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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.

Apr 26: The Chinese city of Wuhan, where the global coronavirus pandemic began, now has no remaining cases in its hospitals, a health official told reporters on Sunday.

"The latest news is that by April 26, the number of new coronavirus patients in Wuhan was at zero, thanks to the joint efforts of Wuhan and medical staff from around the country," National Health Commission spokesman Mi Feng said at a briefing.

The city had reported 46,452 cases, 56% of the national total. It saw 3,869 fatalities, or 84% of China's total.

Jul 1: Hong Kong police moved swiftly on Wednesday against protesters gearing up for the first rally since the introduction of sweeping security legislation, making their first arrest under it and warning of punishment for pro-independence material.

Beijing on Tuesday unveiled the details of the much-anticipated law after weeks of uncertainty, pushing China's freest city and one of the world's most glittering financial hubs onto a more authoritarian path.

As hundreds of protesters gathered downtown for an annual rally marking the 23rd anniversary of the former British colony's handover to China, riot police used pepper spray to arrest at least two people, while one metro station closed.

Police, who earlier banned the rally, cited the law for the first time in confronting protesters and they also made their first arrest under it - a man holding a flag advocating independence.

"You are displaying flags or banners/chanting slogans/or conducting yourselves with an intent such as secession or subversion, which may constitute offences under the ... national security law," police said in a message displayed on a purple banner.

The law will punish crimes of secession, subversion, terrorism and collusion with foreign forces with up to life in prison, heralding a more authoritarian era for the Asian financial hub.

China's parliament adopted it in response to months of pro-democracy protests last year triggered by fears that Beijing was stifling the city's freedoms, guaranteed by a "one country, two systems" formula agreed when it returned to Chinese rule.

Authorities in Beijing and Hong Kong have repeatedly said the legislation is aimed at a few "troublemakers" and will not affect rights and freedoms, nor investor interests.

But critics fear it will crush the freedoms that are seen as key to Hong Kong's success as a financial centre.

"With the release of the full detail of the law, it should be clear to those in any doubt that this is not the Hong Kong they grew up in," said Hasnain Malik, head of equity research, Tellimer in Dubai.

"The difference is that U.S. and China relations are far worse and this could be used as a pretext to impede the role of Hong Kong as a finance hub."

In Beijing, Zhang Xiaoming, executive deputy director of Beijing's Hong Kong and Macau Affairs Office, told reporters suspects arrested by Beijing's new security office in Hong Kong could be tried on the mainland.

He said the mainland's national security office abided by Chinese law and that Hong Kong's legal system could not be expected to implement the laws of the mainland. Article 55 of the law states that Beijing's national security office in Hong Kong could exercise jurisdiction over "complex" or "serious" cases.

Mainland security agencies will also be based in Hong Kong officially for the first time, with powers that go beyond city laws.

"The law is a birthday gift to (Hong Kong) and will show its precious value in the future," Zhang said, adding the law would not be applied retroactively.

On July 1 last year, hundreds of protesters stormed and vandalised the city's legislature to protest against a now-scrapped bill that would have allowed extraditions to mainland China.

Those protests evolved into calls for greater democracy, paralysing parts of the city and paving the way for Beijing's imposition of the law this week.

'INEVITABLE'

Speaking at a flag-raising ceremony to mark the handover anniversary, the city's Beijing-backed leader, Carrie Lam, said the law was the most important development since the city's return to Chinese rule.

"It is also an inevitable and prompt decision to restore stability," Lam said at the same harbour-front venue where 23 years ago the last colonial governor, Chris Patten, a staunch critic of the security law, tearfully handed back Hong Kong to Chinese rule.

Some pro-Beijing officials and political commentators say the law is aimed at sealing Hong Kong's "second return" to the motherland after the first failed to bring residents to heel.

Luo Huining, the head of Beijing's top representative office in Hong Kong, said at the ceremony the law was a "common aspiration" of Hong Kong citizens.

Critics denounced the lack of transparency surrounding the details of the legislation until it was unveiled. It came into force at 11 p.m. (1500 GMT) on Tuesday.

Some pro-democracy activists gave up membership of their groups just before the law came into force, though calling for the campaign for democracy to go on offshore.

"I saw this morning there are celebrations for Hong Kong's handover, but to me it is a funeral, a funeral for 'one country two systems'," said democracy lawmaker Kwok Ka-ki.