I was recently thinking over the recent guidance on social distancing during the current COVID-19 pandemic. Advice from health professionals and Government advisors suggests a 2-meter distance is required to limit the spread of the Coronavirus, as it is unlikely that droplets from a cough or a sneeze will travel further. This made me think, as a scientist who is somewhat obsessed with material characterization, of when I had to measure coughs and sneezes!

This was one of the strangest experiments I’ve ever performed. I was doing a series of US customer visits with a colleague. We were in discussion with a Government institute who wanted to buy a Spraytec to understand the droplet size produced by a human cough or sneeze. We thought such an experiment was possible in theory, but his potential customer asked him to try it.

We were doing some training on a customer’s Spraytec who agreed to let us use their instrument. They would regret this!

First the cough

I did have a bit of a cold, so I was able to cough into the measurement zone of the instrument, allowing us to record this particle size distribution:

Figure 1: Droplet size distribution of a cough

My cough contained some large particles. The median particle size, by volume, was approximately 450 microns.

Then the sneeze

We wanted to test if a sneeze would produce a different particle size. Neither of us could sneeze on demand, so we needed a plan! The next morning, we took some pepper from the hotel’s breakfast buffet and set off to the customer’s site very early to try our experiment. My colleague inhaled pepper…but no sneeze. He inhaled more pepper, until his nose felt like it was going to explode, but still no sneeze! However, as the lab workers started to enter, and the sneeze finally appeared as well, with considerable force We clicked “Measure” and he sneezed towards the laser beam, with no effort at all to cover his face. One of the lab people saw this and exclaimed: “THAT WAS SO DISGUSTING!!“. We told her it was all in the name of science, and we spent 10 minutes cleaning the Spraytec and lab bench with disinfectant and apologizing!

The particle size distribution we recorded for a sneeze is shown here:

Figure 2: Droplet size distribution of a sneeze

As you can see, the droplets ejected during a sneeze are considerably smaller than a cough. This is, perhaps, no surprise as the velocity of air passing through the respiratory tract is much higher when sneezing. A small droplet size will mean that the droplets will hang around in the air for longer. As a result, although a 2-meter distance should still reduce the amount of droplets reaching other people, there is still an increased risk of other people inhaling the droplets. Recently published data by Lydia Bourouiba at MIT (Jama Insights 26th March) confirm this result and show that the turbulent gas associated with coughs can potentially carry very fine droplets over 7 or 8 meters.

So, what was the outcome of our experiment?

Well, the government institute was so impressed that they acquired a Spraytec to support their research. And I was left with a greater understanding of why it is important to cover your mouth and nose when you cough or sneeze, just as we are being advised to do now as we try and control the spread of the coronavirus.

COVID-19 is a very serious health threat, and we hope everyone is staying safe. But it is good to know there’s a lot of top-notch research being done to beat this and I’m sure we will succeed!

Further reading