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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.

Lucknow, Apr 29: Tablighi Jamaat members, who got cured of COVID-19, have been asked by their organisation to donate their plasma for treatment of other coronavirus-infected patients, a move that the outfit thinks will help it shed its “villain” tag.

"Maulana Saad, in an open letter on April 21 to all Jamaatis who have recovered from the coronavirus (infection), has appealed them to donate their plasma to help others. The message has reached all the members," Maulana Anees Ahmad Nadvi, the manager of Tablighi Jamaat’s Lucknow branch, told PTI PTI on Wednesday.

"As per the Health Department data, over 50 per cent of corona patients are Jamaatis. Among them those who have recovered are being contacted and all of them are ready to donate their plasma," claimed Nadvi.

"We have till now contacted 400 Jamaatis. In the entire country, those who have recovered are also giving their plasma. Markaz has given instructions that no one should be left from donating plasma," he said. "Jamaatis are not doing any favour to anyone by giving their plasma.

This is a humanitarian step. This is true that Jamaatis are being presented as villains after coronavirus spread, but Maulana Saad has asked us to forgive those doing this," he said. Tablighi Jamaat leader Maulana Saad Kandhalvi is on the run after an FIR was registered against him by the Delhi police for organising a religious gathering in March this year despite restrictions to combat the coronavirus spread.

With some plasma therapy trial results rekindling hopes of it being a likely cure for COVID-19, the Uttar Pradesh government too had begun contracting patients cured of it for plasma donation, but the move was suspended after the Union Health Ministry on Tuesday said the therapy was only at an experimental stage and there was no evidence yet to support that it can be used as treatment for COVID-19.

Uttar Pradesh Surveillance Officer, Dr Vikasendu Agarwal, said all those who have recovered from coronavirus, including Jamaatis, were being contacted for plasma donation, but the move has been suspended after the Centre’s statement on the issue.

Refusing to divulge the number of cured Jamaatis, he said "We were contacting them. They are not different from us. We were contacting all our patients and asking them that they could donate if they find it appropriate, as it would help other patients." "All of those contacted by us are ready for giving plasma," he said.

Chief Medical Officer, Lucknow, Dr Narendra Agarwal said all 28 Jamaatis, who were admitted in KGMU were contacted to donate their plasma and all of them agreed. "A proposal in this regard has been sent to the government.

After approval, their plasma will be taken," he said. With the plasma therapy gaining a lot of traction as a possible cure for coronavirus, the Union Health Ministry on Tuesday clarified that it is at an experimental stage and there is no evidence yet to support that it can be used as treatment for COVID-19. Till the effectiveness of this mode of treatment is scientifically proven, its application except for research and clinical trial is illegal, Joint Secretary in the Ministry of Health Lav Agarwal said.

Dr Vikasendu said after the Centre's clarification contacting people for plasma donation has been put on hold. A further step will be taken on decision of KGMU which is working on plasma therapy here, he added.