The Best Air Purifier
After spending 180 hours running two laboratory tests of more than a dozen different purifiers by an airborne-particle physicist, we find that the Coway AP-1512HH Mighty remains the best air purifier for most people.
Here are the key facts: Within 20 minutes, our pick reduces airborne particulate pollution by 88 percent—among the best and fastest performance we’ve ever seen. It’s one of the most affordable high-performing HEPA-rated air purifiers available. And on several absolute measures, it outperforms purifiers that cost two and even three times as much. Finally and crucially, the Coway Mighty maintains this exceptional performance long-term, even when we measured its performance using two-year-old filters from our 2014 test—filters that had been run almost continuously for a year beyond their stated lifespan.
If you have severe allergies or other serious health issues related to airborne particles, we have a new step-up pick: the Coway Airmega 300. This is a large but attractive machine; it’s HEPA-rated and designed to clear spaces of about 500 square feet at five complete air-changes per hour (about 1.5 times the coverage of the Coway and Winix), or 1,250 square feet at two complete air-changes. In our test, it gave the third-best absolute performance, 87 percent reduction (not statistically different from the Mighty’s 88 percent) in particulate levels over 20 minutes versus the original measurement. That said, it’s designed more for long-term, large-space, and/or high-intensity use. Its unique twin filters permit very high airflow, allowing it to run on lower settings when the air is relatively clean (quieter, less energy) or to rapidly filter very large volumes of air on the highest setting, as you might when allergens or pollution are elevated or if you need to clear a large space—say, an artist’s studio or loft.
It is expensive according to our calculations and costs about $1,400 (including the purchase price) to maintain over five years. This is in line with most of the high-end models we tested, but it’s more than the cost of a pair of Coway Mightys.
In our tests for odor/molecular removal, its 15-pound activated-carbon filter bested all other air purifiers by a wide margin. (Most air purifiers, including the other picks above, contain no or only token carbon filters and do effectively nothing to remove molecular pollutants—their strength is on larger-particulate pollutants like dust). Its exceptional performance in this area is a big part of why FEMA and the Red Cross chose Austin Air units for deployment at Ground Zero and the surrounding areas in the aftermath of 9/11.
Its annual operating cost ($283, according to calculations at the time of writing) also makes it by far the cheapest high-end purifier to run, but note that this is due to a filter that’s designed to be replaced every five years instead of annually. Against that, its power consumption is rather high due to its high airflow and the airflow-resistance of the carbon filter. Also, the Austin Air was less efficient than all other models at removing particles from the air when running the fan at comfortable sound levels, even though it’s HEPA-rated. But a little extra background noise isn’t too much to ask if serious health concerns over pollutants are an issue.
Table of contents
- Why you should trust us
- What you should know before buying
- What air purifiers do—and don’t do
- Who should consider buying an air purifier
- How we picked
- How we tested and quantified
- Our pick
- Also great
- For large spaces and extreme cases
- How HEPA filters work
- Note on Blueair/Blue
- Note on smart air purifiers
Why you should trust us
John Holecek, MS, guide researcher and co-author, has been involved in researching airborne particles since 1999 for the National Oceanic and Atmospheric Administration and in the private sector. He’s studied atmospheric particles in locations ranging from the continental US to the Arctic to the remote island nation of the Maldives. These field campaigns were government-sponsored research programs aiming to improve our understanding of Earth’s climate in part by determining the sources, transport, and fate of particles. John has also led research to develop aerosol particles with specific optical properties that produce a thick smoke cloud to protect soldiers from sensors and threats.
Tim Heffernan, guide editor and co-author, has been at Sweethome for a year now and has (among other things) worked with John on our lab-tested guide to water filters. His journalistic background is in what you could call big analog—physical machinery, as opposed to “little digital” software and gadgetry. And his education was in biology and economics. All told, it was good preparation for this guide’s mix of science, mechanics, and statistical analysis.
What you should know before buying
Before you jump in, know that effective air purifiers are expensive to purchase and operate. And despite the prolific marketing to the contrary, scientific studies do not support claims that they improve your health. Against this are the extensive (if anecdotal) claims from users of improved sleep, reduced allergies, and/or lessened asthma symptoms. There is mounting evidence, as well, that particulate pollution is a cause of degenerative brain disorders, including Alzheimer’s and Parkinson’s disease. We cover this in greater detail below in What air purifiers do—and don’t do and Who should consider buying an air purifier.
We were, of course, not in a position to perform health studies, but we were able to evaluate the air purifiers’ effectiveness at removing particulate matter and odors. The instruments we used to test these machines are capable of detecting particles as small as 0.010 micron, which is an order of magnitude below the threshold of home air purifiers and the 0.3-micron threshold tested for the HEPA standard. We also conducted a first-of-its-kind odor-control test using a VOC meter.
The downside to all this comprehensive testing is that there’s a whole lot of information to parse. For those who are interested, we lay it all out in How we picked and How we tested, which include our procedures, methods, and results, complete with tables and graphs. If you’re not interested in the nitty-gritty, feel free to skip to Our pick (and the subsequent Also great sections) to read about why we chose the picks that we did.
What air purifiers do—and don’t do
Once all the air in a sealed environment has been filtered, you’re technically left with “purified” air. However, this doesn’t necessarily mean it’s clean air.
For example, it could still contain harmful gases like radon or volatile organic compounds (VOCs) such as formaldehyde—what we’ll call molecular pollutants. By any name, what we mean are substances that exist in the air as individual molecules, rather than as agglomerations and/or physical particles. Both are invisible to the naked eye, but particles are tens to thousands of times larger than molecules, and that allows them to be filtered from the air by HEPA filters in a physical—specifically, an inertial—process. (See How HEPA filters work below.) Molecular pollutants are too small to be removed that way and instead have to be adsorbed—similar to absorbed, but for gases instead of liquids. Some purifiers come with equipment, often an activated-carbon filter, that can remove some of these molecular pollutants by adsorption, but most do not.
Air purifiers also can’t do anything about contaminants not in the air. Many allergens—mites and pet hair, for example—don’t stay airborne; they sink and stick to furniture and other surfaces. They remain in the environment even after the air has been purified. In short, if you want to minimize the particulate pollution in your home, you can’t just rely on an air purifier. You’ll also need a really good dust mop and/or vacuum cleaner.
All this said, purifying your air certainly won’t hurt. Air quality is very important to health and well-being. Degraded air quality from particulate matter is a serious issue in the US and around the world. Elevated levels of particulate matter (PM) are a known cause of acute respiratory issues, aggravated asthma, chronic bronchitis, and even premature death—particularly in the elderly, those already suffering from heart and lung conditions, asthmatics, and children. Fine particles—those less than 2.5 microns in size—are particularly dangerous because they can reach the deepest recesses of the lungs and, recent research suggests, even penetrate the brain and cause degenerative brain disease. Air pollution from US combustion emissions alone results in 200,000 premature deaths a year—more than half of these are attributable to PM from road transportation and power generation, according to a 2013 MIT study. (You can easily check your local air quality currently or annually.) Indoor air quality is especially important, as the average person spends 87 percent of his or her time indoors. But those are all statistics. Whether air purification will actually have any benefits for you personally is a more difficult question to answer.
Who should consider buying an air purifier
Air purifiers can help when it comes to creating breathable air, but they can do only so much. As the EPA says, “The best way to address residential indoor air pollution usually is to control or eliminate the source of the pollutants and to ventilate the home with clean outdoor air.”
That means: Don’t smoke indoors, vacuum and dust regularly, keep your pets outside when possible, and test for radon gas. These measures alone should be enough to ease many people’s symptoms, but they’re insufficient if you don’t have clean outdoor air to begin with. If you have not or cannot take these steps, you should consider an air purifier.
For just about everyone else, it’s a matter of personal preference—if having a purifier gives you peace of mind, by all means get one. Just remember that they are not magical devices.
As for who shouldn’t get one: If you’ve lived your whole life without an air purifier and haven’t had any issues so far, you’ll probably be fine without one. They are expensive to purchase and even more expensive to operate and maintain. Also, you should not expect purifiers to help with easing allergy symptoms if airborne allergens aren’t your concern. If you’re allergic to dust mites, for example, you’d be better served by hypoallergenic bedding and a regular vacuuming/dusting routine than an air purifier.
How we picked
For this guide, we focused on portable air purifiers, which is a blanket term for anything not installed directly into your home HVAC system, or, put another way, any purifier that can be moved from room to room. If you want to protect your entire building from airborne pollutants instead, check out this report from the National Institute for Occupational Safety and Health.
We limited our search to purifiers that meet US HEPA standards. HEPA, which stands for High Efficiency Particulate Arrestance (or High Efficiency Particulate Air), has two definitions; the US definition means that a HEPA filter removes 99.97 percent or more of airborne particles of 0.3-micron diameter. (The EU definition sets that number as low as 85 percent—more than three orders of magnitude worse, and, as far as we’re concerned, useless.)
As explained in more detail below (How HEPA filters work), 0.3 microns is the most difficult particle-size to filter. Larger and smaller particles are easier to filter. Therefore, a US HEPA filter will also remove at least 99.97 percent of virtually all airborne particles.
We also limited ourselves to purifiers rated to clear spaces of at least 350 square feet—larger than most bedrooms and living rooms, though smaller than an average house. Because we spend most of time at home in the living room or bedroom, these purifiers are capable of keeping most of the indoor air we breathe purified.
Almost every purifier we tested is self-monitoring. Self-monitoring purifiers measure the particulate concentration in the cleaned air they deliver to the room and shut off or lower the fan speed accordingly when the concentration is near zero (the exceptions are the Austin Air, Blueair 503, and Blue Pure 211). In practice, we recommend just keeping the units running on the highest fan/noise level you’re comfortable with: Doing so keeps particulate levels at a constant minimum. But the self-monitoring is a useful feature if you prefer to minimize energy costs or are running a purifier in a room you only occasionally use. In 2016, we also considered smart air purifiers and tested three—but for a few reasons, including the fact that most air purifiers are self-monitoring, we aren’t yet sold on them. (More on this below in Note on smart air purifiers.)
For people who suffer from allergies and other particulate-related health issues, HEPA filtration is generally sufficient. However, for people with chemical sensitivities or who live in highly-polluted environments (near an oil refinery or a farm that uses pesticides, for example), we also looked at a subset of HEPA filters that also can remove molecular pollutants. By that, we mean pollutants at the atomic or molecular scale—things like radon and volatile organic compounds (VOCs). These purifiers include a sorbent filter—usually activated carbon—that interacts with molecular pollutants and alters or binds them, rendering them safe or absent from the air. Our 2014 test revealed that five pounds of sorbent is the absolute minimum necessary—and that limited our options to just a couple purifiers.
The speed at which a purifier can filter a room’s worth of air is also important. You could theoretically draw air through a drinking straw with a HEPA filter inside it and eventually remove 99.97 percent of 0.3-micron particles in a sealed room—but in a typical house, with doors opening and closing, leaky windows and chimneys, and cooling and heating ducts drawing air from outdoors, such a tiny system would be totally overwhelmed by the continuous introduction of new particulates.
Finally, we nixed any air purifier that produces ozone to “purify” the air—and there are many that do. The theory is that ozone, a highly reactive form of oxygen, will “neutralize” or “break down” pollutants. Setting aside the vague science behind that claim, even low levels of ozone can be harmful. It’s madness to introduce a new pollutant when the goal is to reduce pollution overall—and even madder to do so when highly effective, non-ozone-creating purifiers are available.
With these standards and limitations in mind, over the course of our 2014 and 2016 research we surveyed hundreds models from the Amazon best seller list, big-box stores (Best Buy, Home Depot), Consumer Reports, specialty air purifier sites catering to allergy or asthma customers such as Allergy Buyers Club or Achoo Allergy, and any review sites we could find that had decent information and methodology. These included the hidden gems Air Purifier Power, written by Edgar V. Sherbenou, and Air Purifier Review, which has a clean and easy-to-use website with surprisingly informative content.
Combining our 2014 and 2016 searches, we’ve now tested more than a dozen different air purifiers that meet our standards. Between them, we are confident we have taken the full measure of what’s available, what’s practical, and what’s worthwhile.
How we tested and quantified
In 2014 and 2016, we tested our selected purifiers under laboratory conditions and with lab equipment. We looked at particle filtration, noise levels, odor (a.k.a. molecular pollutant) filtration, and cost over time (purchase price plus cost of electricity and replacement filters over five years).
Particle counts, pre-, mid-, and and post-filtration, were measured with professional grade equipment used for atmospheric research (specifically a TSI Inc. 3080/3010 Scanning Mobility Particle Sizer and 3321 Aerodynamic Particle Sizer).
Drawing on John’s 12 years of experience measuring airborne particles, in 2014 we selected three sources to produce a dirty, particle-filled room: Combustion from two Diamond safety matches was used to create small smoke particles (0.010-0.300 micron); 5 milligrams of a white, powdered titanium dioxide, commonly used to provide the white pigment in plastics and paints, was used to generate midsize particles (0.3-1 micron); and, in 2014, 25 milligrams of an ISO dust sample was used for the 1-20 micron particles. In 2016, we dropped this last test as all HEPA-rated test models reduced them to effectively zero.
The room we tested in in 2014 was unavailable in 2016. The former was 10 by 13 by 8 feet, or 130 square feet—about the size of a small bedroom and well below our 350-square-foot general standard for test purifiers. In 2016, we tested in a room roughly twice this size, 21 by 10 by 12 feet, but with built-in lab equipment that reduced the effective size to about 240 square feet (equivalent with 8-foot ceilings). We tested our original and current pick, the Coway Mighty, in both rooms, using both the original and brand-new filters in 2016, providing a broad (if imperfectly equivalent) standard of comparison.
In 2016, we followed a similar procedure to previous tests, but increased the amount of particulate pollution significantly, doubling the number of matches and increasing the titanium dioxide by a factor of ten. We artificially increased particulate levels in the air by lighting and burning 4 wooden strike-on-box matches, followed by launching 55 milligrams of titanium dioxide to generate at least a 50-fold increase in particle concentration (to ~50,000 particles per cubic centimeter) over the background (~1,000 particles per cubic centimeter).
The graph above shows the test models’ absolute performance when measured against the initial particle concentration (set at 100 percent for each model independently). Blue is the particle concentration after 10 minutes; red is after 20 minutes. Four key takeaways:
1. Our previous and current pick, the Coway AP-1512HH Mighty, performed exceptionally well and was the third-best performing overall, reducing initial levels to 12 percent (and the difference between it and the second-best model, the Winix 5500-2—our new runner-up—was statistically insignificant). Even more important, the Coway Mighty performed equally well using two-year-old filters and brand-new ones. So not only does it perform exceptionally well—it keeps doing so for years.
2. The new-for-2016 Blue Pure 211 performed extraordinarily well, reducing particulate pollution to just 3 percent of initial levels. Notice, however, that the Blueair 503 only reduced particulates to 18 percent of initial levels. The two machines are made by the same company and utilize the same unique pre-filter, which imparts an electrical charge to particulates to facilitate their removal by electrostatic attraction. In 2014, the Blueair 503 was also a standout performer, reducing particulates to less than 10 percent of starting levels in just 10 minutes. But two years later, its performance had fallen to less than one-quarter that, leaving almost 40 percent of initial particles in the air on the same setting and over the same 10-minute time frame. And new filters, a thorough cleaning, and queries to the manufacturer produced no fix and no explanation. More on this, and on why we nixed Blueair/Blue as a result, below.
3. The Coway Airmega 300 reduced particulates to 13 percent of initial levels—better than any other “large spaces” unit.
4. The Dyson purifier was the poorest performer by a significant margin. It only managed to reduce initial particulate levels to 30 percent of initial levels. That’s worse than any other purifier in our test.
Most air purifiers have three or four fan-speed settings; and all the purifiers we tested evince a huge jump between medium and high. Basically, “high” is for when air pollution has spiked (say, because of a nearby house fire) or when you’ve returned home after a long vacation during which the purifier wasn’t running. Pragmatically, 99 percent of the time you’ll run your machine on one of the low or medium settings. So we measured the noise produced by each machine on the highest middle-speed fan setting. Sound-pressure levels of each unit were measured at a distance of 1 meter away and 0.5 meter above the ground (the standard for hi-fi speakers), using a late-model iPhone running a NoiSee app. (This is one of the top recommended apps in a survey of 192 apps by the National Institute for Occupational Safety and Health, where it came within +/-2 decibels of reference measurements. That’s plenty accurate for our purposes.) We set 55 decibels, about the highest that would not interfere with normal conversation, as our limit; any machine louder than that on its moderate fan setting was eliminated immediately.
The results from the VOC testing were illuminating. While all air purifiers tested had filters which were claimed to be effective at removing odors, most had almost no impact on the initial unfiltered measurement, including the Coway Mighty (our main pick) and Winix 6300 (our previous runner-up). But this was to be expected, since these units only use a thin sorbent filter of activated carbon or zeolites (a class of minerals with a unique physical form that means they can act as “molecular filters”).
However, three premium models stood out. The Austin Air, which contains 15 pounds of activated carbon and zeolite, left only 13 percent of baseline ethanol remaining, twice the performance of the next closest competitor, the IQAir, which has 5 pounds of activated carbon and alumina pellets. The Blueair 503 left 32 percent of VOC and contains 4.2 pounds of activated carbon.
This 2014 test set a baseline for 2016: any unit with less than 5 pounds of chemical adsorbent would fail the odor/molecular-pollution test. And because few air purifiers even approach that amount, we were able to eliminate that test in 2016, too.
Both upfront and operating costs are a significant part of owning an air purifier. These units can be expected to last for several years, so we took a long view at the cost of ownership, including the purchase price, electrical consumption, and recommended filter replacement schedule, using the shortest time for filter replacement if a range was specified.
Power consumption of units was measured at each setting using a Sperry DSA-500 clamp-on ammeter. The table below, and calculations used for cost of ownership, are based on the highest fan setting that didn’t exceed the 55 decibel noise limit. Electricity rates are based on current residential rates in John Holecek’s area: $0.15396 per kWh and operation 24 hours a day. (Rates vary state to state and city to city, of course, but California is in line with much of the country; the Northeast especially, the Mid-Atlantic, and the Pacific Northwest are generally higher.) If you wish to calculate more accurately, see the latest state averages.
After all this testing over two years, we were able to confidently settle on four air purifiers: our two original picks, a new pick for extreme particle-filtering needs, and our original pick for particle-plus-molecular filtration.
In terms of measured particle removal, the Coway is very nearly the best we tested. On the moderate setting—the highest you’re likely to ever run it on for long periods— it purifies better than all but two units, reducing particle concentration to just 12 percent background level after 20 minutes. Of the two machines that bested it, one, our runner-up Winix 5500-2, only beat it by a statistically insignificant 2 percent; the other, the Blue Pure 211, raised significant questions about long-term performance. And again, we measured after just 20 minutes; air purifiers generally run all day or until their built-in sensors measure effectively zero particulate pollution and temporarily shut them off. The net lesson? The Coway Mighty is both highly effective and extremely fast at purifying the air you breathe.
The Coway also maintains this level performance for years, even when pushed far beyond the stated lifespan of its HEPA filters. These, like most HEPA filters, are meant to be replaced once a year. We ran our test model virtually nonstop for two years without replacing them—twice what’s recommended—and it still worked as well as it did on day one. That’s not hyperbole: in 2016 we measured its performance using both the original filters and brand-new filters, and there was no measurable difference between them. That cannot be said for every air purifier, including another of our previous picks, now struck off—again, see Note on Blueair/Blue below. And upon opening the Coway before our tests, we discovered why the Coway excelled: first, its prefilter did a stellar job of removing large particulate matter (pet hair, dust), leaving only fine particles to enter the HEPA filter; second, its HEPA filter is beautifully sealed, with no leaks around the edges that could let fine particles skate through. (See the slideshow above for the dramatic evidence.)
The Coway is also easily the wallet-friendliest particulate air purifiers that met our stringent test criteria, both in upfront cost and in cost of operation and maintenance. To buy and run one 24/7 for five years (and that on the Moderate setting, not Low, where many people would keep it) will set you back about $575, or $115 a year. Most purifiers rated to the same square footage and with similar (in most cases somewhat worse) filtration performance would set you back $1,000 or more; only our runner-up came within $150. Another way to look at this is: You could buy and maintain two Coway Mightys for five years (perhaps one for the bedroom and another for the living room) for less than the cost of some individual competitors.
In an ideal world, none of us would live with an appliance sitting in the middle of our bedroom or living room. But in the very likely event that you can’t install a filtration system in your home or apartment’s HVAC system, the Coway makes the best of the situation, being quite small at 17 by 18 by 10 inches (WHD), about the size and shape of a beach tote. It’s light at just over 12 pounds. That makes it easy to fit into any room and equally easy to move to another.
Finally, in addition to being extremely quiet, its moderate setting—51.1 decibels, where normal conversation averages 55 decibels or more—is effectively silent on low at 42.7 decibels. And, compared to many competitors, it’s reasonably attractive; with smooth curves, minimalist UI, a circular vent, and and all-black and all-white models, it resembles a late-model iPod. Simply put, it’s easy to live with. You may well turn your Coway Mighty on and never think about it again, and that’s about the highest compliment you can give an appliance.
Widely available, well reviewed, cheap to buy and operate, high-performing initially and after years of careless use, physically attractive, and able to disappear visually and aurally into the background of most homes, the Coway AP-1512HH Mighty is our clear winner.
We side with the Coway for proven long-term performance and superior long-term cost and aesthetics. The Winix costs about the same the Coway upfront, but is also a bit less energy-efficient and uses slightly more expensive filters. This means it will end up costing about $130 more over five years’ operation. Because it’s a new model, we don’t have long-term performance data. But if the Mighty is unavailable and you need an air purifier ASAP, it’s a fine choice. (Note again, as above: The 5500-2 replaces our previous runner-up, the nearly identical Winix Plasmawave 6300. The 6300 will phased out within a year, but replacement filters will remain available, so if you bought one, there’s no need to upgrade.)
For large spaces and extreme cases
If you have severe allergies or other serious health issues related to airborne particles, or if you need to purify the air in a seriously large space, we have a new step-up pick: the Airmega 300. This is a large but attractive machine; it’s HEPA-rated and rated to clear spaces of about 500 square feet at five complete air-changes per hour (about 1.5 times the coverage of the Coway and Winix), or 1,250 square feet at two complete air-changes. Its unique twin filters permit very high airflow, allowing it to run on lower settings when the air is relatively clean (quieter, less energy) or to rapidly filter very large volumes of air on the highest setting, as you might when allergens or pollution are elevated. In our test, it gave the overall second-best air-purification performance—more a function of its higher airflow and dual filters than inherently superior filtration. (See Notes on Blueair/Blue purifiers, below, for why the top-performing machine isn’t among our picks.) It is expensive and costs about $1,400 (including the purchase price) to maintain over five years; this is in line with most of the high-end models we tested, but as hinted at above, more than the cost of a pair of Coway Mightys.
The Airmega comes in an even more powerful model, the 400, rated to almost 625 square feet at five air-changes per hour and 1,500 square feet at two air-changes, and each version has an otherwise identical “smart” model (designated by an S after the model number) that allows you to control and monitor the machine via a smartphone app. We actually tested a 300S; six attempts to get the app to work failed. But since it and the “dumb” 300 share the same mechanicals, that didn’t affect its performance. (In general, we’re not yet sold on “smart” models and the price-premium they come with; see Note on smart air purifiers below.)
Its annual operating cost of also makes it by far the cheapest high-end purifier to run, but note that this is due to a filter that’s designed to be replaced every five years instead of annually. Against that, its power consumption is rather high, due to its high airflow and the airflow-resistance of the carbon filter; and though it’s HEPA-rated, when running the fan at comfortable sound levels, the Austin Air was less efficient than all other models at removing particles from the air. But a little extra background noise isn’t too much to ask if serious health concerns over molecular pollutants are an issue.
The Aerus QuietPure Whisper, a “smart” model, had an easy-to-configure app but gave notably inferior performance compared to the otherwise comparable Coway Airmega 300.
The Blue Pure 211 was in absolute terms the best performer in the 2016 test, but for reasons explained above in How we tested and below in Note on Blueair/Blue, we are skeptical it will maintain that performance long-term.
The Blueair 503 was a standout performer in our 2014 test, but as detailed below, it suffered a four-fold loss of particulate filtration when re-tested in 2016. Neither the manufacturer nor our own investigations were able to explain this (though at their request we’ve sent the 503 back to the manufacturer for analysis), and a thorough cleaning of the machine didn’t fix it. At well over $2,300 for purchase and five years’ maintenance—by far the highest in our 2016 test—that’s not acceptable.
The Dyson Pure Cool, now sold as a “smart” version, the Pure Cool Link, was the worst-performing purifier in our 2016 test.
The IQAir HealthPro Plus has long been considered the gold standard. However, in our 2014 test, it proved middling on both particulate and odor/molecular-pollution removal, and it’s incredibly expensive to buy and operate—more than $3,000 over five years as we calculated at the time of writing.
The Rabbit Air MinusA2 SPA-700A earned middling performance, cost-of-ownership, and noise marks in our 2014 test.
The Rowenta PU6020 did not stand out on particulate filtration. It employs a unique formaldehyde-trapping filter, but those with chemical sensitivity should look to the Austin Air for broader odor/molecular-pollution removal. It’s also pricy to buy and run.
The Sharp Plasmacluster FP-A80 performed well on particulates in our 2014 test, but didn’t live up to its claims of molecular/odor removal due to its small (~1 pound) sorbent filter. That plus its high long-term cost (about $900 over five years) puts it in that unhappy middle ground of too much cost and not enough performance relative to true molecular/odor purifiers.
The Winix HR1000, a smart model, has an easy-to-use app but gave unimpressive particulate-filtration performance at a relatively high cost—almost $1,000 over five years, nearly twice that of our main pick, which also comprehensively outperformed it. We like other Winix products, however, including the 5500-2, our new runner-up.
How HEPA filters work
HEPA stands for High Efficiency Particle Arrestance. The technology is the result of an industrial need that became critical in the aerospace age: high volumes of very clean air, vital for the production of microprocessors and other sensitive instruments. Happily, HEPA filtration is also fundamentally simple and cheap, which means it’s available to everyone today.
HEPA filtration is a physical process: it relies on momentum to capture variously-sized microscopic particles when they’re drawn at high speed through a dense, felt-like net with gaps of varying size. It’s not that any given gap in the net’s mesh is fine enough to capture everything; it’s that, cumulatively, almost nothing but air will make it through the thick, multi-layered net without being caught. That’s opposed to what most of us envision when we think of filtration: uniform particles (say, pieces of pasta) trapped by uniform holes (a colander) as the carrying medium (water) slowly drains away.
In HEPA filtration, a dense and seamless sheet of very small fibers (usually fiberglass) is pleated like an accordion and mounted, sealed at the edges, in a frame of metal or plastic, creating an airtight filter. A fan draws air rapidly from side of the filter to the other, and the seamless sheet and the sealing mean that air on one side can’t pass through to the other side without being filtered. (To help ensure a steady airflow, many HEPA filters also interleave the pleats with impervious sheets of aluminum or plastic.)
The fibers in a HEPA filter capture airborne particulates in three basic ways, but they all come down to the comparatively huge differences in size between those particulates—solid, though generally microscopic, things like smoke particles—and truly infinitesimal gaseous atoms and molecules. The latter are so small that they flow more or less freely between the HEPA filter’s fibers, allowing for unimpeded passage of gases (“air”) from one side of the filter to the other. By contrast, particulates, while small on a human scale, are hundreds or thousands of times larger than gaseous atoms and molecules. That means particulates have so much momentum that they cannot simply “go with the flow” of the air. Instead, for three reasons, based on their size, they are virtually guaranteed to slam into the fibers. In short, gases are drawn through quickly and with little resistance while, simultaneously, almost everything that isn’t gaseous gets caught. Thus, High Efficiency Particle Arrestance.
The largest of the particulates—those roughly same diameter of or wider than the filter fibers, about 0.5 microns and above—are captured via impaction: unable to change their course due to momentum, they simply slam into the fibers and stick to them. Particles less than the diameter of the fibers, but not too much less, are captured by interception: they “try” to flow around the fibers but come close enough to touch the fibers on the way by, and again stick. Finally, very fine particles— those below 0.1 microns, or one-fifth or less the diameter of the fibers—get bounced around randomly and slowed by their interactions with atmospheric atoms and molecules, and eventually drift or get bounced into a filter fiber, whereupon (yet again) they get stuck—a process called diffusion. The net result is that virtually all particles get captured.
Crucially, the hardest particles to capture are what you might call the Baby Bears: at 0.3 microns, they’re at the low limit of interception momentum and above the limit of diffusion—in other words, “just right” to get through a HEPA filter. The solution is to make the filter dense enough that even at the 0.3-micron limit, there are enough fibers between the unfiltered and filtered side to capture most of them. And again, according to the U.S. HEPA standard, “most of them” means 99.97 percent—damn near all. Which is the standard we used. The European Union certifies “HEPA” on a numeric scale, and the lowest, and very common, one is less than 85 percent of 0.3-micron particles (“E10”). At the risk of offense: Buy American. And if you have to buy European, insist on a purifier rated to H13 (99.95 percent of 0.3-micron particles removed).
Note on Blueair/Blue
In our 2014 guide, and as a result of our 100 hours of 2014 testing, we recommended the Blueair 503 with the SmokeStop filter package for people with severe health concerns tied to particulate air pollution. It was, and is, an expensive option; nearly $2,600 over five years of operation and maintenance. We recommended the Blueair 503 almost exclusively because of its truly exceptional initial performance on particulate removal. The Blueair employs a unique step in its filtration process: it imparts an electrical charge to airborne particles via metal and nylon “brushes” and attracts them to pre-filters with the opposite charge before running the remainder through its HEPA filter.
This appears to work wonders when Blueair units are new. As we wrote in 2014, “The Blueair was by far the most effective particle collector of the 10 models tested. In our initial test at moderate fan speeds, less than 10 percent of the particles remained after 10 minutes of operation. That’s half the number of particles compared with the next closest model.”
We retained that Blueair 503 for long-term testing, and ran it regularly for the best part of two years.
Then we tested it again—and its performance had nosedived. It was among the worst-performing units in our 2016 test, using both the original filters and brand-new filters. We contacted Blueair and described the dropoff; they suggested it was broken and asked us to pay them $150 upfront to ship them the unit and have them “fix” it—sight unseen. When we further explained the situation and asked for alternate remedies—like cleaning the metal plates that impart the electrical charge—they said there were no such remedies. We then, as basic tinkerer/maker types, physically cleaned the charging plates with plain water and a rag, then retested the unit—and again measured the same dropoff in performance.
In taking the 503 apart for cleaning, we discovered significant dust buildup on the electrical-charging brushes and behind the “Smokestop” and HEPA filters—perhaps the result of the machine’s lack of a prefilter for removing large particulate matter (see the slideshow above). Whatever the cause—and, at their request, we’ve shipped our test model back to Blueair for their own evaluation—those are worrying signs.
In 2016, the Blue Pure 211 was a similar standout, dropping particulates to 3 percent of their initial concentration within 20 minutes, versus 11 percent for our pick, the Coway Mighty. Blue, however, is a subsidiary of Blueair—and the Blue Pure 211 uses the same charging technology as Blueair 503.
Given that the Blueair’s performance collapsed over the course of just two years of use, remained poor even after replacing the HEPA filters, and remained so even after cleaning the charging brushes, and given that the Blue Pure 211 is made by the same company and utilizes the same technology, we can’t recommend either the Blueair 503 or Blue Pure 211.
Note on smart purifiers
It is a commonplace of contemporary life that appliance-makers offer “smart” versions of their every product. Smart fridges, smart stoves, smart washers and driers—and, of course, smart air purifiers.
We tested three smart air purifiers in 2016: the Coaway Airmega 300S, the Winix HR1000 and Aerus Quiet Pure. As noted above, we had trouble with the Airmega’s app. The Winix and Aerus apps, by contrast, were easy to set up and worked well.
However, when it comes to air purifiers, we’re skeptical of the utility, and more broadly, the smartness of “smart.” That’s because air purifiers, like smoke detectors and other “always-on” appliances, are the essence of dumb. That’s not an insult. In fact, it’s praise.
As noted above: almost every purifier we tested has self-monitoring technology and can measure the particulate concentration in the cleaned air they deliver to the room, and shut off or lower the fan speed accordingly when the concentration is near zero. (The sole exception among our picks is the Austin Air.) That means that, unlike such things as air conditioners and space heaters, which alter their activity relative to their users’ presence or preferences, most air purifiers can alter their performance relative to absolute measures. Their job is to create clean air. They do so; then, if set to the self-monitoring mode (a.k.a. “energy saver” or the like) they shut themselves off when they’ve done their job; and then they come back on when external factors (like air coming into a home from outside) mean the air needs cleaning again. We generally recommend simply allowing purifiers to run at the highest fan/noise level you’re comfortable with—doing so ensures a steady, minimal level of particulate pollution in the room. But at any event, we’re dubious of the utility of the roughly-$100 premium you’ll pay for a “smart”, app-ified air purifier that lets you control it when purifiers already do their job automatically.
Sure, a “smart” air purifier app can send you a reminder when it’s time to replace the filter—but so can the digital or paper calendar you already use to remind yourself of long-term maintenance needs. And those cost you nothing.
Of course, if real-time remote monitoring and control of your air quality is a genuine desire or need, $100 or more may be a worthwhile outlay. But for most people, we firmly believe reliability and raw performance are the key factors—not “smart” ability or lack thereof.
(Photos by John Holecek.)
Originally published: August 10, 2016