Airborne vs respiratory droplet diseases

[Dr. Anonymouss]

I have been told that SARS-CoV-2 is an airborne virus, but I've also been told it is respiratory droplet. I really got this stuck in my head after this hospital where I work has deemed it appropriate for all staff to wear surgical masks and not N95s. After doing some research I found out that the general consensus is that pathogens greater than 5 microns are respiratory droplet while those less than 5 microns are small enough to be aerosolized. The SARS-CoV-2 is 50-200 nanometers in diameter or .05-.2 microns which means it is most definitely aerosolized. But by this same logic there are many other diseases which should be considered airborne, yet aren't. I can think of Ebola, pertussis, and strep throat to name a few. The pertussis pathogen has a size of .2-.5 microns, yet the NCBI states there has been no controlled studies in which this bacteria shows airborne transmission. Streptococcus pyogenes also has a size less than 2 microns. I know that this bacteria can form long chains which would be my only rational for why it is not an airborne pathogen. Lastly, Ebola is around .08 microns normally, but can also be between .8-1 micron. I just can't understand why some of these pathogens which are small enough to be transmitted on airborne particles are deemed respiratory droplet diseases. I know this question should be directed toward a pathologist, but when I get something in my head I can't get it out and this has been bugging me. If anyone has some ideas or explanations please enlighten me.

Airborne diseases sizes

Influenza (both) .08-.12 microns
Varicella .18 -.2 microns
measles .1 micron
anthrax .9-1.5 microns
TB 2-4 microns
Mumps .2 microns
Diphtheria 1 micron

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[Patros]

Nothing in life is simple, and unfortunately that includes the SARS-CoV-2 virus. You can't reduce the airborne vs. droplet transmission modes to just the size of the virions, it also depends on other variables, like where the infectious fluid is coming from (e.g. respiratory tract) and how well the pathogen survives drying. Also, the 5uM refers to droplet size, and there isn't a magic line where we can separate droplets that can be suspended in the air and those that won't.

As far as Cov-2 transmission is concerned, most of the guidelines from the WHO and CDC are based on empiric observation, and not so much on controlled experiments, so we aren't yet 100% sure exactly how it works. From what we can tell, the Cov-2 virus is transmitted primarily through large droplets that are not inhaled but physically transferred, but it can also be transmitted through close-range aerosols that can be inhaled (i.e. airborne). It depends on the proximity of an infected person to a susceptible person, but both modes of transmission are possible. See this commentary here for a detailed discussion.

Almost all viruses that infect the respiratory tract have some transmission due to aerosolized droplets. For some pathogens like measles, this is the main mode of transmission and can occur in much longer ranges than Cov-2. But for Cov-2, airborne transmission is probably not the main mode of transmission in the general public. Instead, droplet transmission where someone touches a contaminated surface and then touches their mucosa is probably much more common. Healthcare workers will of course be in much closer range to infected individuals and for longer amounts of time, so aerosols are definitely a concern in a clinical environment.

 

[Money-Moniker]

The reason there is a divergence of viral infectivity through aerosol despite similar sizes is primarily due to differing ability to survive in the aerosolized form. When in a droplet, the virus particle is in a somewhat hospitable environment, while in the aerosolized form, it is just hanging out there in the air, susceptible to desiccation and other means of destruction. Droplet is the oasis, aerosol is the Sahara, basically.

With that said, wearing a mask is better than no mask, and the propaganda bombardment that "masks only work to protect others if you're already sick, they don't protect you bro" is just that, propaganda. I guess it theoretically helps direct masks to those who are at highest risk by trying to trick the sheep into not chasing after masks, but from a selfish, I don't want to get infected perspective, wear the damn mask if you can get your hands on one. It's not going to keep out every single viral particle but if your infectious dose is cut by 90% you're probably less likely to get infected/seriously ill.

 

[ACSurgeon]

A big part of the recommendation to wear surgical masks rather than N95 is because PPE shortage. We don’t have the luxury of wearing the most protective PPE for all these patients. We reserve that for procedures that are high risk.