Step outside your door without a face mask in Hong Kong, Seoul or Tokyo these days, and you may well get a disapproving look.

Since the start of the coronavirus outbreak some places have fully embraced wearing face masks, and anyone caught without one risks becoming a social pariah.

But in many other parts of the world, from the UK and the US to Sydney and Singapore, it's still perfectly acceptable to walk around bare-faced.

Why some countries embrace masks while others shun them is not just about government directives and medical advice - it's also about culture and history. But as this pandemic worsens, will this change?

The official word on face masks

Since the start of the coronavirus outbreak, the official advice from the World Health Organization has been clear. Only two types of people should wear masks: those who are sick and show symptoms, and those who are caring for people who are suspected to have the coronavirus. 

Nobody else needs to wear a mask, and there are several reasons for that.

One is that a mask is not seen as reliable protection, given that current research shows the virus is spread by droplets and not airborne transmission. This is why experts say frequent hand washing with soap and water is far more effective.

Removing a mask requires special attention to avoid hand contamination, and it could also breed a false sense of security.

Yet in some parts of Asia everyone now wears a mask by default - it is seen as safer and more considerate.

In mainland China, Hong Kong, Japan, Thailand and Taiwan, the broad assumption is that anyone could be a carrier of the virus, even healthy people. So in the spirit of solidarity, you need to protect others from yourself.

Some of these governments are urging everyone to wear a mask, and in some parts of China you could even be arrested and punished for not wearing one.

Meanwhile, in Indonesia and the Philippines, where there are suspicions that there are many under-reported cases, most people in major cities have begun wearing masks to protect themselves from others.

For many of these countries, mask-wearing was a cultural norm even before the coronavirus outbreak. They've even become fashion statements - at one point Hello Kitty face masks were the rage in the street markets of Hong Kong.

In East Asia, many people are used to wearing masks when they are sick or when it's hayfever season, because it's considered impolite to be sneezing or coughing openly. The 2003 Sars virus outbreak, which affected several countries in the region, also drove home the importance of wearing masks, particularly in Hong Kong, where many died as a result of the virus.

So one key difference between these societies and Western ones, is that they have experienced contagion before - and the memories are still fresh and painful.

Meanwhile, in South East Asia, especially in more densely-populated cities, many wear masks on the streets simply because of pollution.

But it hasn't caught on everywhere in Asia - here in Singapore, the government has urged the public not to wear masks to ensure adequate supplies for healthcare workers, and most people walk around without one. There is substantial public trust in the government, so people are likely to listen to such advice.

The mask as a social nudge

Some argue that ubiquitous mask wearing, as a very visual reminder of the dangers of the virus, could actually act as a "behavioural nudge" to you and others for overall better personal hygiene.

"Putting on a mask every day before you go out is like a ritual, like putting on a uniform, and in ritual behaviour you feel you have to live up to what the uniform stands for, which is more hygienic behaviour like not touching your face or avoiding crowded places and social distancing," said Donald Low, a behavioural economist and professor at the Hong Kong University of Science and Technology.

Then, there's the idea that every little bit counts in the war the world is waging against the virus.

"We can't say if face masks are ineffective, but we presume they have some effect because that's the protection we give to healthcare workers," said Benjamin Cowling, an epidemiologist with Hong Kong University.

"If face masks are used on a lot of people in crowded areas, I think it would have some effect on public transmission, and at the moment we're looking for every small measure we can to reduce transmission - it adds up."

But there are downsides of course. Some places such as Japan, Indonesia and Thailand are facing shortages at the moment, and South Korea has had to ration out masks.

There is the fear that people may end up re-using masks - which is unhygienic - use masks sold on the black market, or wear homemade masks, which could be of inferior quality and essentially useless.

People who do not wear masks in these places have also been stigmatised, to the point that they are shunned and blocked from shops and buildings.

In Hong Kong, some tabloids have splashed pictures on their covers of Westerners not wearing masks and congregating in groups in the city's nightlife district, and criticised expatriates and tourists for not taking enough precautions.

In countries where mask wearing is not the norm, such as the West, those who do wear masks have been shunned or even attacked. It hasn't helped that many of these mask wearers are Asians.

But those societies that do advocate everyone wearing a mask may have a point and increasingly,experts are now questioning the official WHO advice.

Undocumented cases

Firstly, there is some emerging evidence that there are more "silent carriers", or healthy people with the virus who show little or no symptoms, than experts initially thought.

In China, it is estimated that a third of all positive cases show no symptoms,according to classified Chinese government data seen by the South China Morning Post.

On the Diamond Princess, the cruise ship that docked in Yokohama, about half of the more than 600 positive cases found onboard were found to have no symptoms.

A similar proportion of asymptomatic cases has been reported in Iceland, which says it is testing a higher proportion of citizens than anywhere else in the world.

The prevailing belief has been that because these people do not exhibit symptoms, they are not very contagious. But some are questioning this now. Maybe if everyone wore a mask those silent carriers wouldn't turn into spreaders?

A recently published study of cases in China found that "undocumented cases of infection", or those with either mild or no symptoms, were significantly contagious and could have been responsible for nearly 80% of positive virus cases.

It's just one study though, and future research will no doubt add nuance to the overall picture.

The face mask may be a product of recent history, experience with contagion and cultural norm. But as the scale of this pandemic grows, along with evidence and research, our behaviour may yet change again. BBC