By DJ Rippert
The Bromage Broadcast. Erin Bromage is a professor of biology and a blogger. She will tell you that she’s not an expert epidemiologist but she recently wrote a blog entry that proves she is an eloquent writer when it comes to explaining the physics of Coronavirus to the layman. As Virginia reopens after the lockdown people will have to make personal decisions about what activities to undertake and what activities to avoid. Ms. Bromage’s plain English explanations make a good starting point for making such decisions.
1,000 Particles. The Coronavirus comes to its victims in the form of virus particles. A virus particle is a structure that has evolved to transfer nucleic acid from one cell to another. However, as Ms Bromage explains, different viruses need to transfer different numbers of virus particles to a potential host in order to create an infectious dose. Studies of Coronaviruses other than SARS-CoV2 indicate that relatively few virus particles are required to create an infectious dose. Based on this, experts put the number of virus particles required to form an infectious dose of SARAS-CoV2 as low as 1,000. Obviously, these virus particles need to somehow get to you to infect you.
Breathing. Human respiration creates aerosolized droplets of liquid. These are the vehicles used by virus particles to travel from an infected person to the next potential host. Breathing creates between 50 and 5,000 droplets per breath. The good news is that breathing creates low velocity droplets that fall to the ground quickly. Even better, the amount of viral material expelled through breathing is quite low – 20 to 33 infectious viral particles per minute. It should be noted that this figure is an extrapolation from influenza since the exact numbers for SARS-CoV2 are still unknown. Using 20 viral particles per minute and assuming they all make it to you … you’ll receive an infectious dose of SARS-CoV2 from being in close proximity to a breathing person in 50 minutes (20 X 50 = 1,000). However, it is highly unlikely that all the emitted viral particles will make it to you. Walking past somebody in a grocery store might be fine while sitting next to somebody on a cross country flight might not.
Speaking. Talking ups the generation of virus particles by 10-fold over breathing. In the unlikely event that all of the virus particles are transferred, you could be infected by your talkative neighbor in 5 minutes. When going through the checkout line at the grocery store you may not want to shoot the breeze with the cashier.
Coughing. A cough puts out 3,000 droplets traveling at 50 mph. The droplets are relatively heavy and most fall to the floor but the velocity is sufficient to get some of the viral particles across a room in a few minutes. Coughs (and sneezes) really blow out the viral particles with up to 200,000,000 of the nasties on the 3,000 droplets from a single cough.
Sneezing. Sneezing is the MIRV of viral attack. A sneeze produces 30,000 droplets (10 times more than a cough) traveling at 200 mph (4X faster than a cough). A single sneeze, like a single cough, can transmit 200,000,000 viral particles. If you’re in a room and you hear the “ah” part of an “ah choo” … close your eyes, hold your breath, put your head down and hit the nearest emergency exit like John Riggins in his prime. More seriously, by the time you hear somebody sneeze it’s probably too late. Sneeze droplets are small and will easily travel across a room.
Whatever way the wind blows. Bromage provides two excellent examples of real-world infections, one in a restaurant (see diagram at top of article) and the other in a call center. Both indicate that the odds of being infected change with the airflow in an enclosed space. Unsurprisingly, it’s better to be upwind. Surprisingly, virtually unnoticeable routine air flow turbulence can turn the tables for those seeking upwind advantage. So much for a standard six feet of separation.
Bottom line. Professor Bromage’s article is the first I’ve seen to put COVID-19 infection into somewhat probabilistic terms. It certainly explains why 66% of people recently admitted to New York hospitals were infected while sheltering at home. It doesn’t take a non-sheltering housemate long to pass along thousands of virus particles while explaining what a lousy day they had at work. It also doesn’t take a lot of deep thinking to tell your hair stylist to work quickly and quietly.