Post #617: Slipstream, a followup on aerosol transmission of COVID-19

Posted on April 9, 2020

Four cigars (pr:  SEE-gars).  Source:

I have give David Patariu a cigar for this one.  He gave me the most apt analogy for what I’m about to bring to your attention.

If somebody smoked a cigar in your house, you’d still be able to smell it long after the visible smoke had cleared.  That’s because there would still be some tiny smoke particles floating in the air.  Not enough to see, but enough to smell.

And that’s pretty much how aerosol transmission works.  There are tiny (under 5 micron) droplets floating in the air, produced as a result of anything from breathing (relatively few) to singing (lots).  Not enough to see, but maybe enough to spread viral infections such as COVID-19.

I’ve already discussed this in detail in several posts, and it’s summarized on the front page of this website.  But the upshot is that you should think of this the same way you’d think of avoiding second-hand cigarette smoke.

If the person talking to you was a smoker, they’d be blowing smoke in your face.  If they were singing at full volume, they’d be putting out clouds of cigarette smoke.

And, unsurprisingly, if they are doing any exercise that makes them breath deeply, they’ll be putting out great big clouds of smoke.  And if you happen to be walking/running/biking behind somebody who is sick (and doesn’t know it yet), particularly if they are breathing hard, you need to be aware of the potential for “slipstream” transmission of aerosol virus.

The upshot is that, among all our social distancing rules, I want to add a new one.  If you’re on the W&OD say, you really don’t want to be running or biking or, really, even walking closely behind anyone else. For sure, not if it’s a still day.  And, for sure, not if the wind is coming from directly ahead of you.  The reason is the potential for virus in droplets and aerosols to hang up in a person’s slipstream.  Combine that with higher droplet/aerosol production when breathing heavily, and … you get the picture.

Even a slow walk (4 KPH, about 2.5 MPH) is enough to create such a slipstream effect.  If you want to, you can think of it as the person ahead of you walking out of his or her own cloud of droplets, and you walking into that cloud before they can all settle to the ground.

Here’s an easy way to think of it:  The 6′ social distance is for when you’re standing still.  The faster you are moving, the larger that distance needs to be between you and the person you are walking/biking/jogging behind.  For the simple reason that your movement carries you into the droplet stream of the person ahead of you.  Six feet only works when you’re stationary.  That’s obvious, really, when somebody points it out to you.  But I needed to have this pointed out to me, before I got it.

Or just think of it as the same as walking behind somebody who is smoking a cigarette.  Walk too close, or walk into the wind, and you’ll smell the smoke.  More-or-less the same effect here.

That’s courtesy of a new piece of research out of Belgium.  They concentrate on droplets (conventionally, larger than 5 microns) but the same rules apply.  Some of these droplets (and for sure, aerosols) can get hung up in the slipstream of a walker, runner, or bicyclist, and land on the person behind them.

In general, the slipstream effect is so large that racing bicyclists routinely take advantage of it.  As do runners.  That’s why bike teams travel in a straight line, one behind the other.  You just need to be aware, now, of the theoretical possibility that if the person ahead of you in the slipstream has COVID-19 (and presumably doesn’t know it yet), you can be exposing yourself to it.

The advice from Brussels is that if you go jogging or biking with others, side-by-side is safer than single file.  No proven cases of transmission yet, but it’s not clear how you would ever prove one anyway.

You can see a popular writeup of the research at this link.