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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.

Beijing, Feb 24: The lockdown of Guo Jing's neighbourhood in Wuhan -- the city at the heart of China's new coronavirus epidemic -- came suddenly and without warning.

Unable to go out, the 29-year-old is now sealed inside her compound where she has to depend on online group-buying services to get food.

"Living for at least another month isn't an issue," Guo told news agency, explaining that she had her own stash of pickled vegetables and salted eggs.

But what scares her most is the lack of control -- first, the entire city was sealed off, and then residents were limited to exiting their compound once every three days.

Now even that has been taken away.

Guo is among some 11 million residents in Wuhan, a city in central Hubei province that has been under effective quarantine since January 23 as Chinese authorities race to contain the epidemic.

Since then, its people have faced a number of tightening controls over daily life as the death toll from the virus swelled to over 2,500 in China alone.

But the new rules this month barring residents from leaving their neighbourhoods are the most restrictive yet -- and for some, threaten their livelihoods.

"I still don't know where to buy things once we've finished eating what we have at home," said Pan Hongsheng, who lives with his wife and two children.

Some neighbourhoods have organised group-buying services, where supermarkets deliver orders in bulk.

But in Pan's community, "no one cares".

"The three-year-old doesn't even have any milk powder left," Pan told news agency, adding that he has been unable to send medicine to his in-laws -- both in their eighties -- as they live in a different area.

"I feel like a refugee."

The "closed management of neighbourhoods is bound to bring some inconvenience to the lives of the people", Qian Yuankun, vice secretary of Hubei's Communist Party committee, said at a press briefing last week.

Authorities on Monday allowed healthy non-residents of the city to leave if they never had contact with patients, but restrictions remained on those who live in Wuhan.

Demand for group-buying food delivery services has rocketed with the new restrictions, with supermarkets and neighbourhood committees scrambling to fill orders.

Most group-buying services operate through Chinese messaging app WeChat, which has ad-hoc chat groups for meat, vegetables, milk -- even "hot dry noodles", a famous Wuhan dish.

More sophisticated shops and compounds have their own mini-app inside WeChat, where residents can choose packages priced by weight before orders are sent in bulk to grocery stores.

In Guo's neighbourhood, for instance, a 6.5-kilogramme (14.3-pound) set of five vegetables, including potatoes and baby cabbage, costs 50 yuan ($7.11).

"You have no way to choose what you like to eat," Guo said. "You cannot have personal preferences anymore."

The group-buying model is also more difficult for smaller communities to adopt, as supermarkets have minimum order requirements for delivery.

"To be honest, there's nothing we can do," said Yang Nan, manager of Lao Cun Zhang supermarket, which requires a minimum of 30 orders.

"We only have four cars," she said, explaining that the store did not have the staff to handle smaller orders.

Another supermarket told AFP it capped its daily delivery load to 1,000 orders per day.

"Hiring staff is difficult," said Wang Xiuwen, who works at the store's logistics division, adding that they are wary about hiring too many outsiders for fear of infection.

Closing off communities has split the city into silos, with different neighbourhoods rolling out controls of varying intensity.

In some compounds, residents have easier access to food -- albeit a smaller selection than normal -- and one woman said her family pays delivery drivers to run grocery errands.

Her compound has not been sealed off either, the 24-year-old told AFP under condition of anonymity, though they are limited to one person leaving at a time.

Some districts have implemented their own rules, such as prohibiting supermarkets from selling to individuals, forcing neighbourhoods to buy in bulk or not at all.

"In the neighbourhood where I live, the reality is really terrible," said David Dai, who is based on the outskirts of Wuhan.

Though his apartment complex has organised group-buying, Dai said residents were unhappy with price and quality.

"A lot of tomatoes, a lot of onions -- they were already rotten," he told , estimating over a third of the food had to be thrown away.

His family must "totally depend" on themselves, added the 49-year-old, who has resorted to saving and drying turnip skins to add nutrients to future meals.

The uncertainty of not knowing when the controls will be lifted is also frustrating, said Ma Chen, a man in his 30s who lives alone.

"I have no way of knowing how much (food) I should buy."

Mar 16: A fourth batch of 53 Indians returned to India from Iran on Monday, taking the total number of people evacuated from the coronavirus-hit country to 389.

This comes a day after over 230 Indians were brought back from Iran to New Delhi and quarantined at the Indian Army Wellness Centre in Jaisalmer, the third batch to be evacuated from that country.

"Fourth batch of 53 Indians - 52 students and a teacher - has arrived from Tehran and Shiraz, Iran. With this, a total of 389 Indians have returned to India from Iran. Thank the efforts of the team @India_in_Iran and Iranian authorities," Jaishankar tweeted.

The Indians came in a Mahan Air flight that landed at the Delhi airport at around 3 am, officials said, adding that they were later taken to Jaisalmer in an Air India flight for being quarantined.

The first batch of 58 Indian pilgrims were brought back from Iran last Tuesday and the second group of 44 Indian pilgrim arrived from there on Friday.

Iran is one of the worst-affected countries by the coronavirus outbreak and the government has been working to bring back Indians stranded there. Over 700 people have died from the disease in Iran and nearly 14,000 cases have been detected.

Jaishankar had told Rajya Sabha last week that the government was focusing on evacuating Indians stranded in Iran and Italy as these countries are facing an "extreme situation".