Airborne Transmission. What does it really mean?
Recently a good friend asked me about about the airborne transmission of SARS-2-COV virus responsbile for COVID19. She wanted to understand why COVID19 transmission messaging had changed and what she should do to protect her family. This is my summary of the state of our knowledge to a general audience.
The SARS-2-COV virus is about 0.12 microns in diameter and has three transmission routes.
- Fomites (surfaces). This is the wash hands, wipe surfaces and wait 72 hours advice.
- Droplets. This is the stay six feet apart as droplets fallout of air by that time (not true, more later).
- Airborne. This is wear mask advice and stay out of crowded poorly ventilated indoor spaces.
All in all what I write below doesn't change any of the advice above. We need everyone to do all three sets of things.
So what is a droplet? Well, infectious disease experts have based their droplet definition on a 1934 paper by R. F. Wells. The field misinterpreted the paper to say any particles larger than 5 microns will fall out of air in 2 m. They essentially ignore all the particles that are smaller than that size. The main reason was back in 1934 it was really hard to measure particles smaller than 5 microns. The infectious disease field has been highly reluctant to revisit this rule of thumb of ignoring smaller particles. In reality, when we speak, cough, sing, play musical instruments we produce a continuum of particle sizes, from 0.01 microns up to 100 microns. All of these behave differently in indoor air. Also most virus particles are really liquids that contain the virus and a bunch of salts and water. So in reality particles in the 0.5-5 microns are likely able to carry infectious virus particles, but have been ignored by the traditional infectious disease community. As with most particle sources, the greatest number of particles from our mouth tends to be the smaller sizes, while the greatest mass tends to be the larger sizes.
So let's go back to the 5 micron particles. That is unlikely to settle in 2 m. Mainly due to 1) the hot jet coming out of someone's mouth, 2) the mixing indoors that moves the particle along with air currents (you can see about down to 5-10 micron particles when you see them moving about in a sunbeam coming through a window. You will notice they don't fall out right away). Particles that are smaller will stay aloft even longer. In fact 1 micron particles likely will be removed by the buildings air change faster than they will settle out. With many poorly ventilated spaces this means the particles may linger for hours indoors. How long will the particle be in the air? Think of popping popcorn in the microwave and then see how long you can smell it in your house. Or how long can you smell perfume after the wearer has left your space (elevator) or the smell of a cigarette after someone has been smoking in a room. In other words how long small virus laden particles will be indoors is highly dependent upon the indoor space. This is the point that the 239 particle scientists made in July 2020 when they wrote a letter to WHO asking them to acknowledge airborne transmission.
What is the evidence of airborne transmission of smaller particles? There are more than a dozen cases of super spreader events happening in restaurants, choirs, churches, and bars that can't be explained soley by the transmission of large droplets in less than 2 m. If you think about most of these spaces they have two things in common. Poor ventilation and lots of people. The more asymptomatic people emitting in a space the more virus laden particles will be in the air. The longer the air sticks around and the longer you stick around, the greater chance you have of inhaling the particle.
Where does that leave us? Well it leaves us acknowledging that there is a continuum of particle sizes that we need to be worried about. The issue is that the dominant route is likely to be different in different scenarios. When someone coughs in a doctor's face who is trying to intubate them, large droplets (100 microns) with lots of virus copies are a big worry. When you go to a store that has poor ventilation and lots of unmasked people, then you probably are getting exposed to lots of smaller (0.5-5 micron) particles that may or may not have enough infectious copies of the virus to infect you. But it is unlikely that you will get exposed in the store to the really big droplets that they worry about in hospitals.
And that gets us to masks and avoiding crowded, under ventilated locations (bars, classrooms) or the "airborne route." Will your clothe mask eliminate the risk of airborne infection? Absolute not. If you have a good seal, and 95% of face covers I see don't, your cloth mask may be 50% effective at removing the 2-5 micron particles. Not great, but don't let perfect get in the way of good enough for now. But masks will also reduce the number of particles that you emit if you are asymptomatic, lessing your transmission to others in indoor spaces. This is because unlike breathing in where air and small virus laden particles will sneak around your mask edge, when you cough the air jet will send a significant portion of the particles into the mask where they get removed. Less virus containing particles getting emitted into the air means less dose for the receptor to inhale. There is currently no dose-response relationship for SARS-COV2 as it is a hard thing to quanitify. Do masks work? Yes. Hong Kong didn't shut down only mandated masks, isolation for those shown at risk for contact tracing (7 million people and only 2500 cases, although cases are currently rising), Taiwan bulk produced masks in bulk starting in January and has had 460 cases (numbers through July 27, 2020). The list goes on. But masks are the only one thing.
So should you do anything different now that you know it can be airborne? No. Wash hands, wipe surfaces, stay apart, wear masks in all indoor spaces, avoid crowds and poorly ventilated spaces.
However, things do change a little bit when we are forced to go to places with crowds and poor ventilation (like some classrooms). You can actively remove 0.5-5 microns particles with a HEPA filter or inactivate the virus on them with UV light. I would avoid most UV light solutions that are not attached to a portable air cleaner at this point though as there are too many untrained people selling products that haven't been tested and the proper wavelength to kill the virus can be harmful to humans.
That leaves ventilation and filters. Schools with big budgets and over-designed HVAC systems can increase the outdoor air fraction (to a point, most can not handle 100% outdoor air) or install MERV 13 filters which will remove a vast majority of particles in the 0.5-5 micron range (all filters, including MERV 13 have the most difficulty removing particles around 1 micron). Since that won't be an option for most schools, I have recommended that my teaching sister and sister-in-law get portable HEPA filters (not plasma, ESP or any other "add-on" device) with the highest clean air delivery rates (CADR) and lowest noise they can find for any in-person teaching they have to do in the fall. These effectively will reduce the 0.5-5 micron particle concentrations, but will not eliminate any transmission risk. I might donate one to my kids classrooms this fall.
In summary, wash hands, wipe surfaces, stay apart, wear masks in all indoor spaces, avoid crowds and poorly ventilated spaces. Stay safe.