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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.

Tehran, Feb 28: The coronavirus epidemic in Iran has cost 26 lives, the health ministry announced Thursday, with a vice president becoming the latest top official to be infected as the spread appeared to accelerate.

Health ministry spokesman Kianoush Jahanpour told a news conference that the tally of infections had risen to 245 with 106 more cases confirmed -- the highest number for a single day since Iran announced its first infections on February 19.

The Islamic republic has the highest death toll from the virus outside China, where COVID-19 first emerged.

Among the latest coronavirus sufferers is one of Iran's seven vice presidents, Massoumeh Ebtekar, who oversees women's affairs.

Ebtekar, a former spokeswoman for students who took 52 Americans hostage at the US embassy in Tehran in 1979, is being treated at home and members of her team have been tested, state news agency IRNA reported.

Mojtaba Zolnour, head of parliament's national security and foreign affairs committee, also contracted the virus, appearing in a video posted by Fars news agency saying he was in self-quarantine.

The cleric is a deputy for the Shiite holy city of Qom in central Iran where the country's first cases were detected.

According to media reports, among the deceased in Qom on Thursday was theologian Hadi Khroroshahi, who in 1981 was named Iran's first ambassador to the Vatican.

The announcement by Zolnour comes two days after another top official, deputy health minister Iraj Harirchi, head of the government's coronavirus task force, said he too had contracted the virus.

On Wednesday, Iranian authorities announced domestic travel restrictions for people with confirmed or suspected infections.

They also placed curbs on access to major Shiite pilgrimage sites, including the Imam Reza shrine in second city Mashhad and the Fatima Masumeh shrine in Qom.

Visitors to the shrines will be allowed to visit on condition they are provided "with hand-washing liquids, proper (health) information, masks", Health Minister Saeed Namaki said.

They must "not gather together in groups but just pray and leave", he said.

In a rare move, authorities announced the cancellation of the main Friday weekly prayers in Tehran, Qom and Mashhad as well as in the capitals of 22 of Iran's 31 provinces and other infected areas.

"All of these decisions are temporary and if the situation changes, we might intensify or ease them," Namaki said.