How long do aerosols stay airborne?

Just how long do SAR CoV-2 containing aerosols stay airborne once released into the air?

It’s an increasingly important question as the relevance of airborne transmission of novel coronavirus becomes apparent. Yet despite the science of aerosol dynamics being very well established, it’s surprising just how hard it is to find clear and understandable information on the settling rate of airborne particles.

This became very apparent to me when searching for a simple plot of settling velocity versus particle size that indicates just how slow or fast exhaled COVID-containing aerosols might stick around. I may be missing something but, apart from a bunch of old and quite technical plots and diagrams, there was pretty much nothing.

So I dug into my old aerosol files and created some!

I suspect that the plots below also fall into the category of “quite technical.” But hopefully someone will find them at least marginally illuminating.

They are all based on established aerosol dynamics and draw heavily on Paul Baron’s Aerosol Calculator (which in turn uses Bill Hinds’ still-excellent intro to aerosol technology), together with science that goes back to the mid-1800’s.

Settling Velocity

Figure 1 below shows the terminal velocity of unit density spherical particles in still air — the velocity they reach as they fall under the influence of gravity. This in turn allows an estimate of how quickly particles of different sizes will settle out of the air, and how long they’ll remain airborne after being released.

Settling Time

Figure 2 shows the time it takes for spherical, unit density particles to settle through one vertical meter in calm air.

Assuming this is a representative distance between the point of release and floors or desks (and to be honest, it’s a little on the low side), the horizontal axis provides a rough idea of how long particles of a given size are likely to hang around in the air — again, assuming the air is undisturbed.

Aerosol Concentration Reduction When the Air Isn’t Calm

Of course, in most rooms where people are expelling COVID-containing aerosol, the air (along with the particles) is constantly being disturbed. In this case, a rough idea of the rate at which aerosol concentration reduces over time through settling is given by a well-established model of stirred settling.

Using a ten times reduction in aerosol concentration as the benchmark, figure 3 below indicates how long this is likely to take as a function of particle size:

Using the Plots

Of course, these plots only provide first order estimates of how long particles will remain in the air, as other factors will come into play in practice, including ventilation rates. Yet they do illustrate just how long particles remain airborne, and in the absence of good ventilation, just how long it takes to reduce airborne concentrations.

And the clear indication is that aerosol particles smaller than 10 micrometers or so hang around for some time — certainly long enough to be carried between people who are some distance apart.

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A PDF of this article is available here.

The science behind the calculations can be found in multiple aerosol dynamics textbooks (I still use Hinds’ Aerosol Technology as my primary go-to) – it’s also covered in my aerosol dynamics lecture notes, which are available here. And the calculations themselves are all available in Paul Baron’s Aerosol Calculator.

And given how stressed out people seem to get these days about who’s an expert and who’s not, I thought I should probably add that I cut my scientific and academic teeth on aerosol physics, getting my PhD in this area and leading research groups in the UK and US focused on aerosol sampling. And even though I focus on a myriad other areas these days, I know my aerosol stuff 😊