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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, Mar 6: World health officials have warned that countries are not taking the coronavirus crisis seriously enough, as outbreaks surged across Europe and in the United States where medical workers sounded warnings over a "disturbing" lack of hospital preparedness.

The World Health Organization warned Thursday that a "long list" of countries were not showing "the level of political commitment" needed to "match the level of the threat we all face".

"This is not a drill," WHO chief Tedros Adhanom Ghebreyesus told reporters.

"This epidemic is a threat for every country, rich and poor."

Tedros called on the heads of government in every country to take charge of the response and "coordinate all sectors", rather than leaving it to health ministries.

What is needed, he said, is "aggressive preparedness."

Canberra, May 21: Australian Prime Minister Scott Morrison and his Indian counterpart, Narendra Modi, are looking forward to deepening the countries' strategic relationship, with both sides expected to sign a range of pacts from defence to trade in strategic sectors amid heightened tensions with China over Beijing's response to coronavirus pandemic.

During a virtual summit, scheduled to take place on June 4, both leaders are expected to ramp up efforts to diversify Australia's export markets and find trusted suppliers of vital products and components, a local newspaper, The Australian reported on Tuesday.

The new agreements will focus on reliable supply chains in key strategic sectors, including medical goods, technology and critical minerals, amid heightened tensions with China over Beijing's response to coronavirus pandemic.

The leaders will seal a new defence agreement allowing reciprocal access to bases and co-operation on military technology projects, while a new education partnership will be on the table to help overcome Australian university reliance on Chinese students.

The talks in terms of strategic convergence, now have greater significance as COVID-19 exacerbates the strategic contest between the US and China, and forces like-minded countries to seek out reliable partners.

Australian farmers could also benefit, with talks underway on expanding agricultural exports to India, including barley, as China throws up new trade barriers, media reports stated.

The virtual summit follows the cancellation of Morrison's planned state visit to India in January due to the bushfires.

Morrison said last year, ahead of his planned visit, that India was "a natural partner for Australia", referring to the countries' "shared values" -- a point of differentiation with China.

Former Department of Foreign Affairs and Trade secretary Peter Varghese, who wrote a landmark report on the bilateral relationship in 2018, was quoted by the newspaper as saying that India would be even more important to Australia in the post-COVID world. "If one of the lessons from COVID is that countries need to spread their risk, then finding new markets or building up existing markets is a crucial part of that," he added.

Varghese noted that India, a member of the Quadrilateral Security Dialogue along with Australia, Japan and the US, was a vital strategic partner to Australia in helping "constrain China's ambitions to be the predominant power".

"That shared objective between Australia and India of not wanting to see the region dominated by China is a key component of building up our geopolitical relationship," he told The Australian.

The summit also follows recent talks between Australian Foreign Minister Marise Payne and Indian counterpart Subrahmanyam Jaishankar on the pandemic response and Australia's call for an independent inquiry, which was overwhelmingly backed at the World Health Assembly on Tuesday.

Australia wants to support India to develop a domestic critical minerals processing industry, which would provide Western nations with an alternative to sourcing the materials from China.

Meanwhile, India has strong expertise as a manufacturer of drugs and medical equipment, while Australia is a centre of biomedical research, opening the possibility for closer co-operation in the key sector, the media reported further.