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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.

Mar 13: Canadian Prime Minister Justin Trudeau and his wife announced they were self-isolating Thursday as she undergoes tests for the new coronavirus after returning from a speaking engagement with "mild flu-like symptoms."

Sophie Gregoire-Trudeau's symptoms have subsided since she recently got back from Britain, but as a precaution the prime minister "will spend the day in briefings, phone calls and virtual meetings from home," according to a statement.

Trudeau also cancelled a meeting Thursday and Friday with Canada's provincial and territorial leaders in Ottawa, but still planned to speak with them and world leaders by phone about measures being taken to curb the spread of the virus in Canada.

Gregoire-Trudeau's symptoms had included "a low fever late last night." She immediately sought medical advice and testing.

Trudeau has exhibited no symptoms, and was advised by doctors "to continue daily activities while self-monitoring."

"However, out of an abundance of caution, the prime minister is opting to self-isolate and work from home until receiving Sophie's results," said his office.

Since the novel coronavirus first emerged in late December 2019, 127,070 cases have been recorded in 115 countries and territories, killing 4,687 people, according to an AFP tally compiled at 1200 GMT on Thursday based on official sources.

Canada has so far reported more than 100 cases in six provinces, and one death.

Also Thursday, the Canada's Juno music awards cancelled its upcoming gala show, planned for Sunday evening in Saskatoon, Saskatchewan.

"We are devastated to cancel this national celebration of music, but at this time of global uncertainty, the health, safety and well-being of all Canadians must stand at the forefront of any decisions that impact our communities," organisers said in a statement.

And in Quebec province, Premier Francois Legault unveiled a series of measures to prevent the spread of the coronavirus, including placing all travellers returning from overseas under quarantine for two weeks.

Quebec also banned indoor gatherings of more than 250 people.

Hong Kong, Apr 20: Oil prices collapsed to more than two-decade lows Monday as traders grow concerned that storage facilities are reaching their limits, while equities were mixed, with some support coming from signs that the coronavirus may have peaked in Europe and the United States.

US crude benchmark West Texas Intermediate briefly plunged almost 20 percent to below 15 -- its lowest since 1999 -- as stockpiles continue to build owing to a crash in demand caused by the COVID-19 pandemic.

Analysts said this month's agreement between top producers to slash output by 10 million barrels a day was having little impact on the oil crisis because of lockdowns and travel restrictions that are keeping billions of people at home.

WTI was hit particularly hard as its main US storage facilities in Cushing, Oklahoma, were filling up.

ANZ said "crude oil prices remained under pressure, as projections of weaker demand weigh on sentiment".

"Despite the OPEC+ alliance agreeing to an unprecedented cut in output, the physical market is awash with oil," it said, referring to the Organization of the Petroleum Exporting Countries and non-OPEC partners.

And AxiCorp's Stephen Innes added: "It's a dump at all cost as no one... wants delivery of oil, with Cushing storage facilities filling by the minute.

"It hasn't taken long for the market to recognise that the OPEC+ deal will not, in its present form, be enough to balance oil markets." Stock traders were in slightly more buoyant mood as governments start to consider how and when to ease lockdowns that have crippled the global economy.

Italy, Spain, France and Britain reported drops in daily death tolls and slowing infection rates.

"We are scoring points against the epidemic," said Prime Minister Edouard Philippe, while insisting "we are not out of the health crisis yet".

Meanwhile, in the US, Andrew Cuomo, governor of badly hit New York state, said the disease was "on the descent", though he cautioned it was "no time to get cocky".

Mounting evidence suggests that the lockdowns and social distancing are slowing the spread of the virus.

That has intensified planning in many countries to begin loosening curbs on movement and easing the crushing pressure on national economies.

Adding to the sense of hope was a report indicating promising research on a drug to treat coronavirus.

Hong Kong, Shanghai and Seoul were each up 0.1 percent, while Wellington added 0.4 percent.

However, Tokyo went into the break 0.9 percent lower, while Sydney and Manila dropped one percent apiece. There were also losses in Taipei, Singapore and Jakarta.

"The longer investors have to contemplate future economic issues while they wait for more countries to be on the downward slope of the pandemic curve, the more scope there is of risk assets pricing in a difficult future," Chris Iggo, of AXA Investment Managers UK, said.

Investors are keeping an eye on Washington, where Congress and the White House are working towards a 450 billion economic relief plan for small business to add to the trillions already pledged to support the economy.

Big-name companies including IBM, Netflix and Coca-Cola are due to deliver their earnings reports.