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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.

Feb 4: Americans on Monday kicked off the first vote of the 2020 presidential race as the midwestern state of Iowa began its caucuses, the closely-watched first step in deciding which Democrat will face incumbent Donald Trump in November's election.

The two frontrunners, left-wing Senator Bernie Sanders and former Vice President Joe Biden face a key test in the sparsely populated state, with a handful of others looking to make their mark to give their campaigns momentum.

The Iowa vote is a critical early look at the viability of the 11 Democratic candidates still in the race - even though just 41 Iowa delegates are up for grabs, a fraction of the 1,991 needed to secure the party nomination in July.

Iowa Democrats filed into nearly 1,700 caucus sites - schools, libraries, churches, mosques and meeting halls with Sanders and Biden in the lead in the state, followed by former South Bend, Indiana mayor Pete Buttigieg and Senator Elizabeth Warren, who is also on the left of the party.

But polling has fluctuated and Iowa's quirky caucus system - where voting is not by secret ballot but by public declaration for a candidate - makes the night hard to predict.

Luke Elzinga, a volunteer for Sanders, appeared early at Lincoln High School in Des Moines which was converted into a caucus location.

"I think he really inspires a lot of young people, a lot of disaffected voters who might not otherwise turn out," Elzinga, 28, told AFP news agency shortly before the caucusing began.

"And so I think he's the best candidate to beat Trump."

Three candidates - Sanders, Warren and Amy Klobuchar - have faced the unprecedented scenario of spending much of the past two weeks tethered to Washington for the impeachment trial of Trump instead of on the campaign trail in Iowa.

Even as candidates sought to make 11th-hour impressions on undecided voters, the senators were obligated to return to Washington for the trial's closing arguments on Monday.

Defeating Trump

In a vote scheduled for Wednesday, Trump is almost certain to be acquitted by the Republican-led upper house on charges of abuse of power and obstruction of Congress.

For Democrats, second-tier hopefuls Klobuchar and tech entrepreneur Andrew Yang look to outpace expectations and seize momentum heading into the next contest in New Hampshire on February 11.

Earlier on Monday Biden - who still holds the lead in national polls - brought pizza to a field office in a strip mall near Des Moines to thank volunteers.

"I'm feeling good about today," he said.

Like many candidates, Biden spent the weekend crisscrossing Iowa in a final push to convince undecided voters he is best placed to accomplish Democrats' number one goal: defeating Trump.

The president has not stood idly by. On Sunday he branded Biden "Sleepy Joe" and described Sanders as "a communist," previewing a likely line of attack were Sanders to win the nomination.

Unlike secret ballot voting, caucus-goers publicly declare their presidential choice by standing together with other supporters of a candidate.

Candidates who reach 15 percent support earn delegates for the nomination race while supporters of candidates who fall short can shift their allegiance to others.

Turnout is critical, and candidates and their representatives will seek to persuade voters on issues including healthcare, taxes and ending Washington corruption.

One key candidate who has opted not to contest in Iowa is billionaire businessman Michael Bloomberg, who entered the race in November but has surged into fourth place in RealClearPolitics' national polling average.

The former New York mayor, who has spent more than $300m on advertising, according to Advertising Analytics, is focused on running a national campaign with particular emphasis on states that vote on "Super Tuesday," on March 3.

Singapore, Mar 23: Oil prices fell at the open in Asia on Monday after a trillion-dollar Senate proposal to help the coronavirus-hit American economy was defeated and death tolls soared across Europe and the US.

US benchmark West Texas Intermediate initially tumbled more than three percent but then pulled back some ground to trade 1.5 percent lower, at $22 a barrel.

Brent crude, the international benchmark, fell 4.9 percent to $25 a barrel.

Prices have fallen to multi-year lows in recent weeks as lockdowns and travel restrictions to fight the virus hit demand, and top producers Saudi Arabia and Russia engage in a price war.

The latest drop came after a trillion-dollar Senate proposal to rescue the US economy was defeated after receiving zero support from Democrats, and with five Republicans absent from the chamber because of virus-related quarantines.

The bill had proposed funding for American families, thousands of shuttered or suffering businesses and the nation's critically under-equipped hospitals.

Coronavirus deaths soared across Europe and the United States at the weekend despite heightened restrictions.

The death toll from the virus -- which has upended lives and closed businesses and schools across the planet -- surged to more than 14,300 Sunday, according to an AFP tally.

AxiCorp chief markets strategist Stephen Innes said that "total demand devastation" had set it.

"Oil markets collapsed out of the gate this morning as prices react... to stringent containment lockdown measures," he said.