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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.

The Centre on Sunday extended the COVID-19 lockdown for two more weeks till May 31 with more exemptions as the Centre allowed states more powers for profiling its zones, re-starting of inter-state and intra-state bus travel, plying of autos and taxis and opening of all shops, including in markets but barring those in malls.

Here are the answers to all your questions:

What is 'Lockdown 4.0'?

On March 24, 2020, Prime Minister Narendra Modi announced a 21-day nation-wide lockdown to prevent the spread of the coronavirus outbreak. India follows several countries in its measures to curb the pandemic, which was the first lockdown. Prime Minister then extended the coronavirus lockdown till May 3, which was dubbed as 'Lockdown 2.0'. This lockdown was further prolonged till May 17 which became 'Lockdown 3.0' and now, as the government aims at a staggered re-opening of the country while maintaining the norms such as social distancing, the fourth extension till May 31 is called 'Lockdown 4.0'.

Who issues the guidelines for the lockdown?

The Ministry of Home Affairs (MHA) issues the guidelines for the lockdown.

Are guidelines different during a lockdown compared to normal life?

Well, of course. Guidelines during a lockdown instruct people on all matters from whether you are allowed to leave your house, to whether an MNC is allowed to function and with what percentage of attendance.

Are masks compulsory even now?

Masks are made mandatory in all public places, by the Union Health Ministry. All the states and UTs are to strictly abide by this law. Not wearing masks will attract penalties which are specified by the state.

How would that be determined for an area?

By Lockdown 3.0, all areas of state districts were segregated into containment, red, orange and green zones. In the Lockdown 4.0, states will categorise the areas into red, orange and green zones.

Colourful... but what are red, orange and green zones?

According to the guidelines issued by the Ministry of Health and Family Welfare, states can categorise districts or municipal corporations as red or orange zones.

"States may, however, also choose to categorise a sub-division or ward or any other appropriate administrative unit as red/orange/green zone after detailed analysis at their end, duly taking into consideration the geographical spread of cases, contacts and their zone of influence in terms of disease spread," the ministry said.

With the commencement of the third phase of lockdown, the Union Health Ministry listed 130 districts across the country in the red zone, 284 in the orange zone and 319 in green zones based on the incidence of cases of COVID-19, doubling rate, the extent of testing and surveillance feedback.

Districts were earlier designated as hotspots/red-zones, orange zones and green zones primarily based on the cumulative cases reported and the doubling rate.

A district will be considered under green zone if there have been no confirmed cases of COVID-19 so far or there is no reported case since last 21 days in the district, according to the letter.

Now, what is a buffer zone?

A buffer zone is an area of spread in a 5-kilometre radius (7 Kms in rural areas) of a containment zone.

How do I find out the zone I am in?

You either look at your state or city's municipal corporation pages to avail the zone details. You can also look at your district magistrate's Twitter handle or Facebook account to find out the list of the zone under which your area falls.

Can I leave my home now?

That depends. If you are in a containment zone or a red zone, you may not be allowed to leave your residence. Otherwise, in the other zones, the state governments and the district magistrates will decide upon the level of movement within and outside the zones.

Can I shift from a red zone to an orange or green zone?

You cannot. The residents of a red or containment zone cannot move out of their zones, nobody may enter the zones as well.

What about my office?

The private offices can operate in non-containment zones. The guidelines for offices to work will be listed by the state governments and the DMs (district magistrates).

Will I be allowed to use my bike/car or any other personal vehicle?

There is a likely chance of you being allowed to take out your bike or car or other vehicles (not helicopters or aeroplanes), if you are not in a containment zone. You need to check the rules listed by your state government or DM. The number of people who can ride at one time will also be decided by the state.

Can my driver, house help or neighbour drive me to my office/destination? Will I be allowed to take them to my workplace?

Yes, provided they are not from a red zone which may be risky for the passenger. This facility is prohibited in a containment zone. Also, check with your workplace regarding the norms to follow within the office. For the details on travelling with others in the car, look into the info provided by your state government, DM and Resident Welfare Association (RWA).

Will be able to fill petrol or diesel for my vehicle?

Definitely, yes. All petrol pumps, LPG and oil agencies will continue to be open.

What if I need to take a cab, auto or book one via Ola/Uber?

The same rules apply to them as well. Unless you are in a containment zone, the restrictions for using cabs and autos will be eased.

What if I need to use public transport like buses?

Some states have allowed buses to run, such as in Tamil Nadu in certain areas. You will have to check with the state government or DM's regulations enlisted for knowing the routes and norms to follow inside a bus.

Can I use my city's Metro line?

Unfortunately, metro lines are not allowed to open and will remain closed until further notification from the Centre.

Can I walk around in my area?

Walking will be permitted under the guidelines issued by the state and DM. Walking in groups will be prohibited and social distancing norms are to be followed in public at all times. Movement is allowed between 7 am to 7 pm in any zone - containment, red, orange or green.

Can I take my grandparents/kids out for a walk?

People older than 65 years of age, or younger than 10, persons with co-morbidities, pregnant women are not allowed to venture out of their residences, as they are highly susceptible to the infection.

Can I go out with my friends?

A group of less than 5 people are allowed to walk together. If you are planning to use vehicles such as bikes, every induvial must have their own as more than one person on atwo-wheeler is not allowed in certain areas. Curfew timings are from 7 am to 7 pm as movent is prohibited beyond these timings. Check the regulations issued by your local DM or state government to know further details.

What if I need to see my friends, relatives or others?

The Resident Welfare Association (RWA) will make a decision about allowing visitors inside a zone, barring containment zones. Nobody is allowed within the containment zone or permitted to leave.

Can we now go to restaurants?

Restaurants are still closed irrespective of the zone. Take-away or delivery services will be available, nevertheless.

Can we go to malls?

Malls and restaurants and shops in the malls will remain closed irrespective of the zone, as these are crowd-pulling zones.

Does that mean multiplexes, theatres and drama/concert halls are closed as well?

Cinema halls, theatres, multiplexes and drama/concert halls will remain closed regardless of which zone they are in, till further instructions are sent by the Centre.

Can I go to the beach or a monument/heritage site?

Since such public places will attract a huge crowd that will be tough to control, beaches, monuments, heritage sites and such public places will be closed.

Can I go to coffee shops?

As coffee shops will fall under the category of restaurants, they will also be closed. Take-away and delivery services can be availed from the shops.

What about essentials?

Grocery shops, milk vendors, newspaper circulation are allowed to stay open. Proper sanitisation must be done from time-to-time to ensure customer and vendors’ safety.

I need to repair my phone. Will stand-alone non-essential services be open?

Such non-essential services are allowed to open in non-containment zones. Refer the state governments and DMs rules for knowing the type of shops and state and districts they are permitted in.

What about in-house repairs or services? Can I call a mechanic to my house?

Yes, provided your RWA has permitted to allow mechanics, workmen and labourers inside in non-containment zones.

More than 50 million people in India do not have access to effective handwashing, putting them at a greater risk of acquiring and transmitting the novel coronavirus, according to a study.

Researchers from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in the US found that without access to soap and clean water, over 2 billion people in low- and middle-income nations -- a quarter of the world's population -- have a greater likelihood of transmitting the coronavirus than those in wealthy countries.

According to the study, published in the journal Environmental Health Perspectives, more than 50 per cent of the people in sub-Saharan Africa and Oceania lacked access to effective handwashing.

"Handwashing is one of the key measures to prevent COVID transmission, yet it is distressing that access is unavailable in many countries that also have limited health care capacity," said Michael Brauer, a professor at IHME.

The study found that in 46 countries, more than half of people lacked access to soap and clean water.

In India, Pakistan, China, Bangladesh, Nigeria, Ethiopia, Democratic Republic of the Congo, and Indonesia, more than 50 million persons in each country were estimated to be without handwashing access, according to the study.

"Temporary fixes, such as hand sanitizer or water trucks, are just that -- temporary fixes," Brauer said.

"But implementing long-term solutions is needed to protect against COVID and the more than 700,000 deaths each year due to poor handwashing access," Brauer said.

He noted that even with 25 per cent of the world's population lacking access to effective handwashing facilities, there have been "substantial improvements in many countries" between 1990 and 2019.

Those countries include Saudi Arabia, Morocco, Nepal, and Tanzania, which have improved their nations' sanitation, the researchers said.

The study does not estimate access to handwashing facilities in non-household settings such as schools, workplaces, health care facilities, and other public locations such as markets.

Earlier this month, the World Health Organization predicted 190,000 people in Africa could die of COVID-19 in the first year of the pandemic, and that upward of 44 million of the continent's 1.3 billion people could be infected with the coronavirus, the researchers said.