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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.

Kabul, May 11: Four back-to-back roadside bombs exploded in a northern district of Afghanistan's capital Kabul on Monday, wounding four civilians including a child, police said. Kabul police spokesman Ferdaws Faramarz said a clearance team was at the site of the attacks.

Militants have carried out several roadside bombings and rocket attacks in Kabul and other parts of the country in recent weeks, but Monday's four consecutive explosions appeared to be the first coordinated effort for some months.

The Taliban has not carried out any large attacks in the city since they signed a landmark withdrawal deal with the US in February, meant to pave the way for peace in the country. No group has claimed the attacks. The explosions come as authorities are trying to impose a lockdown in the capital to curb the spread of coronavirus in the country.

Paris, May 10: The number of coronavirus cases worldwide topped four million as some of the hardest-hit countries readied Sunday to lift lockdown restrictions despite concern about a second wave of infections.

Governments around the world are trying to stop the spread of the deadly disease while scrambling for ways to relieve pressure on their economies, which are facing a historic downturn with millions pushed into unemployment.

Despite the intense political pressure to reopen, nations are also keen to avoid second waves of infections that could overwhelm healthcare systems, with reminders over the weekend of the threat posed by the virus.

In the United States, media reported Saturday that the nation's top infectious disease expert, Anthony Fauci, was among three members of the White House coronavirus task force who will self-isolate after potential exposure.

And in South Korea, the capital Seoul shut all bars and clubs on Saturday as more than 50 cases were linked to a man who tested positive after spending time in one of the city's busiest nightlife districts.

Despite the risks, some governments in hard-hit Europe have said are signs of progress that justify cautious steps towards normality.

Officials in France on Saturday said the day's death toll of 80 was the lowest since early April, while nursing home fatalities also fell sharply as the nation prepared to relax curbs on public movement imposed eight weeks ago.

The easing, to begin Monday, has brought mixed reactions.

"I've been scared to death" about the reopening, said Maya Flandin, a bookshop manager from Lyon. "It's a big responsibility to have to protect my staff and my customers."

French health officials have warned that social distancing must be kept up even as restrictions are eased.

In Spain, about half the population will be allowed out on Monday for limited socialisation, and restaurants will be able to offer some outdoor service as the country begins a phased transition set to last through June.

With lingering fears of a resurgence, authorities excluded Madrid and Barcelona -- two COVID-19 hotspots -- from the first phase.

Belgium is also easing some restrictions on Monday, and in some parts of Germany, bars and restaurants reopened on Saturday with further easing set for Monday.

In Britain, Prime Minister Boris Johnson is expected Sunday to lay out a plan for the nation to emerge out of its current lockdown.

Media reports have suggested that Britain may introduce a mandatory 14-day quarantine for international arrivals to stop the spread of the virus.

Global economic figures are pointing to the most acute downturn in nearly a century, with businesses forced to shut and supply lines badly disrupted, and pressure is growing on leaders around the world to find a way out as the worldwide death toll topped 277,000 and infections crossed four million.

In the United States, the country with the highest death toll and where more than 20 million people have lost their jobs, President Donald Trump has insisted that next year would be "phenomenal" for the economy, urging reopening despite the virus still claiming well over 1,000 lives daily in the country.

The scale of the challenge was brought in sharp focus over the weekend as US media reported that top disease expert Anthony Fauci, who has become the trusted face of the government response to the pandemic, is going to self-isolate after possible exposure to an infected White House staffer.

Fauci told CNN that he will undergo a "modified quarantine" as he had not been in close proximity to the staffer, the network reported. He will remain at home teleworking, and will wear a mask for two weeks.

Robert Redfield, the director of the Centers for Disease Control and Prevention, and Stephen Hahn, the commissioner of the Food and Drug Administration, will also self-isolate, CNN added.

All three will still testify at Tuesday's Senate coronavirus hearing, with Redfield and Hahn participating via video link, according to Senator Lamar Alexander, chairman of the chamber's health committee.

It is believed Fauci will attend wearing a mask, CNN reported.

President Trump has faced sharp criticism from his predecessor Barack Obama, who said on a leaked tape that Trump's handling of the crisis was an "absolute chaotic disaster".

With people wearying of being indoors and under economic pressure, anti-lockdown protests have been held in a number of countries in recent weeks, with some demonstrators arguing that such restrictions violate their rights and others promoting conspiracy theories about the pandemic.

Ten people were arrested and a police officer injured in Melbourne, Australia, on Sunday in the latest such protest, where around 150 people gathered to demand an end to the shutdown.

Participants were promoting a number of conspiracy theories, such as linking 5G cellular communications to the disease.

Australian chief medical officer Brendan Murphy said there was "a lot of very silly misinformation out there", including the 5G allegation.

"I have unfortunately received a lot of communication from these conspiracy theorists myself," he said.

"It is complete nonsense. 5G has got nothing at all to do with coronavirus."