A new study has found that respiratory droplets from a cough or sneeze travel farther and last longer in humid, cold climates than in hot, dry ones.
Conducted by researchers from University of California San Diego (UCSD) Indian Institute of Science (Bengaluru) and University of Toronto, the study is published in the journal Physics of Fluids.
How does it arrive at these findings?
The researchers describe a new mathematical model that they say can be used to predict the early spread of respiratory viruses including Covid-19, and the role of respiratory droplets in that spread. The model combines the physics of droplets with the spread of a pandemic based on population-scale interaction. It examines how far and fast droplets spread, and how long they last.
“We have included detailed kinetics model regarding infection as the droplet travels/evaporates,” Professor Saptarshi Basu of IISc, one of the study authors, told The Indian Express. “This is based on molecular collision theory adapted from combustion. Key of this work was to link the droplet physics with the kinetic theory of infection to arrive at reaction or infection rate and subsequent growth.”
Collision theory predicts the rate of a chemical reaction based on the frequency of collisions between molecules. “How frequently healthy people are coming in contact with an infected droplet cloud can be a measure of how fast the disease can spread,” Professor Abhishek Saha of UCSD, one of the study authors, said in a statement.
Using droplets of a salt water solution (saliva is high in sodium chloride) in a levitator, the team applied models for chemical reactions and physics principles to to determine the size, spread, and lifespan of these particles in various environmental conditions.
So, how far do droplets travel?
Depending on weather conditions, it was found that some respiratory droplets travel between 8 feet and 13 feet away from their source before evaporating. At 35°C and 40% relative humidity, a droplet can travel about 8 feet. However, at 5°C and 80% humidity, a droplet can travel up to 12 feet. All this is without even accounting for wind.
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This means that without masks, six feet of social distancing may not be enough to prevent infection, the researchers said.
Should we worry about the winter?
Basu said the life of a droplet depends more on humidity than on temperature. “This means in high humidity (relative humidity) it survives longer and hence travels longer distance before evaporating or settling. Cold temperature will also prolong lifetime but not as much as humidity,” he said.
Are there other significant findings?
The study provides a size range for droplets that carry greater risk. It found that droplets in the range of 14-48 microns take longer to evaporate and travel greater distances. Smaller droplets evaporate within a fraction of a second, while droplets larger than 100 microns quickly settle to the ground.
Wearing masks would trap particles within this critical range.
What are the limitations of the model?
In a statement, Professor Swetaprovo Chaudhuri of University of Toronto referred to “idealized assumptions” and “variabilities in some parameters”. Basu said the next step is to relax a few simplifications and to include different modes of transmission.
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