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Air Cleaners and Filters

By HHI Staff

You've read the ads. They claim to magically remove 99% of the pollutants from the air of your home. Since you want to protect your family's health, you are ready to buy an air cleaner. However, as you begin to compare products, you realize that the subject is quite complicated. What does HEPA mean? How does activated carbon work? Is a portable model big enough? Will it really work?


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To make an educated decision on a specific model, you need to know the answers to these — and many other — questions. In nearly all cases the best approach is not to pollute the air in the first place. Then an air cleaner or filter will be less necessary. It is also important to realize that air cleaners are not substitutes for fresh air because they cannot create oxygen.

There are many low-polluting consumer cleaning products and practices available today that will result in cleaner indoor air. A high-efficiency portable vacuum cleaner, or a central vacuum with an outdoor exhaust, will not blow significant dust particles back into the room through the exhaust port. Banning smoking in your home will also go a long way to improving air quality.

If a mold problem exists, the best solution is to determine how the mold got started and correct it. Some of these approaches to less-polluted air are quite inexpensive. Others can be costly, but so can an elaborate filter system. The bottom line is that source reduction — removing the cause of pollution — is always the first thing to do.

When an Air Cleaner Is the Next Step

Air cleaners don't all remove the same type of pollutants. If your house has a dust problem, it won't do you very much good to get a cleaner that just removes formaldehyde, or vice versa. Most models actually remove pollutants from the air, while a few processes chemically change the pollutants into something else.

There are basically two different types of air pollutants: particles — more properly called particulates — and gases. Until recent decades, no one was very concerned with gases, but today they are becoming a major problem indoors. Formaldehyde, probably the best known, is found in a wide variety of products ranging from the man-made wood products that kitchen cabinets are often made of, to some permanent press clothing.

There may be over a hundred different gases in a typical new house. These are often lumped together and called VOCs (volatile organic compounds). VOCs are given off by fresh paint, new carpeting, adhesives and cleaning products. Human beings even emit some as a part of their metabolism.

The particulates found in the air include pollen, mold spores, house dust, animal dander and smoke. Tobacco and wood smoke are actually comprised of both gases and particulate matter, neither of which is good to breathe.

Evaluate Manufacturer's Claims


Most air cleaner advertisements talk about how efficient their product is, but you may need to ask the manufacturer which testing method they used. For whole-house systems, an efficiency based on "arrestance" will tell you how well the filter performs at removing fairly large particles, something most filters do reasonably well. In order to judge a filter's efficiency at removing fine particles, you need to know its dust spot efficiency or the results of a DOP smoke penetration test. MERV ratings are useful in comparing different whole-house mechanical filtering products. In general, the higher the MERV rating, the more efficiently the filter initially performs in removing smaller particles. The EPA guidance for indoor air quality in schools, for example, recommends filters between MERV 8 and MERV 13. By comparison, the typical fiberglass throwaway filter used in most homes has a rating of MERV 4 or less. 


Clean Air Delivery Rate (CADR) is a performance measurement for portable room air cleaners first standardized by the Association of Home Appliance Manufacturers (AHAM) in the 1980s. The standard measures how well a given air cleaner removes tobacco smoke, pollen and dust from a room, regardless of the removal method used. The resulting performance numbers help consumers to compare models and to match an air cleaner to the room or rooms in which it will be used. All else being equal, higher CADR numbers mean faster contaminant removal. In addition to CADR performance numbers, the certification label carries a suggested maximum room size for the specific model, based on the floor area of the room in square feet. Using an air cleaner in a room that’s smaller than the suggested floor area will result in faster, better performance, at least when the unit is new. Regardless of room size, AHAM recommends closing all doors and windows in the room while an air cleaner is in use.


Note: The standardized efficiency ratings above only compare particulate removal, they do not measure gaseous removal.


In order to compare air cleaners, it is important to realize that there are several different types. Since all air cleaners have limitations, a discussion of what they can and cannot do is important.

Getting the Right Filter

Particulate filters are usually not designed to remove gases. The standard furnace filter is a low-efficiency particulate filter. Unfortunately, it does an extremely poor job of removing anything. It is only capable of capturing fairly large particles. More than one indoor pollution expert has said, with tongue in cheek, that they are good for handling rocks, baseballs and tennis shoes, but little else. Actually, their main purpose is to protect the furnace's fan motor. They aren't designed to protect the health of the occupants.

Medium-efficiency filters come in a variety of styles for commercial applications, but the pleated, accordion-like style is common in residences. While the standard furnace filter is only an inch thick, these may be up to six inches thick. In order to install the thicker ones in a furnace, some minor modifications to the ductwork are necessary. The reason for the extra thickness is because these types of filters have more air resistance. By making them thicker in a pleated shape, designers have given them more surface area, thus lowering their resistance to air flow. Too much resistance means that the furnace fan motor would have to work too hard, possibly resulting in a burned up motor.

High-efficiency filters, called HEPA, were developed by the Atomic Energy Commission to filter out plutonium particles in their labs. HEPA stands for High Efficiency Particulate Arrestance. They are often used in clean rooms in the electronics industry and in hospitals. They can be added to a residential furnace system, but they are even bigger than medium efficiency filters, and they have more air resistance. Such an installation should only be done by a qualified contractor familiar with HEPA filters, and most residential contractors are not. Often a larger fan motor must be installed to handle the extra load.

Many manufacturers are producing portable air cleaners with HEPA filters. These need only be plugged into an electrical outlet and turned on. Since they are self contained, they are very easy to use, and can be moved from room to room. Portable filters are good for people who are renting because they can be taken with them when they change residences.

A special type of medium efficiency particulate filter that has been around for years is called an electrostatic precipitator, or simply an electronic air cleaner. These can be added to a forced air furnace by most residential heating contractors, and portable models are readily available. As dust-laden air passes through an electrostatic precipitator it is given an electrical charge. The charged dust is then attracted to metal collector plates that have the opposite charge. The analogy isn't quite correct, but it can help to think of the dust as being turned into small magnets. The magnets stick to the metal collector plates and are therefore no longer in the airstream. Electrostatic precipitators work better with smaller particles than large particles, so they need some type of prefilter. In fact, most manufacturers of higher efficiency filters recommend the use of inexpensive prefilters that can be changed regularly to extend the life of the more costly primary filter.

The first three filters described work by a mechanical (as opposed to an electrical) process, much like a coffee strainer. As they get clogged, the pores in the filter get smaller. They then become capable of removing even finer particles, thus they become more efficient as they get clogged. However, they also have more air resistance, so less air is passing through them.

With an electrostatic precipitator, as the collector plates become loaded with dust, the unit loses efficiency quickly. A study in a 1973 issue of Respiratory Care found that efficiency dropped to 20% after only 40 hours of use. When an electrostatic precipitator loses efficiency, the air resistance doesn't increase any, but the dust, even though it is given an electrical charge, simply passes through the unit and back into the house.

An electrostatic precipitator may need to be cleaned as often as every week if very much dust is present, although every month may be a good rule of thumb in most situations. This is done by removing the core containing the collector plates and placing it in a dishwasher, or soaking it in a tub of soapy water. The medium-efficiency mechanical filters and HEPA filters can last as long as six months to a year before they need changing.

Electrostatic precipitators have the disadvantage of producing some ozone, a known pollutant. It is not good to breathe. Ozone is released mainly when the units make their characteristic popping sound. This occurs when electrical arcing occurs inside the unit. Wherever there is a spark, there can be ozone produced, so ozone can also be given off by electric motors. Since this is difficult to eliminate entirely, some manufacturers can supply a small activated carbon filter that can be added to their electrostatic precipitator to handle the ozone. The carbon reacts with the ozone to change it back into breathable oxygen.

The last type of particulate filter uses static electricity to capture dust. Some, called electret or filtrete filters, use permanently charged plastic fibers. They will require replacement about every four months. Others rely on the air moving through the plastic filter to generate the static charge. They are referred to as being "self charging" and they can be cleaned and reused. Filters that use static electricity function somewhat like an electrostatic precipitator, but since they use no 110 volt electricity they don't produce ozone. They probably fall somewhere between low- and medium-efficiency filters in their ability to remove particulates. They are used in some portable room-sized filters and can be purchased in one-inch thicknesses to replace standard furnace filters.

Adsorption (yes, spelled with a "d") is a process by which gases adhere to a filter media. The media is usually activated carbon, but it can also be activated alumina, zeolite or other miscellaneous materials. Activated carbon is the most common, and it works something like a sponge. Through a physical/chemical process, gases will adhere to the carbon's surface. The carbon is usually in granule form, and under a microscope it can be seen as very porous, having many tiny chambers. This gives it a tremendous surface area for gases to adhere to. Standard activated carbon can easily remove dozens of gases from the air, but it isn't particularly good at adsorbing formaldehyde. Specially processed carbons are available that are actually designed to attract formaldehyde, and activated alumina works well at adsorbing it. For industrial applications, carbons can be produced that adsorb specific pollutants like mercury gas or ammonia.

Some manufacturers supply a mixture of standard carbon along with something that will handle formaldehyde. Such a mixture will do a good job in most residential applications.

There are a number of portable filters that contain some type of adsorption material. However, because of the increased air resistance, there are only a few whole-house models that can be used in a residential furnace system. The industrial models can certainly be adapted, but they are quite expensive, and not many residential contractors will be familiar with them.

Some companies are now offering a standard furnace filter that is impregnated with activated carbon. These don't have the air resistance problem, but they don't contain a great deal of carbon either. They may work to polish the air in a relatively clean house, but would probably become loaded quickly if there were a lot of VOCs in the air. A good adsorption filter needs to move the air through at least a half-inch to one-inch thickness of carbon (or other adsorption material). In some industrial applications, the carbon will be several feet thick.

It is often a good idea to have a prefilter in front of an adsorption filter to remove the dust that could coat the granules and reduce their effectiveness. As the adsorption capacity of the filter is reached, it must be replaced because the gases will simply pass through and back into the living space. Usually the granules can simply be dumped out and fresh material inserted in the housing by the homeowner.

It is estimated that a pound of activated carbon will adsorb about a half pound of gas, so the more carbon in a filter, the longer it will last. Most portable filters contain enough material to last between three months and a year, depending on how much gas is present.

Most of the particles floating around in the air have a neutral charge, but a variety of processes can cause them to carry an electrical charge. They are then called ions. There is some scientific data available that has found that some people experience a feeling of wellbeing when in the presence of negatively charged ions. Positively charged ions can cause some people to be depressed, anxious or irritable, in some cases suicidal.

Negative ion generators cannot be advertised for their health promoting effects because of FDA rules, so they are often promoted as air cleaners. In fact, they can clean the air. In a way, they act like an electrostatic precipitator. They place a charge on particles, turning them into ions, and blow them back into the room. The particles are then attracted to oppositely charged surfaces, like walls, ceilings, floors or furnishings. In effect, the house itself becomes a giant collector plate. Unfortunately, when the particles lose their charge, they can be reintroduced into the air. For the sooty particles found in smoke, they may stick permanently to the walls, resulting in a dirty film. Some desktop ionizers have left dirty black deposits on nearby walls.

A few manufacturers are combining a negative ion generator with a filter to capture the charged ions. It would seem that a true electrostatic precipitator would do a better job.

Ozone Generators

As has been mentioned, ozone is an air pollutant. It can cause serious lung damage and is irritating to the eyes and respiratory tract. In spite of its drawbacks, machines that produce ozone are being sold as air cleaners. Actually, ozone can help to purify the air. Since it has a powerful bactericidal action, ozone has been used to kill mold and mildew. It works quite well at this, but it is important to note that someone allergic to mold will react to dead mold as well as to live spores.

Being a highly reactive chemical, ozone also has the capacity to chemically combine with some gases and neutralize them. It does this by attacking carbon atoms, something that people as well as VOCs are composed of.

Manufacturers are quick to point out that their equipment is designed to produce less than .05 parts per million (ppm) of ozone. This is the limit allowed by the FDA for ozone generators labeled as medical devices. Some people can detect the odor of ozone as low as .001 ppm, an extremely low level.

To use such a device indoors, one must consider both the benefits and the risks. Operating an ozone generator when the house is unoccupied, then airing it out well before reentering, may pose little risk. However, with the goal of cleaner air, there may be other ways of accomplishing the same result.

Many people have used these devices and have experienced no problems, but an improperly adjusted device placed in a small room can definitely result in unhealthy levels of ozone. They must be used with extreme care.

Other Filters

Research at NASA has indicated that certain house plants, especially spider plants, will clean the air. This work has received widespread media coverage, but unfortunately it doesn't always work in the real world. NASA scientists injected a given amount of formaldehyde into a sealed chamber containing spider plants. The formaldehyde was soon metabolized by the plants and disappeared from the air. Other scientists point out that a house is much more complicated than a test chamber. Houses contain formaldehyde sources, like particle board, that will outgas for several years.

As a plant starts metabolizing formaldehyde from a continually outgassing source, there is an initial reduction in the formaldehyde content of the air, which causes an increased rate of outgassing. Having plants indoors also means that humidity levels will be higher, and formaldehyde outgasses faster at higher humidity levels. While spider plants can consume formaldehyde, and overall they will result in lower levels in houses, the reduction isn't significant. However, plants can have psychological benefits, and they give off a certain amount of oxygen, so there are still good reasons to have them indoors. Removing formaldehyde just may not be one of the reasons.

The air fresheners sold in grocery stores do not clean the air. They simply add additional scents that either cover up air pollution or deaden your sense of smell. This can give you a false sense of security if you believe that the pollutants have gone away. More importantly, the fragrances released are often actually pollutants themselves whose effect on people has not been well studied.

Many manufacturers offer air cleaning devices that combine two or more different filter types. For individuals with conventional allergies to things like dust mites, pollen or mold, a medium-efficiency filter, either electronic or mechanical, can do a reasonable job. HEPA filters are of course much better, but they can cost more also.

For removing gases, some portable air cleaners are an easier solution, because whole house adsorption filter systems, that are a part of the heating system, are costly.

The very small portable filters that are designed to sit on a desktop are probably not worth considering because they simply have too little capacity. Most room sized units are just that — they will only work well in a single room, usually up to 200 square feet. They function best if left running continuously in a room with the door closed. Similarly, a whole house filter that is a part of a ducted heating system will work best if the furnace fan is allowed to run continuously. Many furnaces have a switch on them to allow for this.

Making the Choice

For chemically sensitive individuals, the selection process can be especially difficult. For example, some people will react negatively to odors from the blower motor or to some forms of carbon. Carbon made from coconut husks may be tolerable, but if made from coal it may cause problems. With mechanical particle filters, synthetic resins are often used to bind the fibers together. While these resins are present only in tiny amounts that have no effect on most people, they can bother some sensitive people. When someone is chemically sensitive, it is always a good idea to obtain a sample of the filter media to test for tolerability.

All of the air cleaners on the market will definitely clean the air. The big question is: How good a job will they do? It is not an easy question to answer. With very bad indoor air, it is doubtful if very many of the models can make a dent in the problem. That is why it is important to remove as many sources of pollution as possible before trying to clean the air. In doing so, you may easily get by with one or two portable cleaners, at a cost of a few hundred dollars, rather than a several-thousand-dollar, furnace-mounted system — although a whole-house system is certainly the optimum.


(Note: The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)


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Air Cleaners and Filters:  Created on January 22nd, 2007.  Last Modified on February 28th, 2011


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