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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.

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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 29: Reiterating his offer to mediate on the border dispute between India and China, US President Donald Trump has said that he spoke with Narendra Modi about the "big conflict" and asserted that the Indian Prime Minister is not in a "good mood" over the latest flare-ups between the two countries.

Speaking with the reporters in the Oval Office of the White House on Thursday, Trump said a "big conflict" was going on between India and China.

"I like your prime minister a lot. He is a great gentleman," the president said.

"Have a big conflict …India and China. Two countries with 1.4 billion people (each). Two countries with very powerful militaries. India is not happy and probably China is not happy," he said when asked if he was worried about the border situation between India and China.

"I can tell you; I did speak to Prime Minister Modi. He is not in a good mood about what is going on with China," Trump said.

A day earlier, the president offered to mediate between India and China.

Trump on Wednesday said in a tweet that he was "ready, willing and able to mediate" between the two countries.

Responding to a question on his tweet, Trump reiterated his offer, saying if called for help, "I would do that (mediate). If they thought it would help" about "mediate or arbitrate, I would do that," he said.

India on Wednesday said it was engaged with China to peacefully resolve the border row, in a carefully crafted reaction to Trump's offer to arbitrate between the two Asian giants to settle their decades-old dispute.

"We are engaged with the Chinese side to peacefully resolve it," External Affairs Ministry Spokesperson Anurag Srivastava said, replying to a volley of questions at an online media briefing.

While the Chinese Foreign Ministry is yet to react to Trump's tweet which appears to have caught Beijing by surprise, an op-ed in the state-run Global Times said both countries did not need such a help from the US President.

"The latest dispute can be solved bilaterally by China and India. The two countries should keep alert on the US, which exploits every chance to create waves that jeopardise regional peace and order," it said.

In Beijing, Chinese Foreign Ministry spokesman Zhao Lijian said on Wednesday that both China and India have proper mechanisms and communication channels to resolve the issues through dialogue and consultations.

Trump previously offered to mediate between India and Pakistan on the Kashmir issue, a proposal which was rejected by New Delhi.

The situation in eastern Ladakh deteriorated after around 250 Chinese and Indian soldiers were engaged in a violent face-off on the evening of May 5 which spilled over to the next day before the two sides agreed to "disengage" following a meeting at the level of local commanders.

Over 100 Indian and Chinese soldiers were injured in the violence.

The incident in Pangong Tso was followed by a similar incident in north Sikkim on May 9.

Melbourne, Mar 18: Australian Prime Minister Scott Morrison on Wednesday warned its citizens not to travel abroad and banned "non-essential" gatherings of 100 people or more as the government declared the deadly coronavirus, that has infected over 450 people in the country, a "human biosecurity emergency".

Prime Minister Morrison on Wednesday said Australians should abandon all overseas travel because of the coronavirus epidemic. There have been 40 new cases since Tuesday.

Of the 454 confirmed cases in Australia, 43 have recovered and 5 have died from the COVID-19, according to the official figures.

Morrison declared a national "human biosecurity emergency" and ordered a halt to "non-essential" indoor gatherings of more than 100 people, on top of an existing ban on outdoor events of more than 500.

He said, "We haven't seen this sort of thing in Australia since the end of the First World War. This is a once-in-a-100-year-type event," Morrison said.

"Life is changing in Australia, as it is changing all around the world. Life is going to continue to change. The travel advice to every Australian is do not travel abroad. Do not go overseas."

However, schools across the country were still to remain open.

Morrison said Australia should prepare for "at least six months" of disruption as health authorities attempt to get on top of the coronavirus.

"For those of you thinking about going overseas for the school holidays, don’t. Don't go overseas," he said.

Morrison said there would be no short-term, quick fix to deal with the COVID-19 crisis in Australia, and warned that "tens of thousands of jobs" could be lost.

Responding to the panic buying, the prime minister urged the public to "Stop hoarding."

"I can't be more blunt about it. Stop it. It is not sensible, it is not helpful and it has been one of the most disappointing things I have seen in Australian behaviour in response to this crisis," he said.

On Wednesday, the New South Wales recorded a massive jump in new 59 cases as travellers returned from overseas. The cases jumped from 210 to 269 in the past 24 hours, according to the media reports here.

Queensland's total number of confirmed cases stood at 94.

State Premier Annastacia Palaszczuk said it is now "a different world" from just last week.

For Victoria, the Premier Daniel Andrew said the government could also ban gatherings of fewer than 100 people, noting he could not rule out further changes.

Chief Health Officer Brett Sutton said the spike in cases has been due to people who have travelled overseas.

Sutton said that the transmission in Victoria was not because of community transition like Italy.

Sutton has continued to urge people to wash their hands, socially distance and stay home if they are unwell.

"These are critical measures to be applied regardless of the 100 and 500 threshold,” he said.

In Western Australia, Health Minister Roger Cook said four new coronavirus cases have been confirmed overnight, taking the State's total to 35.

McGowan also condemned panic buyers in supermarkets as "jerks, drongos and bloody idiots."

He said there was "plenty of food to go round" and no need for people to buy in excessive quantities.

The outbreak, which first emerged in China's Wuhan city last year, has marched across the globe, infecting 198,006 people and killing 7,948, according to a tracker maintained by the Johns Hopkins University.