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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, May 28: With 6,566 more coronavirus cases and 194 deaths reported in the past 24 hours, India's COVID-19 tally reached 1,58,333 on Thursday, according to the Union Ministry of Health and Family Affairs.

The number of active coronavirus cases stands at 86,110, while 67,692 people have recovered and one patient has migrated, it said. The death toll due to the infection has reached 4,531 in the country.

Maharashtra is the worst affected state with 56,948 cases. Tamil Nadu has recorded as many as 18,545 cases while Gujarat and Delhi have recorded 15,195 and 15,257 coronavirus cases respectively.

New Delhi, Jan 20: Union Finance Minister Nirmala Sitharaman on Sunday said the kind of cleaning in the system that the BJP-led government had to carry out after coming to power in 2014 was "unbelievable" and it undertook the exercise without any grudge or worry.

Between 2014-16, there were a lot of questions as to why reforms did not come fast and there were comments that the government was incapable of bringing them, she said delivering the Nani Palkhivala Memorial lecture on "Road Map to $5 Trillion Economy" here.

Pointing out that there were allegations and criticism that the government wants to do something but it did not, Sitharaman said, "I am fully willing to buy that." She recalled that Prime Minister Narendra Modi often said he did not believe in incremental changes and the country needed good transformational change. The stage in which India is today, it cannot have little marginal increments, but good transformational change.

"But still one might say in the last five years the government never did. That can be a critical analysis and I am fully willing to buy that. Because post-2014 the kind of cleaning up the government had to do was unbelievable and we undertook that exercise without a grudge without a worry.. we had to do it and it is part of the game," she said. Elaborating, Sitharaman said states have their own views on Land Acquisition Bill and the government could not have done anything because land, after all, is with them.

Commenting on the topic 'Road Map to $5 trillion economy,' she said quoting Prime Minister Narendra Modi's comments, the government would take the route "Sarkar ka abhaav nahi hona chahiye, prabhaav hona chahiye aur dabaav nahi hona chahiye."

"Abhaav and dabaav both of which are not desirable, abhaav is the inadequacy or lack of adequate presence or shortfall. You do not need a shortfall. You need a government where it should be present, where it is expected to function.", she said.

"So there should not be abhaav. Dabaav (meaning pressure) is not something you want from the government. So, you want Prabhaav. It is broadly an influence, facilitation, broadly the philosophy with which it is mandated, she said.

Noting that the government has got the mandate through the election, she said, "The mandate was spelt out in so many different ways in its manifesto. So the route towards $5 trillion is this."

"We have to be there to facilitate. We have to be there to make it easy. We have to be where you need us, where there is no policy (reforms from the government)," she said.

On the Insolvency and Bankruptcy Code (IBC) implemented by the government, the union minister said the approach of the IBC was not to shut business. "IBC takes on the approach in having some kind of resolution where all people who exploited the company do not come back through the "back door," she said.

IBC was done through better management so that the institution is alive and kicking. It is something which she wanted to carry forward from Modi 1.0 to 2.0. "The point I am trying to make on this road to $5 trillion economy is that it is not just an abstraction, this is not how I want India to be. But in micro-level too, we are coming in response to every stakeholder," she said.