Fears Covid is airborne grow as WHO acknowledges 'evidence emerging' that disease passes between people through infected droplets in the air

Scientists in the World Health Organization have admitted there is 'evidence emerging' coronavirus can be spread just by air.

The possibility of airborne transmission - especially in crowded, closed, poorly ventilated places, 'cannot be ruled out', Covid-19 leads at the WHO said.

The UN agency had previously said the virus spreads primarily through droplets expelled from the nose and mouth of an infected person.

These quickly sink to the ground, but can land in the nose or mouth of another person close by. They also land on surfaces, where the virus can be picked up.

But there have been calls for the WHO to update its guidance to warn the bug can linger in the air and be inhaled by others.

It follows 239 scientists in 32 countries writing to the UN agency asking it to acknowledge the growing evidence the virus is airborne.

One professor who signed the paper said there will be concern to label the virus airborne because it may cause panic. 

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Facemasks designed to stop the spread by droplets, as was previously instructed by the WHO. Pictured, people wearing face masks while queueing outside Ikea in Dublin, Ireland, June 8

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Dr Maria Van Kerkhove, the technical lead for Covid-19 at the WHO, confirmed the agency was looking at the theory the bug could be spread via the air

Dr Maria Van Kerkhove, the technical lead for Covid-19 at the WHO, confirmed the agency was looking at the theory the bug could be spread via the air.

She said: 'We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of COVID-19.'

Speaking at Tuesday's briefing in Geneva, Benedetta Allegranzi, the WHO's technical lead for infection prevention and control, said there was evidence emerging of airborne transmission of the coronavirus, but that it was not definitive.

'The possibility of airborne transmission in public settings - especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out,' she said.

'However, the evidence needs to be gathered and interpreted, and we continue to support this.'

There have been over more than half a billion deaths from the virus worldwide since the deadly pandemic gripped the globe. 

Up until now the WHO has been firm that the main route of transmission of the coronavirus is through infected droplets expelled when someone coughs or sneezes.

Because the droplets are large in size, they fall due to gravity within one metre or so. Health chiefs say catching the virus is avoidable by keeping a safe distance - at least 1.5 metres - away from other people.

The virus-laden droplets can land directly into another person's nose or mouth if not caught in a tissue. Or, they fall and land on surfaces, where it can live for up to three days.

If someone else touches that contaminated surface, the virus can transfer onto their hand. It only takes them touching their eyes, mouth, or nose for the virus to enter the body. This is why it is important to regularly wash hands.

Airborne diseases are those where germs from a person's breathe, sneeze or cough can linger in the air and travel distances longer than one metre.

The germs are contained in droplets very small in size - less than 5 μm in diameter compared with respiratory droplets which can be as large as 10 μm in diameter - called aerosols.

Examples of aerosols include dust, cigarette smoke or mist from a deodorant can. 

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Facemasks have been made compulsory for Britons to wear on all public transport. Pictured, a man wearing a mask in central London, June 5

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Any change in the WHO's assessment of risk of transmission could affect its current advice on keeping one metre apart from others. Pictured, people wearing face masks in Thailand, July 8

Measles and tuberculosis are both airborne diseases. They can spread over long distances - more than one metre - and even after the contagious patient has left a physical space, because their aerosols stay airborne for long periods of time.

Any change in the WHO's assessment of risk of transmission could affect its current advice on keeping one metre apart from others. 

Governments, which rely on the WHO for guidance policy, may also have to adjust public health measures aimed at curbing the spread of the virus. 

For months, keeping physically distanced from others and washing your hands regularly has been considered the key to minimising the risk of catching Covid-19.

With airborne diseases, these measures would still be vital. But the use of face masks and proper ventilation indoors, which would dilute virus-carrying aerosols, would likely become more important. 

Ventilation systems in schools, nursing homes, residences, and businesses may need to add new filters to their air conditioning units.

Dr Julian Tang, Associate Professor of Respiratory Sciences at the University of Leicester, said: 'Acknowledging that the virus is airborne would allow and drive various interventions.

'[This would include] enhancing our ventilation systems and keeping windows open - to remove/dilute any airborne virus to prevent its concentration building up indoors; being more vigilant with our PPE.'

Dr Tang added that the wearing of N95 masks - which are worn by healthcare workers and can filter out all sized particles - may be needed across hospitals, and not just for those who work in very close proximity to Covid-19 patients. 

'Other options include the introduction of air-cleansing technologies, like ultra-violet (UV) light (as germicidal UV), reducing over-crowding in public places and on public transport,' he said.

Van Kerkhove said the WHO would publish a scientific brief summarising the state of knowledge on modes of transmission of the virus in the coming days.

'A comprehensive package of interventions is required to be able to stop transmission,' she said.

'This includes not only physical distancing, it includes the use of masks where appropriate in certain settings, specifically where you can't do physical distancing and especially for healthcare workers.'

Dr David Nabarro, a special envoy to the WHO on Covid-19, said that while airborne transmission was 'certainly part of the puzzle', it is not the main route of transmission. 

'The major way in which that this virus is transmitted is through droplets which come out when you cough, sneeze or even shout,' he told the BBC Radio 4 Today programme.

'But there does seem to be the possibility that very small particles containing virus can move further. 

'It is certainly part of the puzzle. The key question is just how important is it. Right now we think it is not the major means of transmission. The major means of transmission is droplets, which is why we talk about one to two metres distance as being the key thing to avoid spread.'

It follows the publication of an open letter to the Geneva-based agency, signed by 239 scientists in 32 countries, which outlined evidence that they say shows floating virus particles can infect people who breathe them in.

Because those smaller exhaled particles can linger in the air, the scientists in the group had been urging WHO to update its guidance. 

The WHO said there is insufficient evidence to prove airborne transmission of SARS-CoV-2 is happening. 

But the letter, published on Monday in the Clinical Infectious Diseases journal, argued there is insufficient proof that airborne transmission does not occur.

They wrote: 'Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond one to two metres from an infected individual.'

Dr Tang, who signed the letter, said: 'This difference is subtle but quite major in terms of how you interpret the evidence - absence of evidence is not evidence of absence.

'Our stance is a more precautionary one than that of the WHO – and is an important difference as countries are now coming out of lockdown with new spikes of COVID-19 cases globally.

'Some of these new cases may be due to aerosol transmission, particularly if new cases are occurring despite people masking.'

Professor Jose Jimenez, a chemist at the University of Colorado who signed the paper, said: 'We wanted them to acknowledge the evidence.

'This is definitely not an attack on the WHO. It's a scientific debate, but we felt we needed to go public because they were refusing to hear the evidence after many conversations with them,' he said in a telephone interview with Reuters.

Professor Jimenez said historically, there has been a fierce opposition in the medical profession to the notion of aerosol transmission, and the bar for proof has been set very high. 

A key concern has been a fear of panic, because an airborne disease would suggest the virus could be caught from someone who has left the room.

'If people hear airborne, healthcare workers will refuse to go to the hospital,' he said. 

Or people will buy up all the highly protective N95 respirator masks 'and there will be none left for developing countries'.

Professor Jose Vazquez-Boland, Chair of Infectious Diseases, University of Edinburgh, said 'there is clearly a danger' that the public will struggle to understand what airborne transmission really means.

He said aerosols may travel longer distances but are more exposed to being dried out by the environment.

'The WHO and other public health bodies interpret that, except in certain circumstances (aerosol-generating clinical procedures during patient care), they play a lesser role in Covid-19 transmission.

'However, this does not mean that under specific conditions of humidity, temperature and ventilation, aerosols formed by exhaled microdroplets could not be a major source of airborne transmission for Covid-19.

'For the public it may be difficult to differentiate between the different situations and technical definitions.' 

Professor Jimenez said the WHO panel assessing the evidence on airborne transmission was not scientifically diverse, and lacked representation from experts in aerosol transmission.

It comes as the US announced it would be leaving the WHO from next July.

The United Nations yesterday revealed the timeline after receiving formal notification of the decision by US President Donald Trump more than a month ago.