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3 Things to Consider When Buying an Air Purifier

Air purifier
You want to know which air purifier is the best? Just consider these 3 things before buying one and you'll be able to make a profound a pick. 

If the city is choked by smog, the air inside is often just as dirty. Air purifiers are useful machines to remove harmful particulates from indoor air. But there are countless brands on the market and due to information and marketing overload, a purchase can quickly end up in confusion. But no worries, air purifiers are no rocket science, just pay attention to the following 3 indicators and you will know which air purifier suits you best.

1: Pay attention to CADR, not to official square meter ratings

The first question is: does the air purifier in question match your room size. Normally, manufacturers help you with this matter by providing official square meter ratings. There is nothing wrong with that, the only problem is that there is no ‘international standard’ governing these square meter ratings, therefore each manufacturer can provide different information. 

CADR will let you know how much air gets cleaned in one hour

Since square meter ratings can differ, we recommend that you focus primarily on the Clean Air Delivery Rate per hour (CADR m3/hr). This indicator tells you how many cubic meters of air the rated machine will clean every 60 minutes. The higher the CADR rate, the stronger the performance of the air purifier. If the CADR is not displayed in the shop or on the machine’s product description, ask the seller. 

Once you know the CADR, you can easily calculate if the machine in question meets your requirements.

Here’s a little example: If a room is 40 sqm and has three meter high walls, we’re talking about 120 cubic meters in total. Therefore, an air purifier with a CADR of 315 m3/hr can provide 2.63 air changes in 60 minutes. This is enough to get rid of all dangerous pollutants within a short time, but since people have different requirements, some might ask for 5 air changes per hour. Hence they need an air purifier with twice the performance and we’d need to consider a machine with a CADR of 600 m3/hr. 

Know the tricks of the manufacturers

Keep in mind that all official square meter ratings and CADR values always refer to the maximum performance of the machine. If you switch your device to lower power settings the CADR will decrease and the cleaning process will take longer.

Xiaomi, for example, advertises their 2s model with a CADR of 310 m3/hr. This cleaning rate, however, can be only enabled in its turbo mode, which cannot be switched on manually but only via the app. Since most people use Xiaomi in auto-mode, they never enable the highest cleaning performance.

2: Pay attention to the noise level

Some companies advertise profusely about how quiet their air purifiers are. This is legitimate, but be wary, because in most cases these specifications only apply to the volume at the lowest power setting. So whenever you see a purifier labeled as “whisper quiet,” rest assured that it’s only part of the truth. In their highest mode, some air purifiers can be extremely loud.

Not sure if an air purifier is loud? Ask for a soundcheck! 

On very polluted days, we are most likely to depend on the maximum performance of our air purifiers. To avoid loud surprises after you unwrap your air purifier at home, better ask in the shop for a little “soundcheck.”  Few things are more irritating than a noisy purifier, negatively impacting your concentration, and worse, keeping you awake during the night. 

When you sleep, you’re dependent on good air quality – otherwise, you would inhale harmful particles for 6 – 9 hours at a time. An air purifier on a low and quiet mode might be pleasant but perhaps not sufficient for adequate air filtration. Depending on your room size and the degree of indoor pollution, you might need a stronger performance of your air purifier. 

Silent “auto-modes” have a downside 

Many manufacturers have reacted to this problem by developing air purifiers with a so-called “auto-mode” which corresponds in most cases to an integrated air sensor. Yes, these functions sound very reassuring, but eventually, they create a worse dilemma. Read here, why auto modes are not reliable and built-in sensors highly inaccurate. (Link)

3: How much are replacement filters? And what is their life-span?

Air purifiers cause running costs because filters need to be replaced at some point. Greatly reduced devices may trigger a buying impulse, but don’t forget the costs of the replacement filters. They might be high and a supposedly cheap device will turn out to be expensive in the long run.

Furthermore, ask how long filters last. But be aware that companies usually have a great financial interest in having their customers change the filters regularly.

Blue Air, for instance, recommends changing the filters every six months. Philips, on the other hand, advises replacing the filters once they get dirty. 

However, this information is problematic and leaves questions open. Is Blue Air’s lifespan for Bangkok air, or Swedish air? Furthermore, where is the empirical justification for this number? And what does Philips call dirty? A light grey or a dark black color?

Official filter life-spans should be questioned

As long as manufacturers don’t provide empirical justification, all official recommendations are not more than questionable guidelines. Basically, the life-span of HEPA filters depends on two factors.

  • Firstly, on the degree of air pollution. The higher it is, the more particles end up in the filter and clog the fibers. In highly polluted cities like Delhi or Beijing, HEPA filters have a shorter lifetime than in Bangkok, where pollution is seasonal. 
  • Secondly, the higher the power setting, the higher the air filtration and the efficiency of the filters decrease faster.

Our partner Smart Air has extensively tested the life span of all filters and made all research results openly available. Under Beijing pollution, the Sqair’s HEPA filter lasts 6 months when using it for 8 hours a day on the highest setting.

The life-span of the Blast Mini HEPA is 15 months, while the one of the Blast lasts at least 22 months. All numbers refer to conditions in Beijing. Since pollution in Thailand is a bit lower, the filters here last a little longer.