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Infectious Diseases

Updated 04 August 2020

Covid-19-infected droplets spread further in cold, humid climates

Scientists have found a new way of modelling the Covid-19 spread via airborne respiratory droplets.

  • A new mathematical model can help predict the spread of viral droplets in a pandemic setting
  • It shows that cold climates and high humidity slow the evaporation process, allowing the droplets to travel further
  • This is another reason why it's so important to wear a mask when leaving the house

Scientists have found a new way of modelling the Covid-19 spread via airborne and contact respiratory droplets.

Generally acknowledged as the main means of transmission for the coronavirus, respiratory droplets spread via talking, sneezing, coughing and singing. They contain small bits of proteins and pathogens that can be passed on to others if they inhale the droplets or touch infected surfaces.

A human can exhale droplets between 1 to 2 000 micrometres, which is why it's so important to know what the evaporation rate of the droplets is.

READ MORE | Worried about catching coronavirus from surfaces? The city you live in may matter 

New mathematical model

Researchers developed a multi-scale model that connects the infection dynamics of the pandemic and the real physics of droplets to measure the role that environmental factors play in the spread of the coronavirus via respiratory droplets. 

They used salty droplets and measured their evaporation time, looking at droplet cooling, heat and mass transfer and the crystallisation of the dissolved salt. 

The model uses a reaction mechanism and collision rate theory to map how quickly the droplets spread in different settings. 

Environment plays a factor

At 35 degrees Celsius and 40% humidity, a droplet could travel around 2.5 metres, but at five degrees Celsius and 80% humidity, the droplet's travel distance increases to 3.6 metres. 

Droplets are far more likely to evaporate faster in the absence of a surface contact, and while smaller droplets can evaporate in a fraction of a second, when they form a cloud they linger longer and are much more likely to infect another person. Bigger droplets have a bigger chance of falling to a surface before evaporating into the air.

Other factors also include the force by which the droplets are expelled, and how fast they decelerate due to lack of momentum. 

"It is prudent to mention again that although we have done the analysis for the single isolated droplet, in reality, coughing or sneezing involves a whole gamut of droplet sizes in the form of a cloud," the authors write.

READ | Physical distancing during the Covid-19 pandemic: Is a 2m gap enough?

Future applications

This model provides important insight into how respiratory transmission of not just the coronavirus, but any future dangerous viruses, help spread the infection.

"We find that the respiratory droplets exclusively contribute to the infection growth parameters and infection growth rate, which decrease with ambient temperature and increase with relative humidity."

This model highlights why it's so important to wear a mask, especially in colder and more humid countries, and could also be used to predict – to some degree – how an infection might spread under very specific conditions.

Scientific debate around airborne transmission

Airborne transmission of Covid-19 is a hotly debated matter in scientific circles, with scientists previously calling on the World Health Organization to acknowledge this mode of transmission more prominently than they have done so far. 

While WHO officials do not deny airborne spread, they reiterate that more research and definitive evidence is needed to confirm it as a major mode of transmission. 

READ MORE | Airborne spread of the coronavirus: Some scientists say yes, but WHO remains unmoved

Image credit: Pixabay