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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.

New Delhi, Jan 15: The Delhi government Wednesday told the high court that execution of the death row convicts in the Nirbhaya gangrape and murder case will not take place on January 22 as a mercy plea has been filed by one of them.

The four convicts -- Vinay Sharma (26), Mukesh Kumar (32), Akshay Kumar Singh (31) and Pawan Gupta (25) -- are to be hanged on January 22 at 7 am in Tihar jail. A Delhi court had issued their death warrants on January 7.

Justices Manmohan and Sangita Dhingra Sehgal were told by the Delhi government and the Centre that the petition filed by convict Mukesh, challenging his death warrant, was premature.

The Delhi government and the prison authorities informed the court that under the rules, it will have to wait for the mercy plea to be decided before executing the death warrant.

They also said that none of the four convicts can be executed on January 22 unless the present mercy plea is decided.

The Supreme Court had on Tuesday dismissed the curative pleas of Mukesh and Vinay.

The mercy plea hearing began Wednesday morning and will continue in the afternoon.

New Delhi, Mar 4: The government on Wednesday permitted NRIs to own up to 100 per cent stake in disinvestment-bound Air India.

The decision comes at a time when the government is looking to sell 100 per cent stake sale in the national carrier.

Union minister Prakash Javadekar said the Cabinet has approved allowing Non-Residents Indians (NRIs) to hold up to 100 per cent stake in Air India.

Allowing 100 per cent investment by Non-Resident Indians (NRIs) in the carrier would also not be in violation of SOEC norms. NRI investments would be treated as domestic investments.

Under the Substantial Ownership and Effective Control (SOEC) framework, which is followed in the airline industry globally, a carrier that flies overseas from a particular country should be substantially owned by that country's government or its nationals.

Currently, NRIs can acquire only 49 per cent in Air India. Foreign Direct Investment (FDI) in the airline is also 49 per cent through the government approval route.

As per the existing norms, 100 per cent FDI is permitted in scheduled domestic carriers, subject to certain conditions, including that it would not be applicable for overseas airlines.

In the case of scheduled airlines, 49 per cent FDI is permitted through automatic approval route and any such investment beyond that level requires government nod.

On January 27, the government came out witha Preliminary Information Memorandum (PIM) for Air India disinvestment. It has proposed selling 100 per cent stake in Air India along with budget airline Air India Express and the national carrier's 50 per cent stake in AISATS, an equal joint venture with Singapore Airlines.

Under the latest disinvestment plan, the successful bidder would have to take over only debt worth Rs 23,286.5 crore while the liabilities would be decided depending on current assets at the time of closing of the transaction.

This is the second attempt by the government in as many years to divest Air India, which has been in the red for long.