What is the fate of a droplet? According to scientists, the indiscernible micro-particles ejected in any cough or sneeze could be travelling much further than we think, and far beyond the limits set by physical distancing requirements.
Given the way 2020 has gone, it's not the first time we've heard warnings like this. All year long, scientists have been telling us just how far coughs can spread, tracking droplet dispersion with stunning levels of precision, and cautioning that physical distancing isn't a silver bullet.
But 2020 isn't over yet. While vaccines are now beginning to emerge, the COVID–19 pandemic is still surging in the US and in places all across the world.
For now, we basically can't have enough science about how the coronavirus is spreading between infected people – especially if sneezes and coughs can propel the pathogen further than we generally think.
Unfortunately, that is the central takeaway from new simulations conducted by scientists from Loughborough University in the UK.
"In the majority of our analyses, the predictions made by our model suggest that the largest droplets consistently exceed the horizontal ranges of two metres [6.5 feet] from the source before settling to the ground," explains mathematician Emiliano Renzi.
In the new work, Renzi and student Adam Clarke modelled the fluid dynamics of expiratory clouds ejected during coughing and sneezing.
The pair found that the evolving shape of a cloud of moisture ejected by a nozzle sprayer matches with a theoretical phenomenon in physics known as buoyant vortex rings, characterising the turbulence and circulation of a torus-shaped vortex in a fluid or gas.
Evolution of buoyant cloud vortex generated by a nozzle sprayer. (Renzia & Clarke, Physics of Fluids, 2020)
The same sort of dynamic is evident in mushroom clouds from nuclear explosions. Its hypothetical existence here suggests that tiny, potentially virus-laden particles in coughs and sneezes could be reaching much further than we tend to realise.
"In some cases, the droplets are propelled in excess of 3.5 metres (11.5 ft) by the buoyant vortex, which acts like a mini atomic bomb," Renzi says.
"Our model also shows that the smaller droplets are carried upwards by this mini-vortex and take a few seconds to reach a height of 4 metres (13 ft). At these heights, building ventilation systems will interfere with the dynamics of the cloud and could become contaminated."
In some cases, the smallest studied droplets (with a diameter of 30 micrometres), which are more easily propelled by the turbulence of the moisture cloud, reached heights greater than 6 metres (almost 20 ft), and remained suspended in the air for the duration of the simulation.
"For diseases capable of transmission via aerosol inhalation, these results begin to show the extent to which droplets may travel in relatively short timescales," the authors write in their paper.
The findings also suggest that the initial direction of the expiratory cloud is a major factor in determining its potential spread. In short, tilting your head downwards while you sneeze or cough is likely to greatly reduce the airborne spread of droplets upwards and across a room.
The researchers acknowledge that their model is based on a number of mathematical assumptions, and point out that there is much we don't yet know about the potential infectiousness of the smallest droplets humans exhale.
Nonetheless, there's more than enough here to warrant further scientific investigation, and to perhaps inform even more changes to how we act and position ourselves around other people, the team thinks.
"Guidelines suggesting two metres physical distancing limits may not be adequate to prevent direct transmission via droplets of large size," Renzi says.
"We recommend behavioural and cultural changes in populations to direct coughs toward the ground, in addition to wearing face coverings, which could help mitigate the risk of short-range direct transmission of respiratory viruses."
The findings are reported in Physics of Fluids.