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London, May 20: The current physical distancing guidelines of 6 feet may be insufficient to prevent COVID-19 transmission, according to a study which says a mild cough in low wind speeds can propel saliva droplets by as much as 18 feet.

Researchers, including those from the University of Nicosia in Cyprus, said a good baseline for studying the airborne transmission of viruses, like the one behind the COVID-19 pandemic, is a deeper understanding of how particles travel through the air when people cough.

In the study, published in the journal Physics of Fluids, they said even with a slight breeze of about four kilometres per hour (kph), saliva travels 18 feet in 5 seconds.

"The droplet cloud will affect both adults and children of different heights," said study co-author Dimitris Drikakis from the University of Nicosia.

According to the scientists, shorter adults and children could be at higher risk if they are located within the trajectory of the saliva droplets.

They said saliva is a complex fluid, which travels suspended in a bulk of surrounding air released by a cough, adding that many factors affect how saliva droplets travel in the air.

These factors, the study noted, include the size and number of droplets, how they interact with one another and the surrounding air as they disperse and evaporate, how heat and mass are transferred, and the humidity and temperature of the surrounding air.

In the study, the scientists created a computer simulation to examine the state of every saliva droplet moving through the air in front of a coughing person.

The model considered the effects of humidity, dispersion force, interactions of molecules of saliva and air, and how the droplets change from liquid to vapour and evaporate, along with a grid representing the space in front of a coughing person.

Each grid, the scientists said, holds information about variables like pressure, fluid velocity, temperature, droplet mass, and droplet position.

The study analysed the fates of nearly 1,008 simulated saliva droplets, and solved as many as 3.7 million equations.

"The purpose of the mathematical modelling and simulation is to take into account all the real coupling or interaction mechanisms that may take place between the main bulk fluid flow and the saliva droplets, and between the saliva droplets themselves," explained Talib Dbouk, another co-author of the study.

However, the researchers added that further studies are needed to determine the effect of ground surface temperature on the behaviour of saliva in air.

They also believe that indoor environments, especially ones with air conditioning, may significantly affect the particle movement through air.

This work is important since it concerns safety distance guidelines, and advances the understanding of the transmission of airborne diseases, Drikakis said.

May 30: A total of 513 domestic flights carrying 39,969 passengers were operated in India on Friday, Civil Aviation Minister Hardeep Singh Puri said on Saturday.

Domestic services resumed in India after a gap of two months due to the coronavirus lockdown. Indian carriers have operated a total of 1,827 flights till Thursday -- 428 on Monday, 445 on Tuesday, 460 on Wednesday and 494 on Thursday.

Puri said on Twitter on Saturday: "Day 5. 29th May till 2359 hrs. Departures 513. 39,969 passengers handled. Arrivals 512. 39,972 passengers handled." A departure is counted as a flight during the day.

During the pre-lockdown period, Indian airports handled around 3,000 daily domestic flights, aviation industry sources said.

In February, when the lockdown was not imposed, around 4.12 lakh passengers travelled daily through domestic flights in India, according to Directorate General of Civil Aviation (DGCA) data.

Airports in West Bengal, Andhra Pradesh, Maharashtra, Telangana and Tamil Nadu have been allowed to handle a restricted number of daily flights as these states do not want a huge influx of flyers amid the rising number of COVID-19 cases.

While domestic services resumed in Andhra Pradesh on Tuesday, they restarted in West Bengal on Thursday.

Though domestic flight operations across the country began on May 25, they could not be restarted in Kolkata and Bagdogra as the West Bengal machinery was involved in relief and restoration work after cyclone Amphan's devastation.

A total of 16 asymptomatic passengers on seven different flights, including 13 who travelled by IndiGo, have tested positive for COVID-19 since the resumption of domestic air services.