droplet precautions already in place (handwashing, distance, masks). It does, however, expand our thinking on the relative safety of different environments. For example, being outdoors in the sun is safer because the virus is infectious longer in darkness than in sunlight. Enclosed environments are the most risky, especially in the combination of crowds, poor ventilation, and prolonged exposure.

Morawska’s paper recommends adequate ventilation (even just opening windows can help), avoiding crowds, air filtration, and UV lights to inactivate viruses. She refers to the guidelines set by ASHRAE, The American Society of Heating, Refrigerating and Air-Conditioning Engineers, who have a very thorough position document, ASHRAE Position Document on Infectious Aerosols (link below).

Looking through the ASHRAE document reminded me that this is not the first time we have encountered airborne diseases and engineering already has strategies which can make the air we breathe safer, not only in buildings, but (from their document) “in a variety of other spaces, such as planes, trains, and automobiles.” They also emphasize that their strategies can influence “dissemination of infectious aerosols” but not “direct-contact routes of transmission . . . such as touching, kissing, sexual contact, contact with . . .” Well, you get the picture. I love that these engineers have a sense of humor.

In conclusion, the question of whether COVID is airborne is not so scary after all. We already know some commonsense ways to avoid it by keeping the air flowing and staying out of crowds. What’s more, technology exists and in some cases is already in place, to address the issue of airborne pathogens.

It is also important to note that the relative importance of aerosol versus droplet transmission is still under investigation and continues to be actively debated and discussed in the media.

We just need to keep asking questions.

Today’s Notable Headlines

“Pandemic exposes scientific rift over proving when germs are airborne,” Reuters, Science News, July 10, 2020. https://www.reuters.com/article/us-health-coronavirus-airborne/pandemic-exposes-scientific-rift-over-proving-when-germs-are-airborne-idUSKBN24B2YG

“Is COVID-19 airborne? WHO issues new guidance,” NBC News, July 10, 2020. https://news.yahoo.com/covid-19-airborne-issues-guidance-125745571.html

“It is Time to Address Airborne Transmission of COVID-19,” Lidia MorawskaDonald K Milton, Clinical Infectious Diseases, July 6, 2020.  https://academic.oup.com/cid/article/doi/10.1093/cid/ciaa939/5867798

Osterholm Update: COVID-19. Episode 15: A Coronavirus Forest Fire, CIDRAP podcast, July 10, 2020. https://www.cidrap.umn.edu/covid-19/podcasts-webinars/episode-15

Additional sources:

“Airborne transmission of SARS-CoV-2: The world should face the reality,” Lidia Morawska and Junji Cao, Environment International, June 2020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7151430/

ASHRAE: https://www.ashrae.org/technical-resources/resources

ASHRAE: Position Document on Infectious Aerosols. https://www.ashrae.org/file%20library/about/position%20documents/pd_infectiousaerosols_2020.pdf

WHO: https://www.who.int/emergencies/diseases/novel-coronavirus-2019/coronavirus-disease-answers?query=aerosol+transmission

Lidia Morawska: https://staff.qut.edu.au/staff/l.morawska

Donald K. Milton: https://sph.umd.edu/people/donald-milton

Why am I doing this?

The coronavirus pandemic will be indelibly written on our memories just as the Great Depression or the Battle of Britain left their mark on past generations. It is my intention to journal the events of these days from three perspectives: as a retired medical technologist, a historian (Ph.D., 2014), and an ordinary person living through an extraordinary crisis.

You are on History’s Edge.

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