I've been thinking about airflow a lot lately because the best way to avoid catching SARS CoV-2 is don't breathe in the virus. Yeah, you can get it by touching stuff and then sticking you finger in your eye, but that's ain't what's driving this thing. It's breathing. It's a respiratory virus, after all.
https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html
(also interesting...not all fomites are created equally. It's hard to get virus on your hands from cloth
https://aem.asm.org/content/79/18/5728)
Not all breathing is created equal. A breath can spew 50-3000 particles. A cough, 3000, and a sneeze, 30,000. A person breathes about 15 times a minute. So, for round numbers' sake, lets just say that minute of breathing equals one cough, and 10 minutes of breathing equals one sneeze. So, while you don't want to be around sneezing and coughing people, it's not great to be around plain old breathing for long periods of time. It's the sneaky way the asymptomatic are getting the job done!
So, people breathe, cough and sneeze and then you breathe in the same air space. You can only get infected if you breathe in enough. So, how much is "enough"? It seems, no less than 1000 virus particles to maybe 10 times that is a minimum infectious dose. That's the range - based on how its kissing cousins behaved (SARS and MERS) If you get less than the minimum dose, your body just throws the bugs out with the rest of the trash. More than than, it can get overwhelmed and you're infected.
So people spew out particles. Wouldn't it be nice if they could just blow away before you could breathe too many of them in? It happens. It can depend where you are. Big volume, big air flow will move and dilute the droplets. Small volume, little flow - not so much.
Airflow is a complicated thing. Engineers try to grasp it working backwards from tests, generating all sorts of equations fitted empirically to test results. Scientists try to grasp it from the physics, working from the fundamentals toward predictions that can be tested. There's a lot going on in a mixed gas like air. Atoms vibrating and banging into each other depending on temperature and pressure. There's a big gap between science and engineering with air flow. What you are trying to find out and what you need the results for will determine what approach you take. There is no "one size fits all" approach for air flow.
Commercial buildings often have rather sophisticated HVAC ventilations systems. Much of the air is recirculated. It has to be. It's the only practical way to deliver enough heat or cooling without installing enormous, prohibitively expensive systems. Still, most are designed to exhaust some air and introduce some outside, make-up air. A typical system will replace about 99% of the air in the building in about an hour (ACH = 4 means roughly 2/3 of the air is changed every 15 minutes. 1/3 ^ 4 = 0.01)
Airplanes have high air change rates, zoned airflow and very good filtration. Some specifics here
https://www.erinbromage.com/post/flying-in-the-age-of-covid-19
Cars have "flow through" ventilation. They used to use this as a selling feature in the 1970s. Now , all cars have it. Air comes in the cowl, exits by vents in the back of the passenger compartment. So, as long as you have the recirculation off and the fan blowing, you have good airflow through the car. It's about 10x the ventilation of a building.
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| Much less airflow in your car if you have this turn on. |
Homes are a mixed bag. The HVAC systems are 100% recirculating. The air change rate depends on how often you open and close doors and windows and leaky your home is, in general. Nice, tight new replacement windows? Not leaky. Old, draftly wood double hung? Leaky.
This nasty virus is really a wimp, unlike the norovirus, which is a tough bugger. Coronaviruses fall apart under all sorts of circumstances. The fat covering dissolves almost immediately with soap, alcohol and other disinfectants. The protein spikes denature with light, heat, rough surfaces. The RNA inside gets blown up by UV light. The virus is most happy when it's floating around protected in a drop of water and that's exactly what our respiratory systems do - blow out drops of water. Very small, to be sure, but drops none-the-less.
Many of these drops are 100 micron range and pretty much fall straight down to the floor when you exhale, but most are around 1 micron and float around for a bit. Some also evaporate and leave the "solid" content - virus and some salt and some other bodily goo, floating around on their own for a quite a while. The longer a person is in a place, the more breathes are being breathed and the more of these droplets are hanging around in the air. They can accumulate until a steady state is reached where the amount dropping to the floor, sticking to the walls and exiting the building is equal to the rate new ones are being breathed out. The smaller the space and the lower the air change rate and the more people are in the space, the greater the steady state density of breath droplets.
How long does a "naked" virus remain effective floating in the air? I can't find a good answer. The trouble is, it's hard to test. Those "it lasts three hours" articles you read are the result of testing for fragments of the RNA. The virus has long lost it's fat coat, spikes and probably intact set of RNA. It's like asking how long cars can last and pointed to a Ford Model A in a junkyard. Yeah, it looks like the husk of a car. It's still made of steel. But I hasn't been able to run in 70 years.
So, maybe a viable virus can hang around in the air, naked, for several minutes? Plus, there are all those viruses still hitching a ride in very tiny water droplets? I needed a good way to visualize what's happening. I thought of soap bubbles. You know, the kind kids play with or are modern day substitutes for throwing rice at weddings. You blow them, they move on the air currents until the water evaporates and the bubble "pops".
Perhaps a good way to think of your risk is to think about everyone around you blowing soap bubbles. Every breathe blows out bubbles. They are carried by the airflow. Your goal is not to have many soap bubble land on you. One or two, not so bad. Dozens and dozens, not good.
So if you are outside and there's a breeze going, with air swirling all over the place, it's gonna be really hard to get any bubbles on you. The very turbulent air flow pushes them up, down, left and right and they have all the open space to flow into. If you're inside, with a lot of people, and the air flow is very low and recirculating, you have lots and lots of bubbles, barely moving at all, with no place to go. Just hanging around - hard to keep them off you!
The longer you hang around the bubble blowers and the closer you get to them, the more they're gonna get on you! So, avoid the bubbles! Stay healthy!
Some really excellent reading in links below. All of these are footnoted with links to studies used in their blog. My blog post here is really a "meta-post" leveraged from these.
https://www.erinbromage.com/post/the-risks-know-them-avoid-them?fbclid=IwAR37zdJrhlCadX28hrt9S4QwXiS5HWIZKWLI5ViSivz5_-sjUgWUecI7ecQ
https://www.facebook.com/jenniferkastenmd/posts/119907962995484
https://www.facebook.com/jenniferkastenmd/posts/120574959595451?__tn__=K-R
