Coronavirus: Could social distancing of less than two metres work?
The government has come under pressure from MPs and businesses to relax the 2m (6ft) rule for social distancing, to make it easier for people to return to work.
But scientists continue to question whether that would be safe, given how little is known about how far coronavirus can spread.
What does the science say?
The simple answer is that the closer you are to someone who is infected, the greater the risk of catching the virus.
The World Health Organization says that a distance of 1m is safe. Some countries have adopted this guidance, while others, including the UK, have gone further:
- 1m distancing rule - China, Denmark, France, Hong Kong, Lithuania, Singapore
- 1.4m - South Korea
- 1.5m - Australia, Belgium, Germany, Greece, Italy, Netherlands, Portugal
- 1.8m - US
- 2m - Canada, Spain, UK
It's not just about distance
Timing is also key. The longer you spend in close proximity with an infected person, the bigger the risk.
Scientists advising the UK government say that spending six seconds at a distance of 1m from someone is the same as spending one minute at a distance of 2m.
Being exposed to someone coughing is riskier. Being 2m away from a cough carries the same risk as someone talking to you for 30 minutes at the same distance.
What's the latest research?
In a study published in the medical journal The Lancet, scientists evaluated recent research into how the coronavirus can spread.
They conclude that keeping at least 1m from other people could be the best way to limit the chances of infection.
The risk of being infected is estimated to be 13% within 1m, but only 3% beyond that distance.
And the study says that for every extra metre of distance up to 3m, the risk is further reduced by half.
Where does the distancing rule come from?
It can be traced back to research in the 1930s.
Scientists found that droplets of liquid released by coughs or sneezes evaporate quickly in the air or fall to the ground.
Most of those droplets, they reckoned, would land within 1-2m.
That is why it is said the greatest risks come from having the virus coughed at you from close range, or from touching a surface that someone coughed onto, and then touching your face.
Can the virus travel further in other ways?
Many scientists regard closeness and surface contacts as the main routes of transmission.
But some researchers are concerned that the coronavirus can also be transported through the air in tiny particles called aerosols.
If that is the case, then the flow of wind from someone's breath could carry the virus over longer distances.
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Professor Lydia Bourouiba from the Massachusetts Institute of Technology (MIT) used high-speed cameras to capture a cough projecting miniature specks as far as 6m.
And a study carried out at hospitals in China, which found traces of coronavirus in Covid-19 wards and intensive care units, estimated that 4m was a better safe distance.
But for the US Centers for Disease Control, the role of aerosols in spreading the virus is "currently uncertain".
And what's still not known is whether any virus that spreads further than 2m can still be infectious.
What else makes a difference?
There is wide agreement that the infection is more easily passed on indoors than outside in the fresh air.
Japanese researchers investigated 110 cases of Covid-19, following up the contacts of the people infected.
They estimated that the odds of the infection being passed on were nearly 19 times greater indoors than when outside.
In many countries, including England and Scotland, people are being encouraged to wear face coverings on public transport and "enclosed spaces where social distancing is not always possible".
Why don't we have definite answers?
It is only a few months since the coronavirus emerged and in that short time scientists have learned a great deal about it.
But there is a long way to go and confirming exactly the right distance is one of the unanswered questions.
It will take careful studies of how the virus can be carried, and how viable it remains, which will all take time.