“Just having the filter is not enough. Transmission of the novel coronavirus is through droplets deposited on surfaces. Air purifiers will not help as they are mainly used to prevent airborne infections such as tuberculosis,” he said.

Guo Xiaoyun, a respiratory medicine specialist at a hospital in China’s Jiangxi province, said there is no direct proof to show household air purifiers can prevent the coronavirus.

“Commercial air purifiers are mainly suitable for removing one or more pollutants from the air. But in the product standards, there is no [removal rate] requirement for pathogens or other pathogenic microorganisms. So air purifiers might not be able to filter out the viruses.”

Despite this, air purifier sales have skyrocketed.

In China, e-commerce firm Suning.com said air purifier sales jumped 2,100 per cent between February 10 and February 13 compared with the previous year. JD, another e-commerce company, said air purifier sales tripled in the month after the coronavirus outbreak compared with a year earlier.

Since the outbreak, Portuguese air purifier maker Airfree Produtos Eletrónicos has sent thousands of devices to Hong Kong and China. US producer A.O. Smith, meanwhile, has donated air purifiers worth 18 million yuan (US$2.5 million) to more than 100 hospitals in China, including Wuhan, the city where the outbreak was first reported.

Swiss air purifier manufacturer IQAir has recorded a 50 per cent year-on-year rise in sales in China during the outbreak. It says many orders have also come from Hong Kong medical organisations.

“The models with the biggest sales growth boast ‘HyperHEPA’ filter cartridges which can effectively filter very minute particulates,” an IQAir spokesman said. “Humanity hasn’t dealt with this new coronavirus so there are no stringent tests and lab reports to prove what effects air purifiers can have on this specific virus. Theoretically, whether an air purifier [can remove virus droplets] depends on the size of the pathogen and the level of the filter cartridge.”

IQAir chief executive Jens Hammes recently told tech site Gadgets 360 that its products have a suction mechanism attached to the machine that extracts droplets from a patient’s cough.

“That [suction] design was created in cooperation with the Hong Kong Hospital Authority. They wanted that design to contain the [Sars] coronavirus [spread] in patients’ rooms. That was their initial idea. We incorporated that into the design,” he said.

“If a person sneezes directly into the inlet of the air purifier, the air coming out of the purifier will be novel coronavirus-free. That much is a certain guarantee … When a patient is coughing when the unit is on, our purifier creates suction. So, all the droplets from the cough are getting sucked in and getting cleared by the filters in the machine.”

A.O. Smith said its air purifiers are 99.9 per cent effective in removing the H1N1 swine flu virus that emerged in Mexico in 2009, infecting an estimated minimum 700 million people worldwide.

“The H1N1 virus is similar in size to the [novel] coronavirus. Air purifiers are very important to ensure the air quality in hospital wards and to help prevent cross-infection,” the company said in a press release.

Amesh Adalja, an infectious disease doctor at America’s Johns Hopkins Centre for Health Security, said while hospitals use special air-management devices for isolation rooms – particularly those containing airborne pathogens such as tuberculosis – he does not believe such devices are required for the general public, even if they have the ability to filter out small particles.

“This is not practical for the general public as people still have to go outside,” he said.

Wu said air purifiers “will help asthma and chronic bronchitis patients in environments where exhaust pollution is serious, such as locations that are close to a main road”.

But even those claims have been challenged.

“The allergens are a similar size to a dust mite and easily attach to the surface of clothes and furniture. They won’t be filtered away easily by purifiers … It’s more important to control the source of pollutants, such as avoiding indoor smoking and ensuring good ventilation.”

How North Korea could benefit from the pandemic

An HKCC spokesman told the Post consumers should not rely on air purifiers to improve indoor air quality.

“Our test on air purifiers focused on their performance in removing common household air pollutants like cigarette smoke, dust, pollen and formaldehyde, but did not test their performance in removing airborne bacteria and viruses.

“Theoretically speaking, some air purifiers equipped with HEPA filters may be able to screen out some larger airborne bacteria or viruses … however, the bacteria or virus trapped on the filter may not be killed. More importantly, studies to date suggest that the virus that causes Covid-19 is mainly transmitted through contact with respiratory droplets rather than through the air.”

Last week, New York attorney general Letitia James ordered air purifier manufacturers AllerAir Industries, Airpura Industries and Sylvane to immediately cease and desist marketing their products as tools that can prevent the spread and contraction of Covid-19.

James slammed the companies for “wrongly lead[ing] people to believe that purchasing an air purifier is enough to protect them from getting the virus and spreading it – a deception that is dangerous to them and to public health”.