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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 31: US President Donald Trump said Saturday he will delay the G7 summit scheduled to take place in June and invite other countries -- including India and Russia -- to join the meeting.

"I don't feel that as a G7 it properly represents what's going on in the world. It's a very outdated group of countries," Trump told reporters on Air Force One.

He said he would like to invite Russia, South Korea, Australia and India to join an expanded summit in the fall.

It could happen in September, either before or after the UN General Assembly, Trump said, adding that "maybe I'll do it after the election."

Americans head to the polls in early November to choose a new president, with Trump keen for a return to normalcy after the coronavirus pandemic and a healthy economy as voters cast their ballots.

Describing the event as a "G-10 or G-11", Trump said he had "roughly" broached the topic with leaders of the four other countries.

Leaders from the Group of Seven, which the United States heads this year, had been scheduled to meet by videoconference in late June after COVID-19 scuttled plans to gather in-person at Camp David, the US presidential retreat outside Washington.

Trump created suspense last week, however, when he announced that he might hold the huge gathering in-person after all, "primarily at the White House" but also potentially parts of it at Camp David.

German Chancellor Angela Merkel became the first leader to decline the in-person invitation outright.

"Considering the overall pandemic situation, she cannot agree to her personal participation, to a journey to Washington," her spokesman said Saturday.

Her response followed ambivalent to positive reactions to the invitation from Britain, Canada and France.

The 65-year-old chancellor is the oldest G7 leader after Trump, who is 73. Japan's Shinzo Abe, also 65, is several months younger than Merkel. Their age puts them at higher risk from the coronavirus.

The G7 major advanced countries -- Britain, Canada, France, Germany, Italy, Japan and the United States -- hold annual meetings to discuss international economic coordination.

Russia was thrown out of what was the G8 in 2014 after it seized Ukraine's Black Sea peninsula of Crimea, an annexation never recognized by the international community.

The work of the G7 is now more important than ever as countries struggle to repair coronavirus-inflicted damage.

The White House had previously said the huge diplomatic gathering would be a "show of strength" when world economies are gradually reemerging from shutdowns.

The United States is the worst-hit country for COVID-19 infections, recording more than 1.7 million cases and over 103,680 deaths.