As coronavirus continues to spread, some governments have restricted travel while, elsewhere, individual travellers have been taking their own precautions.

The BBC has been receiving questions about whether it is safe to fly, go on a cruise or take public transport.

Trains and buses

It's not yet known exactly how coronavirus spreads, but similar viruses are caught from either breathing in droplets from an infected person's coughs or sneezes, or touching surfaces with them on.

We think that coronavirus probably doesn't hang around in the air in the same way as flu does, so, people need to be in closer contact with each other to catch it. The NHS guidance on coronavirus defines "close contact" as being within two metres of an infected person for more than 15 minutes.

So a lot of the potential risk of infection on trains and buses depends on how crowded they are, and this will vary in different parts of the country and on different routes. 

On the London Underground, where there is an extremely high density of people crowded into each carriage, previous research has suggested a link between commuting and the likelihood of catching respiratory illnesses.

Dr Lara Gosce, at the Institute of Global Health, says her research (published in 2018) showed people who used the Underground regularly were more likely to suffer flu-like symptoms.

"Particularly, it shows that boroughs served by fewer lines - where inhabitants are forced to change line one or more times when travelling on the Underground - have higher rates of influenza-like diseases, compared to well-served boroughs where passengers reach their destination by one direct trip," she said.

If you're travelling on a relatively empty train or bus, though, your risks would be different. How well vehicles are ventilated and how long you spend on them will also play a role.

And cleaning will be a factor too. Network Rail says it is continuing with its "business as usual" cleaning schedule but is making plans for extra, specialist cleans of stations if necessary.

Major bus operator Arriva also said it would continue with its normal cleaning regime for the moment.

Transport for London says it has stepped up the cleaning regime on its tube and bus network, including using a hospital grade anti-viral disinfectant.

According to Dr Gosce, "limiting the number of close contacts with potentially infected individuals and objects is important".

"In terms of travel, avoid peak hours if possible," she says, suggesting, where viable, passengers should choose routes involving only one means of transport.

Currently, the UK government is not telling people to avoid public transport. David Nabarro, a special adviser on coronavirus for the World Health Organization (WHO), told the BBC that although public transport was an important thing to look at, the evidence suggested that the kind of "fleeting contacts" people have when travelling together didn't so far seem to be the "most important source of transmission".

Planes

It's a common belief that you are more likely to become ill on an aeroplane, because you're breathing "stale" air.

In fact, the air on a plane may well be better quality than in the average office (and almost certainly better than a train or a bus).

There are more people per square foot on a busy plane, which can increase the risk, but the air is also being changed at a faster rate.

Professor Quingyan Chen at Purdue University, who studies air quality in different passenger vehicles, estimates that the air on a plane is completely replaced every 2-3 minutes, compared with every 10-12 minutes in an air-conditioned building.

That's because while you are on a plane, the air you breathe is being cleaned by something called a high-efficiency particulate air filter (Hepa). This system can capture smaller particles than ordinary air-conditioning systems, including viruses.

The filter sucks in fresh air from outside and mixes it with the air already in the cabin, meaning that at any one time half the air is fresh and half is not. Many ordinary air-conditioning systems merely re-circulate the same air to save energy.

As well as breathing in droplets from someone coughing or sneezing, infections like coronavirus can be transmitted through touching contaminated surfaces with infectious droplets on them - whether that's a person's hand or a door handle.

Vicki Hertzberg, from Emory University in the US, took samples from surfaces on 10 transcontinental flights in 2018 and found they "looked like your living room". In other words, there's nothing notable in the plane samples compared with tests they had done in buildings and other types of transport, she says.

But it's difficult to generalise about the risks on any form of transport because there are various factors that increase or decrease the risk. For example, on a long-haul flight passengers might move around more and, should they have the virus, risk spreading it further.

The WHO guidance is that the highest risk area is the two rows in front, behind or next to an infected person. But during the 2003 Sars outbreak, on a plane carrying one infected person, 45% of those who caught the disease were sitting outside the two-row zone.

The usual advice applies - wash your hands, clean surfaces where possible and sneeze and cough into a tissue.

The main concern about air travel is how it can transport potentially infectious people from one part of the world to another. Currently, the UK government is not advising against taking planes - only travel to specific affected areas, like Italy and Hubei Province, in China. BBC