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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.

Paris, May 10: The number of coronavirus cases worldwide topped four million as some of the hardest-hit countries readied Sunday to lift lockdown restrictions despite concern about a second wave of infections.

Governments around the world are trying to stop the spread of the deadly disease while scrambling for ways to relieve pressure on their economies, which are facing a historic downturn with millions pushed into unemployment.

Despite the intense political pressure to reopen, nations are also keen to avoid second waves of infections that could overwhelm healthcare systems, with reminders over the weekend of the threat posed by the virus.

In the United States, media reported Saturday that the nation's top infectious disease expert, Anthony Fauci, was among three members of the White House coronavirus task force who will self-isolate after potential exposure.

And in South Korea, the capital Seoul shut all bars and clubs on Saturday as more than 50 cases were linked to a man who tested positive after spending time in one of the city's busiest nightlife districts.

Despite the risks, some governments in hard-hit Europe have said are signs of progress that justify cautious steps towards normality.

Officials in France on Saturday said the day's death toll of 80 was the lowest since early April, while nursing home fatalities also fell sharply as the nation prepared to relax curbs on public movement imposed eight weeks ago.

The easing, to begin Monday, has brought mixed reactions.

"I've been scared to death" about the reopening, said Maya Flandin, a bookshop manager from Lyon. "It's a big responsibility to have to protect my staff and my customers."

French health officials have warned that social distancing must be kept up even as restrictions are eased.

In Spain, about half the population will be allowed out on Monday for limited socialisation, and restaurants will be able to offer some outdoor service as the country begins a phased transition set to last through June.

With lingering fears of a resurgence, authorities excluded Madrid and Barcelona -- two COVID-19 hotspots -- from the first phase.

Belgium is also easing some restrictions on Monday, and in some parts of Germany, bars and restaurants reopened on Saturday with further easing set for Monday.

In Britain, Prime Minister Boris Johnson is expected Sunday to lay out a plan for the nation to emerge out of its current lockdown.

Media reports have suggested that Britain may introduce a mandatory 14-day quarantine for international arrivals to stop the spread of the virus.

Global economic figures are pointing to the most acute downturn in nearly a century, with businesses forced to shut and supply lines badly disrupted, and pressure is growing on leaders around the world to find a way out as the worldwide death toll topped 277,000 and infections crossed four million.

In the United States, the country with the highest death toll and where more than 20 million people have lost their jobs, President Donald Trump has insisted that next year would be "phenomenal" for the economy, urging reopening despite the virus still claiming well over 1,000 lives daily in the country.

The scale of the challenge was brought in sharp focus over the weekend as US media reported that top disease expert Anthony Fauci, who has become the trusted face of the government response to the pandemic, is going to self-isolate after possible exposure to an infected White House staffer.

Fauci told CNN that he will undergo a "modified quarantine" as he had not been in close proximity to the staffer, the network reported. He will remain at home teleworking, and will wear a mask for two weeks.

Robert Redfield, the director of the Centers for Disease Control and Prevention, and Stephen Hahn, the commissioner of the Food and Drug Administration, will also self-isolate, CNN added.

All three will still testify at Tuesday's Senate coronavirus hearing, with Redfield and Hahn participating via video link, according to Senator Lamar Alexander, chairman of the chamber's health committee.

It is believed Fauci will attend wearing a mask, CNN reported.

President Trump has faced sharp criticism from his predecessor Barack Obama, who said on a leaked tape that Trump's handling of the crisis was an "absolute chaotic disaster".

With people wearying of being indoors and under economic pressure, anti-lockdown protests have been held in a number of countries in recent weeks, with some demonstrators arguing that such restrictions violate their rights and others promoting conspiracy theories about the pandemic.

Ten people were arrested and a police officer injured in Melbourne, Australia, on Sunday in the latest such protest, where around 150 people gathered to demand an end to the shutdown.

Participants were promoting a number of conspiracy theories, such as linking 5G cellular communications to the disease.

Australian chief medical officer Brendan Murphy said there was "a lot of very silly misinformation out there", including the 5G allegation.

"I have unfortunately received a lot of communication from these conspiracy theorists myself," he said.

"It is complete nonsense. 5G has got nothing at all to do with coronavirus."

Mar 7: Two Malayalam news channels, Asianet News and Media One, which were banned by the information and broadcasting ministry for their coverage of the recent violence in Delhi on Friday evening, were allowed to resume telecasting on Saturday morning.

While Asianet News appeared to have begun operations around 7am on Saturday, Media One was screening content by 9.30am.

The ministry of information and broadcasting had imposed a 48-hour ban on Asianet News and Media One for their coverage of the Delhi violence for 48 hours from 7.30pm on Friday. Both Asianet News and Media One were barred under Rule 6(1 c) and Rule 6(1e) of the Cable Television Networks Act, 1994.

The ministry of information and broadcasting alleged Asianet News and Media One were "biased" and critical of the RSS and Delhi Police.

The ban on Asianet News and Media One triggered a torrent of criticism of the move. Congress MP Shashi Tharoor asked how "Malayalam channels inflame communal passions in Delhi?" and alleged some English news channels were continuing "their brazen distortions" with impunity.

In a statement issued on Friday after the ban, Media One termed the move "unfortunate and condemnable" and called it a "blatant attack against free and fair reporting". Media One called it "an order to stop free and fair journalism".