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How an air purifier works: Do you really need one for your allergies? Can it filter out bacteria and viruses?

In this final installment of CNA Lifestyle's Household Hacks series, we look at air purifiers. Mechanical or electronic? Can your air-conditioner double up as one? Read on.

An air purifier, regardless of brand, price and design, typically has a fan to draw in air and a filtering medium to remove airborne particles, explained Dr Papia Sultana, a senior lecturer at the School of Engineering at Republic Polytechnic.

What sets one model’s performance apart from another is really the type of filter used, she said, and there are two broad categories: Mechanical and electronic.

“Mechanical filters use a fibrous medium coated with an odourless, adhesive substance, so that any particles that come into contact with the filter’s surface stick to it,” said Dr Sultana. Examples of such filters include the pleated, accordion-like high-efficiency particulate air (HEPA) filters as well as the ultra-low penetration air (ULPA) filters.

(Photo: iStock/Jomkwan)

The electronic filter, on the other hand, is controversial. It uses electrostatic charge to sieve out contaminants such as dust, smoke and pollen. However, “most of these filters use high voltages, and there is the concern of ozone gas emission and its associated toxicity”, said Dr Sultana. Examples of electronic filters include electrostatic and ionised filters.

But isn’t ozone protective? Yes, in the upper atmosphere, where it filters out the sun’s damaging ultraviolet radiation, according to the United States Environmental Protection Agency. But in the atmosphere that we live in, even low amounts of ozone can cause chest pain, coughing, shortness of breath and throat irritation.


Naturally, the mechanical filter is a safer bet for your health, said Dr Sultana. Look for filters that can remove particles smaller than 0.3 microns, she said. That’s the particle size that is the toughest to filter out and is also known as the most penetrated particle size (MPPS).

“Most of the infectious airborne microbial agents such as viruses or bacteria fall under this category of less than 0.3 micron.”

But while regular HEPA filters can remove these germs from the air, they can’t kill or inactivate them. It may be a different story if your HEPA filter comes with a bio-antibody or antimicrobial coating, said Dr Sultana, which may potentially provide germ-neutralising abilities. Otherwise, replace the filters about every six months or at a frequency recommended by the manufacturer.

A new HEPA filter (left) versus a used, dirty one that needs immediate replacing. (Photo: iStock/Pranay Chandra Singh)


There are a couple of standards, along with their acronyms, to look for. Here’s a quick rundown, according to Dr Sultana:

Air change rate per hour (ACH): The number of times the total volume of air in a room gets completely filtered by the air purifier in an hour. For instance, an ACH of 5 means that the entire room’s air gets cleaned five times per hour.

Does that mean the higher the ACH, the better the air purifier? Not unless you want the hulking presence of an air purifier ruining the interior aesthetics of your room. You’re better off getting the right ACH to match your needs. For normal use, an ACH of 2 will do just fine. But if you or your family members have allergies, you might want to consider a 5 instead. For smokers, you might even want to go up to 8 or 10.

Minimum efficiency reporting value (MERV): The MERV scale goes from one to 20, and higher values mean a greater percentage of particles is captured. For example, a MERV 16 filter would capture more than 95 per cent of particles. But note that the higher the MERV rating, the more energy is needed to operate the purifier. According to the National Environment Agency (NEA), at least MERV 14 is recommended.

(Photo: iStock/RgStudio)

Clean air delivery rate (CADR): This rating tells you how quantitatively fitting an air purifier is for the room. The manufacturer may provide the CADR in cubic metre per hour (cmh) or cubic feet per minute (cfm) for the removal of smoke, dust and pollen from the air, according to NEA.

Because the CADR is based on the room volume, you need to know your room's specifications to calculate it:

  • In cmh, multiply room volume by 5
  • In cfm, divide room volume by 12

For example, a room with a floor area of 24sqm (258sqft) and a ceiling height of 2.6m (8.5ft) will give you a room volume of 62.4 cubic metre (2,193 cubic feet). This means the CADR is:

  • 62.4 x 5 = 312 cmh
  • 2,193 ÷ 12 = 183 cfm

Got more questions about the air purifier? Find out the solutions here:

SCENARIO 1: You’re not sure if the air-conditioner can double up as an air purifier as well

Technically, you can. “If you are a heavy air-conditioner user, you may want to consider upgrading the air-conditioner’s filter with a HEPA one,” said Dr Sultana. That could save you the money you’d otherwise spend on a standalone air purifier.

“Or mask the air inlet with bio-antibody filters for particle filtration enhancement. You can talk to the air-conditioner supplier regarding these options,” she said.

But if you’re looking at neutralising a very obvious and localised source of indoor pollutant, you are better off getting an air purifier, she said, especially if you’re combating allergy issues.

(Photo: iStock/golfcphoto)

SCENARIO 2: The air purifier won’t work when switched on

Check that the filter and air intake port aren’t clogged. “When the filter is too dirty, it becomes very hard for the fan to overcome the pressure created by the clogged filter,” explained Dr Sultana.

The average filter needs to be replaced every six months on average, she said, but if the indoor air is particularly dusty or smoky, it may require more frequent changing.

Another possible reason could be a frozen motor shaft. To test, make sure the air purifier is switched off and unplugged. Rotate the fan blades by hand to check for tightness, said Dr Sultana. If the rotation feels stiff, some motor lubricant might help.

(Photo: iStock/tonefotografia)

SCENARIO 3: There is little or no airflow when the air purifier is switched on

The conditions leading to “no air flow” and “low air flow” are quite different, said Dr Sultana. In the former, it could be due to blocked air intake and outlet ports. “If there’s no obvious blockage, check the fan. If the fan’s motor is defective, you may need to contact the supplier about replacing it.”

If there is low or little airflow – despite switching to a higher speed or deactivating the quiet mode – it may not be a technical fault, she said. “Assess the location of the air purifier in the room. The air intake may be blocked by furniture or other features in the room.”

To help the air purifier along, create air movement in the room by switching on the fan or air-conditioner, or opening the door or windows. “If there is still no improvement, contact the supplier or get your air purifier serviced.”

Don't place the air purifier in a corner. (Photo: iStock/PrathanChorruangsak)

SCENARIO 4: The air in the room doesn’t feel any different even though the air purifier has been running for an hour or so

There could be three possible reasons, said Dr Sultana. One: The location of the air purifier. Mounting it on the wall is the best position, she said. But if you need to keep your air purifier mobile, place it 1m to 1.5 m above the floor, and avoid leaving it near a corner of the room, wall, furniture or other objects that can block the air inlet and outlet ports. “Keep your air purifier away from everything at least 1m in all directions.”

If the air purifier is in a humid area such as the bathroom or kitchen, the humidity can also cause the air purifier to work harder and eventually lead to less airflow output, said Dr Sultana.

Two: The size of the air purifier and the room it is in; the air volume may be too big for the air purifier to cover. “It is important to choose an air purifier with a sufficient CADR,” she said. Check the specifications for the device’s rated CADR and coverage area.

Three: Filter maintenance. As mentioned earlier, check that the filter is not covered with debris so that it can deliver clean air.

SCENARIO 5: The air purifier emits a strange smell when it’s working

Check for the source of smell. Odours from the nearby kitchen or bathroom can be trapped in the air purifier and they get dissipated when the fan creates air movements, said Dr Sultana.

And remember, an air purifier doesn’t create fresh air; it can only clean the existing air in the room. So try to encourage frequent air exchanges between the indoor and outdoor air by opening the door and windows sometimes. “However, if the odour is very localised to the air purifier’s air intake or outlet ports, check if the deodorising option is faulty,” she advised.

Source: CNA/bk