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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.

Tehran, Jan 12: Iranian police dispersed students chanting “radical” slogans during a Saturday gathering in Tehran to honour the 176 people killed when an Ukrainian airliner was mistakenly shot down, Fars news agency reported.

News agency correspondents said hundreds of students gathered early in the evening at Amir Kabir University, in downtown Tehran, to pay respects to those killed in the air disaster. The tribute later turned into an angry demonstration.

The students chanted slogans denouncing "liars" and demanded the resignation and prosecution of those responsible for downing the plane and allegedly covering up the accidental action.

Iran said Saturday that the Ukraine International Airlines Boeing 737 was “unintentionally” shot down on Wednesday shortly after taking off from Tehran's main airport. All 176 people on board died, mostly Iranians and Canadians, many of whom were students.

Fars, which is close to conservatives, said the protesting students chanted “destructive” and “radical” slogans. The news agency said some of the students tore down posters of Qasem Soleimani, the Iranian general killed on January 3 in a US drone strike on Baghdad.

Fars published pictures of demonstrators gathered around a ring of candles during the tribute and a picture of a torn poster bearing the image of a smiling Soleimani. It said that police "dispersed" them as they left the university and blocked streets, causing a traffic jam.

In an extremely unusual move, state television mentioned the protest, reporting that the students shouted "anti-regime" slogans.

A video purportedly of the protest circulated online showing police firing tear gas at protesters and a man getting up after apparently being hit in the leg by a projectile. It was not possible to verify the location of the video, or when it was filmed.

Iran's acknowledgement on Saturday that the plane had been shot down in error came after officials had for days categorically denied Western claims that it had been struck by a missile. The aerospace commander of Iran's Revolutionary Guards accepted full responsibility.

But Brigadier General Amirali Hajizadeh said the missile operator acted independently, shooting down the Boeing 737 after mistaking it for a "cruise missile".

Mumbai, Jun 1: Singer-composer Wajid Khan of music director duo Sajid-Wajid, popular for their work on superstar Salman Khan's films such as "Wanted", "Dabangg" and "Ek Tha Tiger", died in early hours of Monday in a city hospital due to complications arising from a kidney infection. He was 42.

Music composer Salim Merchant, who confirmed the news of Wajid's death, said the composer was hospitalised few days ago at Surana Hospital, Chembur where his condition deteriorated.

"He had multiple issues. He had a kidney issue and had a transplant a while ago. But recently he got to know about kidney infection... He was on the ventilator for the last four days, after his situation started getting worse. Kidney infection was the beginning and then he got critical," Salim told PTI.

The music composer duo made their Bollywood debut with Salman's 1998 movie "Pyaar Kiya Toh Darna Kya" and went on to work on actor's various films including "Garv", "Tere Naam", "Tumko Na Bhool Payenge", "Partner" and the popular "Dabangg" franchise.

Wajid also did playback for Salman in chartbusters like "Mera He Jalwa", "Fevicol Se" and for Akshay Kumar in "Chinta Ta Chita Chita" from the film "Rowdy Rathore", among others. He recently co-composed Salman's songs "Pyaar Karona" and "Bhai Bhai", which the actor released on his YouTube channel.

Wajid along with his brother Sajid served as mentor on singing reality shows "Sa Re Ga Ma Pa 2012" and "Sa Re Ga Ma Pa Singing Superstar".

The composer duo also scored IPL 4 theme song, "Dhoom Dhoom Dhoom Dhadaka", which was sung by Wajid.

Soon after the news of the composer's demise broke, many from the film fraternity took to Twitter to pay tributes.

Actor Priyanka Chopra said she will always remember Wajid's smile.

"The one thing I will always remember is Wajid bhai's laugh. Always smiling. Gone too soon. My condolences to his family and everyone grieving. Rest in peace my friend. You are in my thoughts and prayers," she posted on Twitter.

Actor Parineeti Chopra said Wajid was one of the "nicest" men in the industry.

"Always singing. All heart. Always positive. You will truly be missed Wajid bhai," she tweeted.

Actor Varun Dhawan shared a picture of his filmmaker-father David Dhawan with Wajid and remembered the musician as one of the most positive people in Bollywood.

"Shocked hearing this news @wajidkhan7 bhai was extremely close to me and my family. He was one of the most positive people to be around. We will miss you Wajid bhai thank you for the music," he wrote.

Music composer-singer Vishal Dadlani said he was "heartbroken" after hearing the news.

"Both @SajidMusicKhan and @wajidkhan7 have been close & true friends. The kind who might see the light on and show up at our studio in the middle of the night just to meet and talk and share a laugh. Can't believe Wajid and I will never speak again," he wrote.

Singer-composer Shankar Mahadevan said he was still unable to come to terms with the news of Wajid's demise.

"Shocking ! Good bye dear brother.. love you .. till we meet on the other side ! Prayers for your peaceful journey Wajid bhai," he tweeted.

Singer Javed Ali posted, "Feeling Deeply Saddened after hearing shocking news of the sudden demise of Wajid Khan. May his Soul Rest in Peace. My heartfelt condolences to his family. May Allah give strength to the family."