As South Korea scrambles to control an outbreak of the killer MERS virus, its fearful citizens have donned surgical masks en masse -- but the jury is out on whether they actually protect against the invisible enemy lurking in the air.

Across Asia, masks have long lost their stigma to become an everyday sight in the street or on the subway, despite some experts believing they do little more than provide psychological reassurance against diseases such as MERS, which has already left 23 people dead in South Korea.

The virus, which arrived in the country with a businessman who had been travelling in the Middle East, has sparked a rush of orders at small Japanese mask-maker Clever, similar to that in the 2002-2003 Severe Acute Respiratory Syndrome (SARS) outbreak in China.

"We have received 100,000 orders for our masks, 10 times more than the same time last year," company president Tsuyoshi Nakagawara told AFP.

"Half of them are from overseas, of which 70 percent are from South Korea, while Hong Kong and (mainland) China account for the rest."

The company's most popular mask, a 9,980 yen ($80) model, is made with several layers of filters, which the firm claims are fine enough to block pollen, infectious viruses, PM2.5 -- tiny airborne pollutants small enough to penetrate deep into the lungs -- and even radioactive particles.

While fear over MERS is at the root of the latest boom in South Korean mask wearing, the practice is widespread in East Asia, even in relatively normal times.

- Runny noses -

In Japan it is common to see small children, businessmen, pregnant women and others in a white surgical mask as they go about their daily life -- on trains, in offices and in the street.

Masks -- there were almost four billion in Japan in 2013 -- are particularly abundant during the winter 'flu season and remain so through spring as millions of hayfever sufferers grapple with runny noses and sneezing.

Masks are used in China chiefly as a way of protecting the wearer from the sometimes-choking pollution of the big cities, while South Koreans wear them routinely for a mixture of reasons.

"Masks are most prevalent in the winter season and in the spring, supported by a government campaign," said Kim Mi-Kyung, a health management professor at Kyung-In Women's University.

"Nowadays, different types of masks are widely used for sanitation and protection against the cold, or to prevent the inhalation of pollutants," she said.

However, scientists -- and even mask manufacturers -- say wearing a mask every day does not provide the level of protection many people seem to think.

"Wearing a mask is not perfect to prevent a wearer from picking up a virus," said Mitsuo Kaku, professor of infectious disease at Tohoku University.

"It can be said, though, that wearing a mask and washing your hands could reduce the spread of micro-organisms," he said.

Norio Ohmagari, director of Japan's Disease Control and Prevention Center, said improper use of masks further lessens their efficacy.

"In principle, masks should be used only once," he said, adding a wearer must wash their hands after disposing of the used mask.

"People don't know how to handle them properly."

In the US, the Centers for Disease Control and Prevention says face masks block "large-particle droplets, splashes, sprays or splatter that may contain germs... from reaching your mouth and nose."

But they do not "filter or block very small particles in the air that may be transmitted by coughs, sneezes or certain medical procedures."

- Disguise -

In fastidious, polite Japan, where masks have been in use since at least the early part of the 20th century, covering your face is often a matter of etiquette.

A survey by Kobayashi Pharmaceutical Co., found that more than 70 percent of Japanese people believe it is simply good manners to cover your mouth and nose with a mask when you have a cold or the 'flu.

But not all of the people wearing a mask are ill. For some of them, it's a kind of disguise.

Not uncommon is the sight of a young man slumped on a train seat wearing a mask, a baseball cap, sunglasses and a pair of earphones -- the perfect defence against any messy interaction with strangers.

Masks also offer an easier option for anyone who may be running late.

"It's good to cover your face when you don't put makeup on," says fashion model Zawachin, who sells her own range of designer masks -- including a raspberry-flavoured one.

While white is the regular colour for plain masks, they are also available in a range of hues -- including black -- to enable the wearer to co-ordinate with their outfits.

Masks for these non-medical purposes are described collectively as "date masuku", which literally means "masks just for show".

Tamagawa-Eizai Co, a pharmaceutical firm, began running a beauty mask pageant in 2013, saying surgical masks are now fashion items in Japan, like glasses without prescription lenses.

However, spokesman Taichi Ono admits they are trying to steer customers back to a more usual usage.

"A surgical mask is a hygiene item to start with," he said, "So we are also trying to raise awareness of how to use it properly."

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