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Explained: How droplet evaporation affects spread of Covid-19

The study cites the example of Delhi in July, when temperature and RH were both high, yet cases saw a surge. The number of cases grew by more than half from 87,000 at the end of June to 1.35 lakh at the end of July.

Coronavirus news, coronavirus research, Covid moisture, Covid transmission through moisture, indian expressHealth workers take swab samples for Covid-19 testing at a government school in New Delhi. (Express Photo: Amit Mehra)

The effect of temperature and humidity on the novel coronavirus has been studied a number of times since the pandemic took hold. So have the fluid dynamics and heat transfer aspects of the evaporation of droplets.

A new study has now looked at evaporation of respiratory droplets — specifically those that contain virus. Conducted by University of Nicosia researchers, the paper is published in the American Institute of Physics journal Physics of Fluids.

Effect of relative humidity on coronavirus respiratory droplet cloud at same temperature and windspeed, and different humidity. (Talib Dbouk & Dimitris Drikakis/Physics of Fluid)

When temperature is high and relative humidity is low simultaneously, the study found a significant reduction in virus viability. On the other hand, when RH is high, then the distance travelled by the droplet cloud, and the virus concentration remain significant — at any temperature.

This, the study notes, is in contradiction with what was previously believed by many epidemiologists.

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The study cites the example of Delhi in July, when temperature and RH were both high, yet cases saw a surge. The number of cases grew by more than half from 87,000 at the end of June to 1.35 lakh at the end of July, according to government records.

The research took into account humidity, temperature, and wind speed. The researchers developed new theoretical correlations for the unsteady evaporation of coronavirus-contaminated saliva droplets. It introduced the thermodynamic properties of virions (the complete virus) as a liquid.

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The key finding is that evaporation is a critical factor for the transmission of the infectious particles immersed in respiratory clouds of saliva droplets.

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