Aerosols vs. Droplets
and why you should care about the difference
Why should you care about the difference between an aerosol and a droplet? Because it’s affected your life for the last 2 years. A number of our public health dictats have been partially based on the difference.
Droplets and aerosols are both little bits of water that travel through the air. Droplets are “big” and aerosols are “small”. There is no hard-and-fast cutoff in casual terms - it’s like the difference between a brook and a river, or a puppy and a dog. The transition is a grey area. Scientifically, aerosols are defined as particles that are 100 micrometers or less. Bigger than that and it’s a droplet.
If you’ve ever seen a dog shake after swimming, you’ve seen droplets. They fly in a given direction for a certain distance but fall relatively quickly. Run away a few steps and you can stay dry.
Smoke - aerosolized carbon - is the prototype aerosol that everyone can easily picture because it’s visible. Aerosol particles are very small and unlike droplets, can circulate in the air for long periods of time. They are not directional like droplets - they circulate. Larger aerosol particles settle out faster, smaller ones last longer. Many substances can aerosolize, from poisonous and harmful, to healthy and necessary things like water. There is always some water vapour in the air but we don’t typically see it unless it’s very thick (think clouds or fog).
Why it matters to COVID
When COVID first reared it’s spiky head, it was thought to be droplet-transmitted. Thus the 6-foot rule. 6 feet was felt to be a safe distance because droplets fall to the ground fairly quickly. Stay 6 feet from your dog if he is shaking, or 6 feet from someone spewing COVID droplets. Six feet away, it was thought, those nasty COVID viruses should be underfoot and not in your face by the time they reach you. Interestingly, 6 feet was not determined by scientific research. Most countries picked one metre. A few picked 2 metres. Some split the difference. If you weren’t in a metric country, The Science™ changed and droplets settled out in 3 or 6 feet, instead of one or two metres.
Viruses have different modes of transmission that work best for them. Some are mostly droplet-transmitted - influenza, most common colds, and RSV (which fills pediatric wards every winter) are examples. Others can pass on by aerosol and are generally more transmissible as a result of being suspended in the air much longer than droplets - measles is the prototype for an aerosol-transmissible virus. Some viruses are more efficiently transmitted by fomites (things we touch like countertops, toilets, or towels) - Norwalk viruses spring to mind (and to mouth!). Other viruses require close contact, either sexual or otherwise, as they are very unstable once they leave the host. (This is the reason HIV is not airborne, nor fomite-transmitted, as was originally feared. Almost forgotten now is the fact that Anthony Fauci, the major purveyor of COVID fear porn, was once the major purveyor of HIV fear porn.)
“…perhaps there will be a certain number of cases …who are just living with and in close contact with someone with AIDS, or at risk of AIDS, who does not necessarily have to have intimate sexual contact or share a needle, but just the ordinary close contact that one sees in normal interpersonal relations,” - Anthony Fauci
The details of why these viruses differ in their optimum mode of transmission have to do with how stable a virus is when it dries out, gets hit by UV light, or cools below body temperature. My understanding is that droplet-transmitted viruses need to stay “wetter” than aerosol-transmitted, and thus denature quickly if the water droplet they get transmitted on is too wee. (Some smart virologist could probably talk for an hour on this specific topic, and explain it without using the word “wee”.)
Even the best masks are less than perfect
When breathing out hard, or if the mask doesn’t fit perfectly, or if you have a beard, much of the air you breathe in or out can go around the edges of the mask - where none of the viruses get filtered. When medical staff are fitted for N95’s, we are expected to be clean-shaven. Even a day or two stubble makes the mask FAR less efficient, as do things like talking, turning your head, or looking up or down. They get less efficient when they are wet from your moist breath. Prior to COVID, N95’s were not suggested for all-day use, but rather for short periods of time in high-risk situations.
Beyond the fit issue, pore sizes vary greatly between various masks, so even if all the air goes through them, only some viruses get filtered. Aerosolized water vapour (presumably with viruses attached) can pass right through even several surgical masks. Cloth masks (which generally have larger pore sizes) are probably the least likely to be useful, which has even been recognized and stated publicly by some who support mask mandates - you know, the mandates that forced us to wear these “facial decorations” for the last 2 years.
Even an N95 mask can have pore sizes up to 65micrometers, while aerosols carrying COVID can be as small as 20 micrometers. This issue has spawned many memes, and may be one of numerous reasons that scientifically-sound real-world data does not show universal masking reduces COVID rates.
If masks mandates don’t work for droplets, how do they magically work for aerosols?
If you consider the physics of how masking works, they must logically be better at preventing droplet-transmitted viruses (like influenza) then they would at preventing aerosol-transmitted viruses (like COVID). So I was actually excited when the consensus changed from droplet to aerosol-spread for COVID, as I thought the universal mask (and especially shield!) mandates would be dropped. Decades of data already resulted in recommendations against universal masking for droplet-transmitted diseases. So how did The Science™ show that masks prevent COVID spread?
A medical intervention should be scientifically plausible
We wouldn’t send a firefighter into a burning building with a medical mask and a plastic face shield. That would make no sense, since smoke is an aerosol. Why would wearing masks and shields prevent transmission of an aerosol-transmitted virus? Aerosols gonna aerosol. They will find their way under, over, around, and through masks, as well as over and under face shields. Anyone who ever sanded drywall and can attest that even an N95 mask doesn’t keep that fine dust out of your nose very well. (PS: kudos to drywallers. I might have a fair amount of post-secondary education but drywalling is an art/science/skill that I never cottoned on to).
We wouldn’t make a rule that everyone should wear their shirts inside-out to reduce COVID transmission rates. To justify a universal masking rule – which is clearly more intrusive than an inside-out shirt rule, has a great impact on comfort, and clearly has negative side effects (more in a later post) - masking should not just plausibly help, but should have compelling scientific data supporting it. As I’ve laid out in this post, even plausibility is questionable, let alone evidence.
As with so many COVID-related issues, when considering masking we ignored plausibility, proof of efficacy, and side effect concerns, and instead jumped straight to mandates, shaming, and appeals to morality.
In my next article I’m going to back up the bus and look at the question: is there good data supporting the utility of universal masking?
Next Post: Don’t just do something, stand there! Evidence should inform policy, not vice-versa.