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The coronavirus can stay aloft for hours in tiny droplets in stagnant air, infecting people as they inhale, mounting scientific evidence suggests.

This risk is highest in crowded indoor spaces with poor ventilation, and may help explain superspreading events reported in meatpacking plants, churches and restaurants.

It’s unclear how often the virus is spread via these tiny droplets, or aerosols, compared with larger droplets that are expelled when a sick person coughs or sneezes, or transmitted through contact with contaminated surfaces, said Linsey Marr, an aerosol expert at Virginia Tech.

Follow latest updates on the Covid-19 pandemic here

Aerosols are released even when a person without symptoms exhales, talks or sings, according to Marr and more than 200 other experts, who have outlined the evidence in an open letter to the World Health Organization.

What is clear, they said, is that people should consider minimizing time indoors with people outside their families. Schools, nursing homes and businesses should consider adding powerful new air filters and ultraviolet lights that can kill airborne viruses.

What does it mean for a virus to be airborne?

For a virus to be airborne means that it can be carried through the air in a viable form. For most pathogens, this is a yes-no scenario. HIV, too delicate to survive outside the body, is not airborne. Measles is airborne, and dangerously so: It can survive in the air for up to two hours.

For the coronavirus, the definition has been more complicated. Experts agree that the virus does not travel long distances or remain viable outdoors. But evidence suggests it can traverse the length of a room and, in one set of experimental conditions, remain viable for perhaps three hours.

How are aerosols different from droplets?

Aerosols are droplets, droplets are aerosols — they do not differ except in size. Scientists sometimes refer to droplets fewer than 5 microns in diameter as aerosols. (By comparison, a red blood cell is about 5 microns in diameter; a human hair is about 50 microns wide.)

From the start of the pandemic, the WHO and other public health organizations have focused on the virus’s ability to spread through large droplets that are expelled when a symptomatic person coughs or sneezes.

These droplets are heavy, relatively speaking, and fall quickly to the floor or onto a surface that others might touch. This is why public health agencies have recommended maintaining a distance of at least 6 feet from others, and frequent hand washing.

But some experts have said for months that infected people also are releasing aerosols when they cough and sneeze. More important, they expel aerosols even when they breathe, talk or sing, especially with some exertion.

Scientists know now that people can spread the virus even in the absence of symptoms — without coughing or sneezing — and aerosols might explain that phenomenon.

Because aerosols are smaller, they contain much less virus than droplets do. But because they are lighter, they can linger in the air for hours, especially in the absence of fresh air. In a crowded indoor space, a single infected person can release enough aerosolized virus over time to infect many people, perhaps seeding a superspreader event.

For droplets to be responsible for that kind of spread, a single person would have to be within a few feet of all the other people, or to have contaminated an object that everyone else touched. All that seems unlikely to many experts: “I have to do too many mental gymnastics to explain those other routes of transmission compared to aerosol transmission, which is much simpler,” Marr said.

Can I stop worrying about physical distancing and washing my hands?

Physical distancing is still very important. The closer you are to an infected person, the more aerosols and droplets you may be exposed to. Washing your hands often is still a good idea.

What’s new is that those two things may not be enough. “We should be placing as much emphasis on masks and ventilation as we do with hand washing,” Marr said. “As far as we can tell, this is equally important, if not more important.”

Should I begin wearing a hospital-grade mask indoors? And how long is too long to stay indoors?

Health care workers may all need to wear N95 masks, which filter out most aerosols. At the moment, they are advised to do so only when engaged in certain medical procedures that are thought to produce aerosols.

For the rest of us, cloth face masks will still greatly reduce risk, as long as most people wear them. At home, when you’re with your own family or with roommates you know to be careful, masks are still not necessary. But it is a good idea to wear them in other indoor spaces, experts said.

As for how long is safe, that is frustratingly tough to answer. A lot depends on whether the room is too crowded to allow for a safe distance from others and whether there is fresh air circulating through the room.

What does airborne transmission mean for reopening schools and colleges?

This is a matter of intense debate. Many schools are poorly ventilated and are too poorly funded to invest in new filtration systems. “There is a huge vulnerability to infection transmission via aerosols in schools,” said Don Milton, an aerosol expert at the University of Maryland.

Most children younger than 12 seem to have only mild symptoms, if any, so elementary schools may get by. “So far, we don’t have evidence that elementary schools will be a problem, but the upper grades, I think, would be more likely to be a problem,” Milton said.

College dorms and classrooms are also cause for concern.

Milton said the government should think of long-term solutions for these problems. Having public schools closed “clogs up the whole economy, and it’s a major vulnerability,” he said.

“Until we understand how this is part of our national defense, and fund it appropriately, we’re going to remain extremely vulnerable to these kinds of biological threats.”

What are some things I can do to minimize the risks?

Do as much as you can outdoors. Despite the many photos of people at beaches, even a somewhat crowded beach, especially on a breezy day, is likely to be safer than a pub or an indoor restaurant with recycled air.

But even outdoors, wear a mask if you are likely to be close to others for an extended period.

When indoors, one simple thing people can do is to “open their windows and doors whenever possible,” Marr said. You can also upgrade the filters in your home air-conditioning systems, or adjust the settings to use more outdoor air rather than recirculated air.

Public buildings and businesses may want to invest in air purifiers and ultraviolet lights that can kill the virus. Despite their reputation, elevators may not be a big risk, Milton said, compared with public bathrooms or offices with stagnant air where you may spend a long time.

If none of those things are possible, try to minimize the time you spend in an indoor space, especially without a mask. The longer you spend inside, the greater the dose of virus you might inhale.

Washington, May 30: President Donald Trump said Friday he would strip several of Hong Kong's special privileges with the United States and bar some Chinese students from US universities in anger over Beijing's bid to exert control in the financial hub.

In a day of concerted action, the United States and Britain also raised alarm at the UN Security Council over a controversial new security law for Hong Kong, angering Beijing which said the issue had no place at the world body.

In a White House appearance that Trump had teased for a day, the US president attacked China over its treatment of the former British colony, saying it was "diminishing the city's longstanding and proud status."

"This is a tragedy for the people of Hong Kong, the people of China and indeed the people of the world," Trump said.

Trump also said he was terminating the US relationship with the World Health Organization, which he has accused of pro-China bias in its management of the coronavirus crisis.

But Trump was light on specifics and notably avoided personal criticism of President Xi Jinping, with whom he has boasted of having a friendship even as the two powers feud over a rising range of issues.

"I am directing my administration to begin the process of eliminating policy that gives Hong Kong different and special treatment," Trump said.

"This will affect the full range of agreements, from our extradition treaty to our export controls on dual-use technologies and more, with few exceptions," he said.

Secretary of State Mike Pompeo on Wednesday informed Congress that the Trump administration would no longer consider Hong Kong to be separate under US law, but it was up to Trump to spell out the consequences.

China this week pressed ahead on a law that would ban subversion and other perceived offenses against its rule in Hong Kong, which was rocked by months of massive pro-democracy protests last year.

US restricts students

In one move that could have long-reaching consequences, Trump issued an order to ban graduate students from US universities who are connected to China's military.

"For years, the government of China has conducted elicit espionage to steal our industrial secrets, of which there are many," Trump said.

Hawkish Republicans have been clamoring to kick out Chinese students enrolled in sensitive fields. The FBI in February said it was investigating 1,000 cases of Chinese economic espionage and technological theft.

But any move to deter students is unwelcome for US universities, which rely increasingly on tuition from foreigners and have already been hit hard by the COVID-19 shutdown.

China has been the top source of foreign students to the United States for the past decade with nearly 370,000 Chinese at US universities, although Trump's order will not directly affect undergraduates.

Critics say Trump has been eager to fan outrage about China to deflect attention from his own handling of the coronavirus pandemic that has killed more than 100,000 people in the United States, the highest number of deaths of any country.

Chuck Schumer, the top Democrat in the Senate, called Trump's announcement "just pathetic."

Eliot Engel, a Democrat who heads the House Foreign Affairs Committee, noted that Trump treaded lightly on Hong Kong during last year's protests as he sought a trade deal with Xi.

"Now, the president wants to shift the blame for his failures onto China, so he's doing the right thing for the wrong reason," Engel said.

Trump's order could also trigger retaliation. China in March expelled US journalists after the Trump administration tightened visa rules for staff at Chinese state media.

Clash at UN

The United States and Britain earlier in the day urged China to reconsider the Hong Kong law during talks at the UN Security Council, where China wields a veto -- making any formal session, let alone action against Beijing, impossible.

The Western allies raised Hong Kong in an informal, closed-door videoconference where China cannot block the agenda.

They said China was violating an international commitment as the 1984 handover agreement with Britain, in which Beijing promised to maintain the financial hub's separate system until at least 2047, was registered with the United Nations.

"The United States is resolute, and calls upon all UN members states to join us in demanding that the PRC immediately reverse course and honor its international legal commitments to this institution and to the Hong Kong people," said US Ambassador Kelly Craft, referring to the People's Republic of China.  

China demanded that the United States and Britain "immediately stop interfering in Hong Kong affairs," saying the law did not fall under the Security Council's mandate.

"Any attempt to use Hong Kong to interfere in China's internal matters is doomed to fail," warned a statement from China's UN mission.

"There was no consensus, no formal discussion in the Security Council, and the US and the UK's move came to nothing," it said.

Bengaluru, Jun 22: Areas having three to four COVID-19 cases being termed as clusters will be totally sealed and maximum testing will be conducted there, moreover only critical cases will be shifted to COVID hospitals in Bengaluru, said Karnataka Home Minister Basavaraj Bommai.

Speaking to media persons, Karnataka Home Minister Basavaraj Bommai said, "It has been decided that clusters (areas having 3-4 #COVID19 cases) will be totally sealed and maximum testing will be done there. Only critical cases will be shifted to Covid hospitals. I demanded that police personnel are tested on priority."

"A lot of issues were discussed about the spike in COVID-19 cases. As far as hospitals are concerned there are two types of patients, asymptomatic and symptomatic, moderate and critical. Moderate and normal cases can be shifted to COVID centres," he added.

Bommai further added that beds in hospitals need to be reserved for corona warriors as well.

"There has to be a coordination between BBPM who brings the patient and the hospital, the moment a positive case is reported to avoid the waiting period. Even in hospitals, some beds have to be reserved for corona warriors. Community transmission is not yet there but we have to be prepared," he added.

Earlier today, Karnataka Chief Minister BS Yediyurappa had called an emergency meeting of concerned officials and departments to discuss measures to control rising COVID19 cases in Bengaluru.

As the numbers of COVID-19 cases are increasing in Bengaluru at an alarming rate, the Chief Minister opined that this can be contained only if preventive measures were implemented strictly.

He directed the officials to implement lockdown strictly in the clusters, which reported more number of cases, especially KR Market and surrounding areas such as Siddapura, VV Puram, Kalasipalya. It was decided to seal the adjoining streets, where the cases are reported.

He said that stringent action would be taken against those who violate quarantine and FIR would be filed if necessary.

Officers were directed to fix rates for treatment of COVID patients in private hospitals to make coronavirus treatment accessible to all.

Yediyurappa further instructed officials to set up fever clinics in all wards and maintain hygiene and provide other basic amenities to the people who were quarantined in social welfare hostels and other government institutions.

"COVID-19 should be contained without affecting the economic activities in Bengaluru, which resumed recently," he said.

"COVID War Room shall have real-time information on the availability of beds in various COVID hospitals and shall facilitate the infected person to avail treatment without losing any time," added Yediyurappa.