Not all air purifiers are equal. They don’t all sanitise air in the same way either. Most of the models sold in stores are the kind with a Hepa (high-efficiency particulate air) filter. In these machines, air is drawn in and passed through an ultra-fine filter that traps mould, dust mite spores, and most importantly, PM2.5 – microscopic air particles which are prevalent during hazy days – before it is emitted back into the room.

In addition, some purifiers may come with other features to clean the air:

Activated carbon filter Also known as a deodorising filter, this helps to remove odours and gases such as airborne formaldehyde, which could be released by furniture and cigarette smoke.

Electric plates Particles are drawn through the machine, attracted to the plates and removed from the air.

Ionisers An ioniser charges air molecules electromagnetically to create ions that stick to dust, smoke, pollen and other allergens. The ions become heavier as they “stick” to more particles, clinging to walls or falling onto the floor, where they can be swept or vacuumed away.

Ultraviolet light Some purifiers emit UV light to kill harmful bacteria passing through them to “sterilise” the air.

BUYING AN AIR PURIFIER
Dr Ang Keng Been, president of Indoor Air Quality Society Singapore, believes that an air purifier with two filters – Hepa and carbon – is enough to clean indoor air. While features like electric plates, ionisers and UV light complement the filtration process, they don’t replace it.

Dr Ang points out that the more features an air purifier has, the higher the cost. Get one that suits your needs, but don’t be taken in by features you may not require.

Check the cost of replacement filters too – both Hepa and carbon filters need to be replaced, usually at intervals of between one and two years, depending on usage.

You should also ensure that the air purifier is 100 per cent ozone-free. Ozone is a by-product of some air purifiers with ionisers, and in high concentrations, it could cause lung inflammation and trigger asthma.

For light sleepers, check the decibel level of the machine so you don’t end up buying a noisy one that keeps you awake at night. The quieter, the better.

HOW MANY DO I NEED FOR MY HOME?
There should be one for each room, as air purifiers do not work across walls. But as they are portable, they can be placed in the living room in the day, and the bedroom at night. Check the coverage area of the machine and choose one that is right for your room size.

WHAT IS THE MOST EFFECTIVE WAY OF USING AN AIR PURIFIER?
For truly clean indoor air, it’s best to use the machine in an air-conditioned room, with the doors and windows shut. A constant stream of air flowing into your home from the outside makes it difficult for the purifier to completely remove dirt and other pollutants even at full capacity. In such ventilated rooms, the machine will reduce the amount of particles in the air, but not eliminate them.

If you’re unlikely to have the air-conditioning on 24/7, even on hazy days, it’s a good compromise to keep the windows just slightly open – to ventilate the room, yet minimise air circulation – while keeping the air purifier on.

HOW LONG SHOULD I LEAVE IT ON FOR?
It is not necessary to keep the air purifier on when you are out, as it’s a waste of electricity to clean the air when you are not around to breathe it. Just turn it on whenever you are at home, says Dr Ang. 

WHAT TO BUY
For the bedroom

Samsung AX022HC
$429, from major electrical stores
The machine combines the Hepa and deodorising filters into one, so you’re saved the hassle of replacing two filters later on.
Coverage area: 22 sq m

For the master bedroom or living room

Winix WAC-5500
$499, from Isetan Scotts
On top of its carbon filter and Hepa filter, this purifier cleans the air with an ozone-free technology called Plasmawave, which works the same way an ioniser does.
Coverage area: 33 sq m

Novita Puriclean Complete 3-in-1 Air Purifier NAP 2000H
$799, from estore.novita.com.sg
For families who need the full package of features, this is an air purifier with an ioniser and humidifier.
Coverage area: 36 sq m

For the living and dining rooms

Philips AC4014
$519, from major electrical stores
A basic unit that covers a rather large space, it has a pre-filter, Hepa filter and activated carbon filter.
Coverage area: 55 sq m

Hitachi EP-A8000
$999, from major electrical stores
If dry skin is a problem for family members with eczema or other allergies, this purifier comes with two humidifying modes to adjust the humidity in an air-conditioned room to 50 or 60 per cent.
Coverage area: 55 sq m

This article was originally published in Simply Her October 2014.

All product and pricing information was correct at the time of the magazine’s publication. We advise readers to check with the stores for the latest availability and pricing information.