Tech Guides

The top 5 things to look for when buying an air purifier

By Alvin Soon - 29 Sep 2019

The top 5 things to look for when buying an air purifier

Holy smokes!

Note: This article was first published on 9th September 2015 and it's still relevant today.

It looks like the haze is making a comeback and there’s not much we can do except stay indoors, wear masks outdoors, and drink lots of fluids. Another way to cope is to get an air purifier for the home, but knowing which features you need and which you don’t can get confusing. If you haven’t already bought one, here’s what you need to look out for.


1. The most important thing is a HEPA filter

You can skip on everything else, but the one thing you must have in an air purifier is a High Efficiency Particulate Air (HEPA) filter, which can remove more than 99.97% of 0.3-micron particles.

HEPA filter illustration. Image source: LadyofHats via Wikipedia.

That 0.3-micron figure is important. There are two ways of measuring particle matter (PM): PM10 and PM2.5. PM10 measures particles from 2.5 to 10 microns in diameter, while PM2.5 measures particles 2.5 microns in diameter or less. A human hair has a diameter of 40 microns, so these are extremely small particles that you can’t see with the naked eye. To put it another way; just because you can’t see them doesn’t mean that they’re not there.

Image source: US Environmental Protection Agency.

PM10 particles can be trapped by your nose or throat, but PM2.5 particles are so small that they can slip through and lodge in your lungs, causing all sorts of health problems.

To trap and filter PM2.5 particles, you’ll need that HEPA filter. Without it, any air purifier is really just decoration. We recognize that air purifiers with HEPA filters can cost more, but if you want to get something to do a job, then you need to know that it actually does that job.

Beware of air purifiers claiming to be “HEPA-type,” “HEPA-like,” or “99% HEPA,” which are not true HEPA filters and not as effective.


2. Match the air purifier to the size of your room

The second thing to look for is an air purifier that’s built for the size of your room. If you put an air purifier built for a small room into a large one, it won’t be able to filter air as well.

Not your typical Singapore bedroom, but the only picture of one we could find on Wikipedia. Source: Tim Collins, via Wikipedia.

Some manufacturers list the recommended area in their specifications sheet, which is an easy way to find out if that model fits your room. Some list a number called CADR, which stands for Clean Air Delivery Rate.

CADR is an independent test result from the Association of Home Appliance Manufacturers (AHAM) in the US. The number is expressed in cubic feet or meters per minute, and it measures for tobacco smoke, pollen and dust filtration. The higher the CADR number, the more air it filters per minute for that particle.

Note that CADR doesn’t measure the effectiveness of the size of particles being filtered, that’s where you still need to have that separate HEPA filter specification.

How to get the right CADR rating for your room

AHAM recommends a ‘two-thirds’ rule when it comes to the tobacco smoke rating, that is, you’ll want to find an air purifier with a CADR (tobacco smoke) that is at least two-thirds of your room’s area.

The problem is that the number is mostly listed in square-feet, not square-meters, so you’ll have to do the conversion yourself. For example, if you have a room that is 120 square-feet, you’ll need a CADR (tobacco smoke) rating of at least 80 (two-thirds of the room’s area). Yup, some math is required.

If you truly cannot afford to get a HEPA air purifier sized to your room, then you can consider placing a smaller-rated air purifier closer to the important areas, like near your bed, or near the sitting area of your living room.

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