Pedestrians in disposable face masks have become a defining image of the coronavirus outbreak in China. Some areas have reported shortages as people rush to prepare for the worst.

Experts say masks can be helpful in preventing the spread of the virus – if worn properly, under the right conditions.

For now, public health officials say there’s no need to wear face masks in the United States, but do recommend them in China, especially for people who think they may be sick. Demand for masks is surging, and manufacturers in China and the United States are increasing production.

Here are the key facts.

Q: How do they work?

A: The most commonly worn cheap and disposable masks, known as the surgical masks, will limit – but not prevent entirely – the chance of inhaling large, infectious particles circulating near the face, said Marybeth Sexton, an assistant professor at Emory University School of Medicine. Even with perfect use, these masks aren’t foolproof because a virus or pathogen can still slip through the sides or enter the body through the eyes.

All in all, the science suggests they are a flawed but valid line of defense.

The outer fabric of surgical face masks is usually yellow or blue with one elastic or wire edge for tightening. To put it on properly, the mask’s absorbent side must be worn facing in and the colorful side facing out. The mask needs to securely cover the mouth, chin and nose. That’s why the edge with the strip is put on facing up, so that the elastic can easily be tightened by the nose.

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Taking off the mask correctly is equally important. The mask should be treated as if it’s contaminated and pulled off by the straps around the ears, and never just lowered from the mouth.

The Centers for Disease Control and Prevention (CDC) recommends that any health-care workers interacting with coronavirus patients or suspected cases wear a stronger kind of mask, known as the N95 respirator, along with other precautions like gloves and eye protectors.

The N95 filters out 95% of pollutants and is “highly effective” in preventing the transmission of viruses, said Sexton. However, these masks must be specially fitted and therefore aren’t commonly worn outside of the health care setting.

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Q: Should I wear one?

A: Infectious-disease experts said that there’s little need to wear face masks in the United States, where just two cases of coronavirus have been confirmed and the patients are in stable condition in hospitals.

Given the low threat level, covering one’s face and nose isn’t necessary when outside or in a place with good ventilation, said Collen Kraft, associate chief medical officer for Emory University Hospital, who helped treat the first U.S. Ebola cases in 2014.

“Wearing a mask walking around isn’t going to do any good but if you’re in a situation where you’re highly exposed, a mask is helpful,” Kraft said. “You may wear a mask when someone is going to cough directly on you or a place with a lot of ill people. In a hospital, we wear a mask with patients who have influenza.”

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Kraft said that people in the United States should instead be taking the same precautions they would to avoid contracting the flu, such as being vigilant about washing hands regularly and cautious about touching their faces and possibly infected surfaces.

In high-risk areas like Wuhan, China, where the outbreak began, wearing masks pays off, experts say.

Q: Are there enough?

A: Chinese authorities have been encouraging people to wear face masks covering their mouths and noses, which has led to a surge in demand.

Chinese face-mask manufacturers have reported that they’re running factories despite the Lunar New Year holiday break to keep up. Companies are even offering to increase workers’ wages dramatically to entice them to return to work and stay for longer hours.

“From what I have heard, the mask shortage is much, much more severe than what the public knows,” Cao Jun, general manager of Chinese mask manufacturer Lanhine, told Reuters. “Almost all hospital workers nationwide are facing a huge shortage of masks, not just in Wuhan. That’s very terrible.”

Usually Cao’s company produces 400,000 masks daily, but demand has skyrocketed to 200 million.

As part of China’s emergency efforts, state media reported that the country’s minister of industry has ordered factories “to overcome labor difficulties during the Spring Festival, accelerate production and do their utmost to increase supply to the market,” face masks included, Reuters reported. On social media, hospitals and health workers have been making urgent pleas for supplies, face masks included.

E-commerce platforms popular in China such as Taoboa have warned sellers not to increase prices on face masks. Nonetheless, news sites have reported increases over five times the usual price. The Straits Times, a newspaper in Singapore, reported that Chinese nationals there are stocking up on protective gear to send to family members in China.

U.S. face mask manufacturers also told The Post they’re experiencing a surge in demand.

“We are experiencing a surge in demand for our protective face masks in North America, Europe and China,” Honeywell International, based in North Carolina, said in a statement. “We are increasing production at multiple facilities globally, and we are fulfilling all current orders.”

3M, another major manufacturer based in Minnesota, also reported an increase in production after a rise in demand from China for respiratory protection products.

This is part of why the U.S. government keeps a face-mask stockpile for emergencies.

As The Post’s Lena Sun reported, the U.S. government has several secret storage facilities holding drugs and supplies that are part of a $7 billion Strategic National Stockpile ready to be used in the case of a bioterrorism or nuclear attack or an infectious-disease outbreak. Nationwide, the repository’s medical contents would cover more than 31 football fields if laid out.