All About Air Purifiers

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All About Air Purifiers

Can you believe the promises made by manufacturers of portable air purifiers?

Posted on Jan 19 2018 by Martin Holladay

If you’re concerned about indoor air quality, you may have noticed ads for a type of appliance called a portable air purifier. Purchasers hope that these boxes will suck in dirty air and discharge clean air, but few homeowners know how these appliances operate.

In this article, I’ll try to answer a few basic questions about portable air purifiers:

  • How many kinds of portable air purifiers are on the market?
  • Do they work?
  • Who needs one?

Exaggerated claims

If you Google “air purifiers,” you’ll quickly discover retailers who make outlandish claims. Air purifiers are the radiant barriers of the IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness. world. In other words, air purifiers often work, but the field seems to attract a lot of charlatans.

For example, in , a company called Dyson claims that its appliance “automatically purifies to remove allergens, pollutants and gases from the air.” Well, if the device removes gases from the air, that must mean that the device simply emits particulates — probably by spraying the room with soot. I think I’ll pass. I’d rather have an air purifier that removes the solids from the air and lets the gases pass right through.

Types of air purifiers on the market

Portable air purifiers are devices that sit on the floor of a room. A portable air purifier is designed to clean the air of a single room, not an entire house. Most air purifiers include a fan; the device pulls air into the appliance, does something to the air, and then discharges it. Most models cost between $100 and $1,000.

Most air purifiers include a particulate filter — a filter that in some cases meets the standard for a HEPA (high efficiency particulate air) filter. Other air purifiers claim to clean the air with an activated charcoal filter or with ultraviolet (UV) light. Still others claim to clean the air by emitting negative ions or ozone.

Let’s clear the air right from the start:

  • Most people don’t need an air purifier.
  • The most likely reason to buy an air purifier is if someone in your family has a special medical condition (for example, a respiratory illness or allergies).
  • If you think you need an air purifier, you want to buy one that is described as a HEPA (high efficiency particulate air) filter. This type of air purifier will remove small particulates from the air.
  • If someone in your family smokes tobacco indoors, and you can’t convince that person to go outdoors to smoke, you may want to purchase an air purifier that includes active charcoal filtration.
  • You don’t want to buy an air purifier that includes UV treatment, ozone generation, or the production of negative ions. These features are useless or dangerous.

Filters with a MERV rating

If you are thinking of upgrading the filter on your furnace or air handler, you may have noticed that many furnace filters have MERV ratings. (MERV stands for "minimum efficiency reporting value.") MERV ratings range from 1 to 16; the higher the number, the smaller the particles that the filter can trap. MERV filters rated above 13 will remove particles as small as 0.3 micron.

If you want to buy a MERV-rated filter for your furnace, remember two points: (1) The filter mustn’t be so thick that it won’t slide into your filter slot, and (2) The filter mustn’t be so effective that it reduces air flow significantly.

I had a long conversation about air purifiers with Terry Brennan, the president of Camroden Associates in Westmoreland, New York. Brennan is a widely respected consultant on issues of indoor air quality, mold, and ventilation. “If a client has an air handler or furnace, I’ll definitely tell them to put in a filter with the highest MERV rating they can fit into their filter slot,” Brennan told me. “That’s typically a 1-inch thick filter. That type of filter is relatively inexpensive and has a pretty low pressure drop. A 1-inch thick filter will probably be a MERV 11. It won’t reduce the air flow enough to matter. There is no down side to that type of filter, and there is some benefit. But you have to consider the run time on your air handler. It’s only running for probably 20 percent of the time, so you might say that this approach is only 20 percent effective. You don’t want to leave your blower on for 24 hours a day — if you do, now you are looking at 300 to 500 watts, at least for a system that doesn’t have an ECM motor.”

A HEPA filter makes sense

Let’s say you’ve decided to buy a portable air purifier. The type you’re looking for is a HEPA filter. It will filter particulates from the air. To meet the federal standard, a HEPA filter must remove 99.97% of particles that have a size of 0.3 microns (µm) from the air passing through the unit.

A HEPA filter is different from a pleated media filter inserted into an air handler or furnace. Because it does a better job of reducing very fine particles, a HEPA filter will reduce air flow unless it includes a powerful fan.

All HEPA filters exceed the ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. test protocol (52.2) used to determine MERV ratings, so HEPA filters don’t receive MERV ratings.

Although the most important criterion for an air purifier is to make sure it meets the HEPA filter standard, the information can be surprisingly hard to find. Even reputable manufacturers of HEPA filters often fail to describe their air purifiers as HEPA filters. When in doubt, call up the manufacturer and ask to speak to a technical help representative. (Note: Beware of deceptive marketing materials that proclaim that a unit uses "HEPA-type" filters. There is no such thing. Either it meets the HEPA standard or it doesn't.)

Unlike ozone generators or negative ion generators, HEPA filters are effective at the task they are designed for. The size of the particles removed by a HEPA filter covers common allergens, including mold spores, animal dander, dust mites, and pollen. A HEPA filter will also remove some of the particles found in tobacco smoke or dirty urban air.

Most manufacturers of portable HEPA filters report the unit’s clean air delivery rate (CADR). You’ll probably be buying a unit with a CADR that ranges from 100 cfm to 200 cfm.

What about ozone generators?

What about advertised features that go beyond filtration of particles? When I brought up the topic with Brennan, he said, “Now we come to the land of miracles.” We’re entering a shady realm of marketing — the realm occupied by insulating paint.

Some air purifiers trumpet the fact that they generate ozone. Avoid these units. Ozone is a respiratory irritant that can worsen rather than relieve allergy or asthma symptoms. , “When inhaled, ozone can damage the lungs. Relatively low amounts can cause chest pain, coughing, shortness of breath and throat irritation. Ozone may also worsen chronic respiratory diseases such as asthma and compromise the ability of the body to fight respiratory infections. People vary widely in their susceptibility to ozone. Healthy people, as well as those with respiratory difficulty, can experience breathing problems when exposed to ozone.”

Although the EPA warns consumers about the dangers of ozone generators, plenty of retailers (including push these dangerous devices. claims that the $163 device “Removes powerful odors and cleans your air with its powerful Ozone Generator.”

Ozone levels can be increased by several types of air purifiers — including not only those that describe themselves as ozone generators, but also those that describe themselves as electrostatic precipitators or ionizers. According to a , “introduction of any amount of ozone into indoor spaces may result in increased levels of formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen.", ultrafine particles, and other pollutants due to the reaction of ozone with terpenes (fragrance compounds such as pinene and limonene) and other chemicals emitted from modern consumer products and building materials.”

So don’t buy an ozone generator.

No, you don't want a negative ion generator

Some marketers of air purifiers call themselves “negative ion generators.” These devices should be avoided.

For example, Walmart sells a $27 appliance describes as . Wow! The device produces “a constant stream of harmless negative ions.” I think I’ll pass.

No, you don't need ultraviolet light

What about portable air purifiers that tout the advantages of ultraviolet (UV) light? They are almost certainly ineffective, because the percentage of the air in a room that ends up being treated by the light is too small.

According to the , “Some [air purifier] units fitted with a ultraviolet (UV) light kill viruses and bacteria but even this does not purify all the air you breathe. The best any air cleaner can do is remove small particles that pass through the filter.”

discusses “electronic particle air cleaners, ionizers, and UV light devices” under the headline, “Mixed Reviews on Safety and Effectiveness.”

The article notes, “We would use extra caution with these devices, as it is the ionization process that converts beneficial oxygen molecules (O2) into highly reactive Ozone (O3) and most of these devices emit some level of ozone. Many of the popular brands, such as the Winix PlasmaWave, use a ‘corona discharge’ purifier, which creates ozone. Since the emitted ozone is less than the 0.05 ppm recommended by the EPA, these devices can be marketed as emitting no ozone. However, it is possible they fall just slightly under the EPA’s limit and furthermore, while the device emission itself may be considered safe, the danger lies in the buildup of ozone in the room while the device is constantly running. … Other devices that emit ozone include ultraviolet air purifiers, which use shortwave ultraviolet light to bombard oxygen molecules in the air.”

Will this device remove odors?

What about air purifiers that claim to remove odors?

According to a , “Odor removers usually don’t work.”

Brennan provides more qualified advice on odors. “There are occasional reasons for activated carbon filters, maybe for dealing with tobacco smoke or formaldehyde,” Brennan told me. “For issues of outdoor air pollution, activated carbon can help, or for someone who is sensitive to odors. Of course, if we’re talking about smoking, the best thing is: Don’t freaking smoke in your kid’s face.”

While some manufacturers of charcoal filters claim that these filters can reduce VOCVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production. levels, don’t believe the claims. , “Many purifiers claim to reduce VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production., or volatile organic compounds, using an activated charcoal filter. ‘Very few of them actually do,’ Mr. [Tim] Heffernan added. ‘It comes down to the mass of the activated charcoal filter.’ In order to be effective, the activated charcoal filter would have to be a minimum of five pounds to have any statistically significant effect, he said. Most contain a tiny fraction of that.”

Focus on source control and ventilation

It’s worth repeating this : “The three most common approaches to reducing indoor air pollution, in order of effectiveness, are:

  • Source Control: Eliminate or control the sources of pollution;
  • Ventilation: Dilute and exhaust pollutants through outdoor air ventilation and
  • Air Cleaning: Remove pollutants through proven air cleaning methods.

“Of the three, the first approach — source control — is the most effective.”

An example of a source control strategy is the use of a range hood exhaust fan. An example of ventilation is a heat-recovery ventilation(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. system. These approaches are much more effective ways to reduce indoor air pollution than the use of a portable air purifier. For more information on this topic, see All About Indoor Air Quality.

Health benefits are unproven

Remember, a portable air purifier isn’t required unless someone in your family has a medical condition.

Moreover, it’s also worth mentioning that there isn’t any evidence that portable air purifiers improve health. According to , “Despite prolific marketing to the contrary, scientific studies do not support claims that air purifiers improve your health, in part because it’s exceptionally difficult to disentangle their impact from the many other factors that influence your health.”

Two other sources make the same point, to varying degrees. According to , “Studies that definitively link air purifiers to long-term health benefits are rare, because long-term health is affected by multiple factors. (This is also why air purifiers may not be marketed as medical devices in the United States.)”

According to , “While air cleaning devices may help to control the levels of airborne allergens, particles, or, in some cases, gaseous pollutants in a home, they may not decrease adverse health effects from indoor air pollutants.”

Brennan’s advice

If someone in your family has a respiratory illness, and you’re interested in buying a portable air purifier, where do you start?

Brennan told me, “I have most frequently recommended them for people whose symptoms or existing medical conditions make them susceptible to fine particles. If it is someone with eye, nose, and throat irritation or allergies, put a portable air purifier with a HEPA filter in the bedroom. That’s where your biggest exposure probably is — in the bedroom. A portable 100 to 200 cfm HEPA filter in the bedroom can make a big difference. If you’re buying one, look for the Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. label. I’ve always liked the air purifiers from Austin Air and BlueAir.”

Unless you’re a physician…

If you’re a builder, architect, or home performance contractor, remember that you shouldn’t be giving medical advice or promising to relieve symptoms. If a client says, “My daughter has asthma — what should I do?” your answer should always be, “I think your daughter should visit a doctor.”

Brennan elaborated on this point. “Our job is to fix the building, not to diagnose or fix the medical problem. If someone is telling me about their symptoms, I usually respond with, ‘Have you seen a physician?’ And I usually recommend that people see someone who is board certified in occupational health.”

Martin Holladay’s previous blog: “Will Thick Cellulose Cause Your Ceiling to Sag?”

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Image Credits:

  1. BlueAir

Jan 19, 2018 12:27 PM ET

AWESOME article
by Armando Cobo

Thanks Martin, one of your best! The main reason I got into high-performing houses, 20+ years a go, was because of IAQ. This is a very comprehensive and detailed explanation about the "fake" claims made by manufactures of air-purifiers, and becomes a good one-stop-read resource to show our clients the advantages of installing ERVs and HRVs in their homes.

Jan 19, 2018 12:53 PM ET

Informative and humorous
by Robert Opaluch

This was a fun read. I have breathing allergies so have researched this topic. Agree this article is a very informative good summary. And good humor.

Jan 19, 2018 9:30 PM ET

Yes, a good read
by Malcolm Taylor

Living on the West Coast during the 1980s I was exposed to every imaginable form of purifying filters and burnt offerings. I think I've almost recovered.

Jan 20, 2018 10:16 AM ET

Something I didn't see here
by Andy Kosick

Something I didn't see here was if there is any scientific evidence that high performance filters (HEPA and otherwise) in air handlers and ventilators have significant health benefits? Or is it still too mixed up with other factors? I realize it makes good sense, just wondering if there's any hard evidence.

Jan 20, 2018 10:31 AM ET

Response to Andy Kosick
by Martin Holladay

As far as I know, the amount of evidence showing health benefits from HEPA filters attached to air handlers is the same as the amount of evidence showing health benefits from HEPA filters in portable air purifiers. There is none.

Jan 22, 2018 5:16 PM ET

insidious advertising
by Rachel Wagner

I, too, appreciated this article. It is clear and useful. And as I scrolled through the comments, a goog*e ad for this popped up:

I clicked - couldn't help myself - I was oh so curious ... and learned that Martin neglected to discuss miracle-molecular air purifiers!

Jan 22, 2018 6:11 PM ET

Edited Jan 22, 2018 6:20 PM ET.

Response to Rachel Wagner
by Martin Holladay

Thanks for the link! As expected, lots of fancy words, but not much explanation. We learn that "Molekule’s patented technology, Photo Electrochemical Oxidation (PECO), works at the molecular level to eliminate indoor air pollution."

Wow! It works on the molecular level! Just like -- well, fire, evaporation, breathing, or most everyday processes.

Jan 22, 2018 10:47 PM ET

Worth clarifying
by Malcolm Taylor

If it works at the molecular or molekular level.

Jan 24, 2018 3:49 PM ET

Edited Jan 24, 2018 4:10 PM ET.

Proof of long-term health benefits may be too high a bar
by Derek Roff

There has never been a study of the long-term health benefits of avoiding motor vehicle accidents. We've had plenty of studies on traffic injuries, but no careful studies of the reverse. That's because the study of "long-term health benefits", as Martin's article notes, is very difficult, expensive, and "affected by multiple factors". While it is good to be forewarned that there is no scientific proof that removing particulates from the air provides a long-term health benefit, dismissing every product without such proof is roughly equivalent to dismissing every product.

Can anyone provide a scientific study proving the long-term health benefits from installing a MERV-rated furnace filter? An ERV? An HRV? A range hood? I'm betting that the answer is "no". Yet we recommend all those things. We know the problems that these products address, and we don't demand proof of long-term health benefits from using them, before considering installing them in our homes.

In helping to avoid fraudulent and misleading marketing, let's not apply a product selection "filter" to one product, that we ignore for all the others. There are solid reasons for avoiding most of these air purifier products, without demanding that they clear the bar on proof of long-term health benefits.

Jan 24, 2018 4:11 PM ET

Response to Derek Roff
by Martin Holladay

Your point is well taken. That said, the health benefits of some environmental factors have been studied, and the results are convincing. People who never smoke cigarettes have much lower rates of lung cancer than people who smoke. Children who grow up in homes with lead paint are at much higher risk of lead poisoning than children who grow up in homes that are free of lead paint. These findings are stark and unambiguous.

While it was taken for granted by two generations of parents that keeping children clean helped keep them healthy, the hygiene hypothesis has turned that assumption on its head. We now know that small children who are exposed to dirt -- and especially children who are exposed to barns and animal manure -- at an early age are healthier than small children who are kept clean. Who knew?

In other words, assumptions can be tricky.

Jan 24, 2018 9:55 PM ET

Edited Jan 24, 2018 10:21 PM ET.

by John Semmelhack

The most recent study of outdoor fine particulate matter (PM2.5) and death rates seems to conclude that there's no "safe" level of outdoor PM2.5.

(Video summary)

If there's no known safe level of outdoor PM2.5, it's not a big stretch to suppose that there's probably no known safe level of indoor PM2.5, either. In fact, there was a 2016 workshop at the National Academy of Science to "...discuss the state of the science on the health effects of indoor exposure to particulate matter." It looks like many of the indoor air quality bigwigs in North America attended...including Terry Brennan. For those that want to dig into this, all of the presentations and videos are available for viewing here:

Regarding room air "purifiers": at the workshop, presenter Bill Fisk pointed to research that shows that MERV 16 or HEPA level filtration on stand-alone room filtration systems is an effective method for reducing indoor PM2.5, and in a typical home (with a poorly designed HVAC system) could reduce PM2.5 for a fraction of the energy compared to a high-MERV filter inline with the ductwork.

It's a shame that there's so much snake oil in this product market. There are some super effective room filtration products on the marketplace, but end users have to wade through the muck to find them.

Jan 24, 2018 9:59 PM ET

by John Semmelhack

Martin - regarding the Dyson unit: a quick look past the slick advertising and the insane price ($500) yields some useful information - the Dyson unit uses a HEPA filter for particles and graphite filters for VOCs...and it uses an efficient DC motor as well.

Jan 25, 2018 5:55 AM ET

Edited Jan 25, 2018 6:53 AM ET.

Response to John Semmelhack
by Martin Holladay

Thanks for your comments. In my 2016 article, All About Indoor Air Quality, I advised GBA readers:

"Most of the chemicals that show up in the questions posted on GBA don’t make the list [of worrisome substances created by LBNL researchers]. What matters most? It turns out that fine particulates — PM2.5 — cause the most health damage. In fact, these fine particulates are three times more concerning (from a health perspective) than the next most worrisome contaminant on the list. Some of these particulates come from outdoor air, due (for example) to diesel exhaust or wood smoke. The rest come from indoor activities like cooking and burning candles. (For more information on the researchers' findings, see )

"If your home has a supply ventilation system (for example, a central-fan-integrated supply ventilation system), it's a good idea to include a MERV 13 filter. This filter should be installed at your air handler or furnace. If your home has a balanced ventilation system (an HRV or ERV), choose a ventilating appliance that offers a MERV 13 option. A MERV 13 filter will eliminate most of the PM2.5 particles entering your home.

"According to Terry Brennan, a building scientist who serves on the ASHRAE 62.2 committee, “The kitchen range is the biggest stationary source of fine particulates in a residential setting. It's the pyrolizing and burning of food that makes most of the particulates and contaminants.” So install a good range hood fan, and use it."

This remains good advice, in my opinion, even in the absence of research that nails down any connection between these MERV 13 (or MERV 16) filters and improvements in occupant health.

I think we can agree as well that (as this article explains), when it comes to a portable air purifier, a smart purchaser only cares about one thing: Is it a HEPA filter?

Finally, I don't doubt that the Dyson air purifier removes particulates from the air. I just wish the company's marketing department knew the difference between solids and gases.

Jan 26, 2018 11:45 PM ET

Box fan with filter
by John Semmelhack

There are a lot of DIY-ers here on GBA. For those that want a super low-cost, high MERV room air filter for just a handful of bucks, the box fan filter "hack" is the way to go. $20 fan, ~$15-$30 2-in to 4-in MERV 13 filter, and a little tape. The woodworkers among the crowd will build a fancy-looking enclosure for it.

Linda Wigington's ROCIS group has been researching these setups and has found they can reduce PM2.5 more quickly than room air purifiers, due to their higher volume of air.

Jan 27, 2018 12:22 AM ET

Personally I don't doubt that
by Jon R

Personally I don't doubt that indoor air pollution has negative health effects - of unknown significance. It would be interesting to see an analysis of the cost effectiveness of interior filters vs source control (eg low VOC building materials) vs increased outside ventilation air. Or even compared to other non air quality related housing health/safety options.

Jan 27, 2018 12:38 AM ET

John Semmelhack
by Malcolm Taylor

Thanks for the link. That's a great simple setup.

Jul 18, 2018 1:40 PM ET

by Eesan Kesavan

Valuable information

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