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There are only a few things that really matter regarding an air purifier purchase:
- Clean Air Delivery Rate (CADR) (basically how large an area the purifier can process effectively)
- Noise (in decibels)
- Purchase price
- Replacement filter availability and price
There are a few other aspects of a device, such as the ability to use timers, remotely turn on/off, and measure the air pollution in the room (this is not as great a feature as one might expect).
How to Know if the Purifier is Working
The first thing one needs to know, and independently verify, is does the unit actually work. Without an independent source of measuring the actual air quality in the room (and not just next to the device), one really doesn't know. Regardless of whether the AQI sensor is correlating with a locally verified data source, the main point is that how does one know if:
- The air quality the device is displaying, once the air is being purified, is accurate.
- The air quality throughout the room (and not just next to the air purifier) is being improved.
I personally bought a new air purifier and diligently turned it on, and moved it from room to room for a few weeks. Then I finally received a particle counter I had ordered, and low and behold, the particle counts did not change with this air purifier on. I got out the manual and found out how to open the device, and it turns out I had not removed the plastic wrapping over the filter. Let this be a lesson, actual data is needed to know if something is working!
Independent Stand-Alone Air Quality Monitors
The term air quality monitor is marketing-speak for particle counters (though some do also measure volatile organics, such as formaldehyde). The only way to know the answer to these two questions above (accuracy of measurement, throughout a room) is to have an independent and separate device that does the measuring, apart from the air purifier itself. If the device can be validated by a third party device, then all to the good (perhaps borrow one from a friend).
Laser Egg Particle Counter
The least expensive, independently tested reliable device is Kaiterra's Laser Egg 2 and Laser Egg 2+ (which also detects TVOCs (see Ebay for possible deals).
Chiang Mai Makerspace Air Tricorder
Chiang Mai Makerspace offers an Air Tricorder for 2,500 THB. However, it appears to be little more than a rebranded, cheaper product available from AliExpress and shipped from China for a little over 1,000 THB.
Richard Barrow reviewed the air tricorder, but the testing on that review consists of visiting different locations and seeing the measurement provided (no comparison is made to other, higher accuracy measurements). There is no attempt to assess the accuracy of the device.
The Air Tricorder may nevertheless be accurate, but there is no evidence for that, online or elsewhere. The marketers of the device do not offer any testing detail or data. The problem is that, while particle counters do not necessarily have to be expensive to be accurate, they do include a laser which requires some amount of calibration. Therefore the very low component price makes accuracy much less likely. Only actual testing will validate or invalidate the accuracy of these devices.
Certainly there are other air quality monitors than the Laser Egg(2 2+) and the Air Tricorder. However, as far as testing data goes, they are more expensive or not as reliable, or both.
What an Air Purifier Purifies
Most air purifiers tackle particulates of pm10 and pm2.5, some handle a slightly different size such as pm3, and some go down to pm0.3 which starts to get into the gaseous range of air pollution. Some purifiers also deal with total volatile organic compounds (TVOCs) such as formaldehyde, carbon monoxide, and a long list of other pollutants/hazards from a variety of sources.
How Should an Air Purifier Purify
There really is only a single technology that is affordable and recommended by researchers. For particulates, that is a HEPA filter or similar, and for TVOCs that is an activated charcoal or similar technology. There are certainly special application areas where things like ultraviolet can be used to help attack viruses (such as hospitals), but most technologies are not as safe or cost-effective as a HEPA filter, nor are they measurably better. Many low-cost air purifiers have an ozone or ionizer option, but this actually creates ozone at ground level, which is not necessarily safe. Ionizers are not recommended by air pollution researchers for this reason.
Clean Air Delivery Rate (CADR)
For Energy Star-rated appliances (and most quality devices are Energy Star-rated), the CADR is performed at a specific watt efficiency. This means it is somewhat simple to calculate the CADR (other things being equal, such as the HEPA filter size) and that is essentially a range of from 1/2 to 1 times the wattage is equivalent to the square meter coverage.
A 300m3/h CADR is effective for 30-60m2 area, depending on the level of air pollution.
CADR is a combination of airflow and, to some degree, filter size, along with the fluid dynamics of the air from fan intake to clean air output. What is actually more important, and very site specific, is the rate at which the particle count drops for a given room at a given level of particle count inside as well as outside.
If the particle count outside is 400, it will be more difficult to attain a lower ambient particle count, than if the particle count outside is 200, given a fixed room size and CADR. The sizing of a CADR for a given metric area presumes a max external particle count.
What is needed for each location (each room in a building), is actual performance data, measuring changes to the range of particle counts over time (for example, every 15 minutes), using a reliable or at least consistent particle counter.
Noise of an Air Purifier
The noise level of an air purifier should be tested and evaluated on high or high and medium, rather than low. Of course air purifiers are quieter when on low. It is rather the medium and high settings that will make a difference.
To my ears, 60db is a bit noisy, much like a somewhat noisy 18" consumer-grade fan on high. Indeed, since air purifiers are largely filters plus a fan, only truly innovative quiet fans will drop down into the whisper category of 50db, or less.
Purchase Price and Maintenance Costs of an Air Purifier
Regardless of purchase price, HEPA filters do have a fixed lifespan, degrading in effectiveness over time. If one cannot find replacement HEPA filters, or those are costly, then the overall cost of the system remains high. Smaller filters need to be replaced more often, but they are not necessarily much less expensive than larger filters. It would be best overall to be able to use a variety of filters rather than a single manufacturer's make and model. This will make it possible to have multiple sources and use price comparisons to keep replacement costs low.
Below are four real-world examples with costs comparison, assuming a 5 year lifespan.
Example costs 1 - Obscure low-cost brand:
- Nobico 15m2 air purifier purchase price: 2,500 THB
- Replacement HEPA filters purchased were fake (did not meaningfully reduce air particles), no other sources available
- Result: ~1 year lifespan with cost of 2,500 THB/year for 15m2, initial price of 2,500 THB
- Note: Don't buy obscure brands, the replacement filter situation is not reliable
Example costs 2 - Local manufacturer low-cost brand:
- Gruenluft 15m2 air purifier (VK-6011) purchase price: 1,790 THB (purchased 01Feb19, price as of 19Mar19 is 4,099)
- Replacement Gruenluft VK-6011 HEPA filters available on Lazada for 360 THB (x 4)
- Result: ~5 year lifespan with cost of 3,230 THB = 646 THB/year for 15m2, initial price of 1,790 THB
- Note: While this brand is not widely known, it is a German brand with local manufacturing in Thailand. The risk is that the particular model stops being manufactured and replacement filters become unavailable.
Example costs 3 - Popular, medium-cost brand:
- Xiaomi 30m2 air purifier purchase price: 4,500 THB
- Replacement HEPA filters available at 1,000 THB (x 4)
- Result: ~5 year lifespan with cost of 8,500 THB = 1,700 THB/year for 30m2, initial price of 4,500 THB
- Note: A popular option, though there are questions as to the actual effectiveness of Xiaomi brand air purifiers and the Xiaomi particle counters
Example costs 4 - Generic brand, DIY:
- Hatari (or other brand) 18" portable fan: 1,000 THB
- Initial and replacement HEPA filters (generic, 30cm2) available at 300-500 THB (avg. 400 THB x 5)
- Additional bit of plastic string (virtually free)
- Result: ~5 year lifespan with cost of 3,000 THB = 600 THB/year for 30m2, initial price of 1,400 THB
- Note: A smaller Hatari (or other brand) 14" fan can be purchased starting around 600 THB for a smaller room. This would yield a savings of 80 THB/year for a yearly cost of 520 THB/year for 15m2, initial price of 1,000 THB
Fan + HEPA Filter = Air Purification
A fan and a HEPA filter (and maybe an activated charcoal prefilter) is the sum total of elements which make up an air purifier. Combine this with a reliable particle counter, and one can measure the actual, real world performance of air purification, and at a better price.