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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, Apr 28: US President while addressing a news conference on coronavirus pandemic said his administration has launched "very serious investigations" into China's response to the novel coronavirus.

"And we are not happy with China, we are not happy with that whole situation, because we believe it could have been stopped at the source," he said. "It could have been stopped quickly and it wouldn't have spread all over the world," the BBC reported.

Trump has been critical of China as the pandemic has progressed and has frequently touted his decision to close the US borders to China in an effort to curb the outbreak. Some health experts have said the effort bought time for the US to prepare, but the Trump administration has been accused of squandering the opportunity.

"Nobody except one country can be held accountable for what happened," Trump said.

"Nobody's blaming anybody here, we're looking at a group of people that should've stopped it at the source."

The US will never forget those who were "sacrificed for a reason of incompetence or something else other than incompetence," he added.

"They" - referring to China - "could've protected the whole world - not just us - the whole world," he said.

At the starting of the conference Trump said COVID-19 cases are declining or stablising across the country.

"In all cases getting better," he said. "Really a horrible situation that we've been confronted with, but they're moving along."

He added there's a "hunger" to get back to work.

"Ensuring the health of our economy is vital to ensuring the health of our nation - these goals work in tandem."

The president has suggested an unnamed individual "a long time ago" could have stopped the "unnecessary death[s]" due to COVID-19.

"There has been so much unnecessary death in this country," he said.

"It could've been stopped and it could've been stopped short, but somebody a long time ago, it seems, decided not to do it that way and the whole world is suffering because of it."

He did not say who he was referencing and gave no other details.

Trump was asked if he has considered delaying the November presidential election.

"I never even thought of changing the date of the election," he answered. "Why would I do that? November 3rd. That's a good number."

Trump called the suggestion "made up propaganda" and said that "sleepy Joe" Biden - his presumptive Democratic rival - was likely unaware his campaign had put the statement out.

Former Vice-President Joe Biden said at a virtual fundraiser last week that he thought Trump would "try to kick back the election somehow".

The president has frequently levelled insults at his Democratic opponent by questioning the former vice-president's mental fortitude.

"I look forward to that election," Trump said.

The president responded to a question criticising Health Secretary Alex Azar's early downplaying of the disease by saying it was "unfair". He claimed Democrats did the same, including Speaker of the House of Representatives Nancy Pelosi.

"I was very fortunate through luck or whatever that we closed the border, we put a ban on China," he said.

"But I could tell you that Nancy Pelosi was dancing in the streets in Chinatown. She wanted to go, let's go out and party. That was late in February."

Back in February, Pelosi had encouraged people to visit San Francisco's Chinatown to help struggling businesses. She did not propose any parties, as the president suggested.

The city issued a stay-at-home order in March.

A reporter asked: If an American president loses more Americans over the course of six weeks than died during the entirety of the Vietnam war, does he deserve to be reelected?

Trump took the question in stride.

"So, yeah, we've lost a lot of people but if you look at what original projections were, 2.2 million, we're probably heading to 60,000 - 70,000," he said.

"It's far too many - one person is too many for this. I think we've made a lot of really good decisions," he added. "The big decision was closing the border, doing the ban on people coming in from China."

He also brings up the "unbelievable" job his administration did with ventilators.

"I think we've done a great job. And I will say this - one person is too many."

New Delhi, Feb 1: Air India's jumbo B747 plane, evacuating 324 Indian nationals from the novel coronavirus-hit Wuhan in China, landed here on Saturday morning, officials said.

The plane reached Delhi around 7.30 am, they said.

There were five doctors from Ram Manohar Lohia (RML) Hospital and one paramedical staff on board, said an Air India spokesperson.

The Indian Army has set up a quarantine facility in Manesar near Delhi to keep those evacuated from China's Hubei province.

Officials said they would be monitored for any signs of infection for a duration of two weeks by a qualified team of doctors and staff members.

"With 324 passengers, special flight has taken off for India from Wuhan. It may reach Delhi at 7.30am," said the Air India spokesperson at 1.19 am on Saturday.

The flight had departed from Delhi airport at 1.17 pm on Friday to evacuate Indian nationals from China, where more than 250 people - none of them Indian - have died due to novel coronavirus.

On Friday evening, the Air India spokesperson had stated that another special flight may take off from Delhi airport on Saturday to evacuate Indians from Wuhan.

The death toll from the novel coronavirus outbreak in China has risen to 259 with total confirmed cases surging to 11,791 amid stepped up efforts by a number of countries to evacuate their nationals from Hubei province, the epicentre of the virus, officials said on Saturday.

About Friday's flight, the spokesperson had said earlier during the day, "A team of five doctors from RML hospital, one paramedical staff from Air India, with prescribed medicines from doctors, masks, overcoats, packed food are in the aircraft. A team of engineers, security personnel are also there in this special aircraft. Whole rescue mission is being led by Captain Amitabh Singh, Director (Operations), Air India."

The spokesperson had added that there were five cockpit crew members and 15 cabin crew members on Friday's flight.

Before departure at Delhi airport, Air India Chairman and Managing Director Ashwani Lohani had said, "No service will take place in the plane. Whatever food is there will be kept in seat pockets. As there will be no service, there will be no interaction (between cabin crew and passengers)."

"Masks have been arranged for the crew and passengers. For our crew, we have also arranged a complete protective gear," he had added.

"Total five doctors from the Health Ministry are also going... The plane will be there (at Wuhan airport) for 2-3 hours," Lohani had said.

Air India has done such evacuations earlier also from countries such as Libya, Iraq, Yemen, Kuwait and Nepal.