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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.

Washington, May 20: The United States recorded another 1,536 coronavirus deaths over the past 24 hours, the Johns Hopkins University tracker said.

That figure, tallied as of 8:30 pm (0030 GMT), raises to 91,845 the total number of COVID-19 deaths in the US.

The US tops the global rankings both for the highest death toll and the highest number of infections, with more than 1.5 million cases.

Seoul, Apr 26: A train presumed to belong to North Korean's Kim Jong-un has been spotted at a station in the state's eastern coastal town of Wonsan amid speculation about the leader's health, a US monitor said on Sunday, citing commercial satellite imagery on the region, Yonhap news agency reported.

According to 38 North-- a website devoted to analysis about North Korea, the imagery showed a train "probably belonging to Kim Jong Un parked at the Leadership Railway Station servicing his Wonsan compound since at least April 21."

"The approximately 250-metre long train, although partially covered by the station's roof, can be seen at a railway station reserved for use by the Kim family. It was not present on April 15 but was present on both April 21 and 23," it said.

"The train's presence does not prove the whereabouts of the North Korean leader or indicate anything about his health, but it does lend weight to reports that Kim is staying at an elite area on the country's eastern coast," it added.

The report came as rumours about his health have spread as Kim apparently skipped an important annual visit to the Kumsusan Palace of the Sun on the occasion of the April 15 birthday of late state founder and his grandfather, Kim Il-sung.

CNN intensified the speculation by reporting earlier last week that the United States is looking into intelligence that Kim is "in grave danger" after surgery.

Seoul officials have disputed recent media reports about Kim, saying there have been no unusual signs from the North. Some said that Kim is presumed to be staying in Wonsan for unspecified reasons.

Washington has also dismissed the reports, with US President Donald Trump calling such reports "incorrect" in a press briefing late last week.

On Saturday, other media reports stated that China has dispatched a team of medical doctors and officials to North Korea "to advise on" Kim, citing multiple unnamed people familiar with the situation.

North Korea's state media, however, has not made any mention of Kim's public activity for two weeks since he was last seen in April 11 presiding over a major party meeting, though it has reported on his handling of routine state affairs, such as sending diplomatic letters.
But not all speculation has proven to be false.

When he was absent from public for about a month in 2014, speculation arose about his health and a political crisis in the secretive state. He later reemerged with a cane and a limp reportedly after having a cyst removed from his ankle.

The 36-year-old leader is known to have various health problems apparently caused by obesity and heavy smoking. He took office as leader of the communist state after his father, Kim Jong-il, died of a heart attack in late 2011.