Let's talk about transmission of respiratory infectious diseases.
Respiratory infections are transmitted between two people when one is infectious and the other susceptible to infection.
When the infected person talks or coughs they release a mist of particles made out of
respiratory fluid that contains the virus.
They can throw between 900 and 300 000 liquid particles from their mouth.
These particles are less than one micron up to hundreds of micron in size.
By comparison, the thickness of a human hair is about 100 microns.
One way to get infected is by contact transmission:
When you touch a surface that has been contaminated by a virus and then touch your eyes or nose, you can get infected.
There is not much evidence at all that this is the way you can get COVID-19.
The second way is when a spray of large particles (like a drop of rain) fly out of the infected
person's mouth and lands on your eyes, nose, or mouth.
To get infected by this route you must be close up to a person.
The third way you can get infected is by inhaling virus-containing particles that are floating in the air.
This transmission route is called airborne--or aerosol--transmission.
Some people think of an aerosol spray can of deodorant when they hear the word aerosol,
but really an aerosol is just a bunch of particles suspended in the air.
Getting infected this way could happen by either being close up to a person,
called short-range aerosol transmission, or when you're far away from a person,
called long-range aerosol transmission.
animal studies, air sampling, and superspreading show that SARS-CoV-2 is transmitted by breathing in aerosols.
SARS-CoV-2 is an over dispersed pathogen, like SARS and measles.
This means that as few as 10% of people transmit 80% of the infections and many people barely transmit.
Most infections happen within households, where one or two other people may be infected within your own bubble.
Also unique to SARS-CoV-2 is that many transmissions are pre-symptomatic
or asymptomatic, so you literally have no idea who is infectious.
People are most infectious right before they have symptoms of the disease.
To prevent infections we must control both short-range and long-range airborne transmission.
Short-range is when you happen to be in close contact.
If you are as close as eight inches, you can get infected by a spray of large particles landing on your face.
Between eight inches and six feet, you will inhale the virus.
Long-range aerosol transmission also happens.
This is how super spreading occurs.
When you have a series of unfortunate events that happen all at once.
A highly infectious person is indoors for a long time shedding lots of virus and there are also many susceptible people in the same space.
The airborne virus begins to spread through the indoor space and builds up to high levels
because there is poor ventilation, minimal surface deposition, and no air cleaning.
Why is this important?
Because the way to minimize the risk is with different strategies.
You will never prevent short range aerosol transmission by improving your ventilation.
The only way to prevent it is by wearing a good fitting mask, staying socially distanced from the person,
and quarantining upon possible exposure.
And definitely when you start to show symptoms.
The closer you are to somebody, the more virus you will encounter.
So try to stay at least six feet apart.
To minimize the risk of long range aerosol transmission and super spreading
we must treat this with ventilation and air cleaning.
And still wear masks to reduce the amount of virus that you breathe in or that you may release.
Reduce the number of people indoors, so that the chance that somebody is infected goes down.
You cannot ventilate your way out of a short range transmission scenario,
but you must ventilate your way out of a long range aerosol transmission situation.
And always use multiple strategies in case one fails.