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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.

May 22: A Pakistan International Airlines (PIA) flight on its way from Lahore to Karachi, crashed in the area near Jinnah International Airport on Friday, according to Civil Aviation Authority officials.

Geo News reported that the plane crashed at the Jinnah Ground area near the airport as it was approaching for landing. There were more than 90 passengers on board the Airbus aircraft. Black smoke could be seen from afar at the crash site, say eye witnesses.

There were no immediate reports on the number of casualties. The aircraft arriving from the eastern city of Lahore was carrying 99 passengers and 8 crew members, news agency AP said, quoting Abdul Sattar Kokhar, spokesman for the country’s civil aviation authority.

Witnesses said the Airbus A320 appeared to attempt to land two or three times before crashing in a residential area near Jinnah International Airport.

Flight PK-303 from Lahore was about to land in Karachi when it crashed at the Jinnah Garden area near Model Colony in Malir, just a minute before its landing, Geo News reported.

Local television reports showed smoke coming from the direction of the airport. Ambulances were on their way to the airport.

News agency said Sindh’s Ministry of Health and Population Welfare has declared emergency in all major hospitals of Karachi due to the plane crash.

It’s the second plane crash for Pakistani carrier in less than four years. The airline’s chairman resigned in late 2016, less than a week after the crash of an ATR-42 aircraft killed 47 people. The incident comes as Pakistan was slowly resuming domestic flights in the wake of the coronavirus pandemic, Bloomberg reported.

Jan 30: The death toll rose to 170 in the new virus outbreak in China on Thursday as foreign evacuees from the worst-hit region begin returning home under close observation and world health officials expressed “great concern” that the disease is starting to spread between people outside of China.

Thursday’s figures cover the previous 24 hours and represent an increase of 38 deaths and 1,737 cases for a total of 7,711. Of the new deaths, 37 were in the epicenter of the outbreak in Hubei province and one in the southwestern province of Sichuan.

The news comes as the 195 Americans evacuated from Wuhan, the Hubei province city of 11 million where the outbreak originated, are undergoing three days of testing and monitoring at a Southern California military base to make sure they do not show signs of the virus.

A group of 210 Japanese evacuees from Wuhan landed Thursday at Tokyo’s Haneda airport on a second government chartered flight, according to the foreign ministry. Reports said nine of those aboard the flight showed signs of cough and fever. Three of the 206 Japanese who returned on Wednesday tested positive for the new coronavirus, Prime Minister Shinzo Abe said during a parliamentary session. Two of them showed no symptoms of the disease.

France, New Zealand, Australia and other countries are also pulling out their citizens or making plans to do so.

The World Health Organization emergencies chief said the few cases of human-to-human spread of the virus outside China — in Japan, Germany, Canada and Vietnam — were of “great concern” and were part of the reason the U.N. health agency’s director-general was reconvening a committee of experts on Thursday to assess whether the outbreak should be declared a global emergency.

The new virus has now infected more people in China than were sickened there during the 2002-2003 SARS outbreak.

Dr. Michael Ryan spoke at a news conference in Geneva on Wednesday after returning from a trip to Beijing to meet with Chinese President Xi Jinping and other senior government leaders. He said China was taking “extraordinary measures in the face of an extraordinary challenge” posed by the outbreak.

To date, about 99% of the cases are in China. Ryan estimated the death rate of the new virus at 2%, but said the figure was very preliminary. With fluctuating numbers of cases and deaths, scientists are only able to produce a rough estimate of the fatality rate and it’s likely many milder cases of the virus are being missed.

In comparison, the SARS virus killed about 10% of people who caught it. The new virus is from the coronavirus family, which includes those that can cause the common cold as well as more serious illnesses such as SARS and MERS.

Scientists say there are many questions to be answered about the new virus, including just how easily it spreads and how severe it is.

In a report published Wednesday, Chinese researchers suggested that person-to-person spread among close contacts occurred as early as mid-December.

“Considerable efforts” will be needed to control the spread if this ratio holds up elsewhere, researchers wrote in the report, published in the New England Journal of Medicine.

More than half of the cases in which symptoms began before Jan. 1 were tied to a seafood market, but only 8% of cases after that have been, researchers found. They reported the average incubation period was five days.