Hepa air filters were popular in Davos but what are they and do they work?

The air purifiers were said to be found in most rooms at the World Economic Forum, possibly to reduce the transmission of the coronavirus

People inspect a Philips smart air purifier. A Hepa filter is expected to remove particles of a certain size with an efficiency above 99.9 per cent. Sarah Dea / The National

When the great and the good gathered recently in Davos for the World Economic Forum, it was not just the state of the world that came under the spotlight.

There was also chatter about the fact that Hepa (high-energy particulate air) filters seemed to be present in most rooms.

Some online observers commented that the often wealthy delegates were being protected from the spread of the coronavirus by air purifiers, a luxury that average members of the public may not enjoy in their day-to-day life.

But do air purifiers or air cleaners actually remove pathogens and cut disease transmission?

Certainly many think so, because sales reportedly surged by more than half in the US in 2020 when concerns over Covid-19 were running high.

Among those interested in air cleaners and their effectiveness is Prof Nicola Carslaw, an expert in indoor air chemistry at the University of York in the UK.

"We’ve been interested in them for years. With Covid coming along, that led to an explosion … certainly in the number being purchased," she said.

How do Hepa filters work?

Hepa filters, which are found in some but not all filtration air cleaners, contain mats of fibres that can trap, among much else, viruses, bacteria, pollen and the types of fine particles that may be given off by cooking or vehicle exhausts. A Hepa filter is expected to remove particles of a certain size with an efficiency above 99.9 per cent.

Prof Carslaw said that, in the right circumstances, air cleaners with Hepa filters "absolutely" could reduce the spread of pathogens in rooms with several people, so they may be beneficial in classrooms.

"If you have a room with a lot of occupants or for some other reason you can’t ventilate properly, Hepa filters probably are the way to go," she said. "There seems to be some evidence they can reduce the chance of transmission."

They may also prove useful for people who live near a busy road and want to clean air that may contain particulate matter or other exhaust pollution.

Aside from filtration air cleaners, an array of other devices are on the market.

To give just two examples, some create an electrical charge that causes particles to fall onto surfaces, while others generate ozone, which studies have said can kill bacteria, fungi and other pathogens on surfaces or in the air.

Ozone, however, can be harmful to people, being a lung irritant that can also create other potentially hazardous air chemicals.

Universal standards for air cleaners

A 2021 plan announced by the government in Wales in the UK to supply 1,800 ozone-generating air purifiers to schools with the aim of reducing the spread of the coronavirus was scrapped following a backlash over the safety of the devices.

It speaks to wider concerns that Prof Carslaw and others have about a lack of set standards for air cleaners.

"The concern from me and lots of other people that work in the indoor air quality field is the complete lack of regulation around them," she said.

She said that any company could produce and sell devices and give figures for their efficiency at removing harmful substances from the air, yet there was no central laboratory to verify such statistics and analyse the potential production of other chemicals as by-products of the cleaning.

"Quite often it will be all focused on, ‘Our air cleaner removes Covid,’ or, ‘Our air cleaner removes particulate matter,’ but it doesn’t say, potentially, what it could also make," she said.

"Hepa filters generally don’t have all these secondary pollutant production issues, which is why they are often the preferred air cleaner."

Prof Carslaw noted that for many households, air cleaners were probably not necessary, as letting in fresh air from outside did the job just as well.

Where they can offer benefits – such as in a classroom or a household in a polluted area – multiple factors determine their effectiveness.

The size of the air cleaner and its location will influence how well it cleans a particular space, but it may be difficult for the average person to ensure a device is used most effectively. Proper maintenance, including changing filters if it is a filtration device, is also important.

"It’s not a simple thing, which is why I worry about a member of the general public buying one. You need to understand what you’re buying, and that information isn’t always provided by manufacturers," Prof Carslaw said.

"You need to know about the building you’re putting it in, you need to know about the air pollution levels, you need to know about ventilation.

"I feel there’s a lot of onus on the person buying an air cleaner to understand the science, with very little help provided by the manufacturers in most cases."

How does air filtration work on planes?

Hepa filters are used on planes, where air flows from the ceiling to floor, leaving through grilles before being filtered and mixed with outside air. The air in a cabin is renewed every few minutes.

Boeing, the US aircraft manufacturer, states that filters should be changed every 12 to 18 months, but not more frequently, because the filters become more effective over time, up to a point, as particulate matter on the surface increases the surface area for filtration.

Smaller aircraft might have just one, albeit large, Hepa filter, but a Boeing 747 has around 10.

"The air is forced through them and the small particles are held back," said Prof John Oxford, emeritus professor of virology at Queen Mary University of London. "The trouble is, they’re a bit noisy because you have to force the air through them."

While standards for air purifiers sold to the public may be lacking in most locations, the California Air Resources Board (CARB) tests devices and publishes its results online.

For more than a decade, all indoor air cleaners sold in California must be certified by CARB, although its tests are limited, covering issues such as electrical safety and ozone emissions.

In addition to providing certification, the organisation offers tips to buyers, including that they select a device with an appropriate clean air delivery rate (CADR) for the space it is placed in.

The cost of air purifiers varies widely, but a Hepa filtration device aimed at cleaning an average-sized living room may cost more than Dh1,000 ($270).

In future, an additional type of air cleaner may become available, because an emerging approach uses far-ultraviolet light, a type of low-frequency light that is invisible to humans, to kill pathogens.

Unlike some other types of UV, is not harmful to people, so it could be used to decontaminate a room, for example.

Other types of UV can only be used for decontamination in enclosed environments, such as laboratory cabinets, because they harm the skin and eyes.

Prof Carslaw said researchers had yet to determine if far-UV caused unwanted changes to the chemistry of the air in the way that some air cleaners do.

If it does turn out to be safe, the world of air cleaners, which already has many and varied players, could yet see another entrant into the market.