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Air Purification > Product Basics
  Indoor Air
  Air Cleaning
  Purifier Types
  Purification Technologies 
An air purifier, also commonly referred to as an air cleaner, is an appliance which removes impurities from the air in an indoor location. While not all air purifications technologies are created equal, the benefits of the best-of-breed products are unmistakable. Whether you get seasonal allergies, or suffer from asthma, or are simply proactive about your health, employing an air purifier in your home can result in a profound difference in the make-up of the air that you breathe.
Indoor Air
The air that circulates through homes which do not contain an air purifier is not treated for chemicals, pollutants, or other impurities. As a result, the air that is breathed on a regular basis by the residents contains various substances which filter in from the outside as well as from the full range of indoor spaces, including basements, attics, crawl spaces, and so forth.

The air inside most homes is actually up to 10 times dirtier than the air outside. The reason for this surprising fact is that the same tight seals and insulation measures which make the home comfortable and energy efficient also serve to trap the indoor air and reduce circulation, preventing various pollutants generated in the course of daily life from escaping to the outside.

There are many different airborne pollutants and contaminants which are present in the air of many homes. The list begins with airborne particles of all kinds, including pet dander, dust mites, pollen, plant spores, fungi, and tobacco smoke. Then, there are household odors and gases, including cooking odors, kitty litter, pesticides, and aerosols. Then, there are volatile organic compounds, so-called "VOCs", which escape into the air in the process of using such common goods as paints, varnishes, cleaning supplies, disinfectants, and glues. Finally, there are microorganisms, including viruses, bacteria, germs, and mold.

The residents of the home, including the pets, bring in various airborne particles and microorganisms both inside and outside their bodies each time they leave and return. Various impurities and microscopic chemicals settle on skin, clothes, and fur and are then unsettled into the air. Each time a chemical-based household product is used, such as a cleaning solution, paint, wax, powder, bleach, and so forth, VOCs are released into the air. In addition, impurities are constantly seeping into the home from the outside air. Dust and pet dander build up over time. Thus, the air in most homes is in fact far dirtier than the air outside, where these chemicals have the opportunity to dissipate into the atmosphere.

The possible health effects of air which has not been purified are myriad, and some of these can be quite serious. The presence of VOCs can lead to irritation of the eyes, nose, and throat, headaches, and nausea. If present in sufficient quantities, VOCs can lead to permanent damage to the nervous system, as well as carcinogenic effects. VOCs are also known to exacerbate asthma. Microorganisms such as bacteria and viruses can lead to various airborne infections and sickness, ranging from the common cold to more serious conditions. In addition, the presence of airborne particles can exacerbate a broad range of allergies, inflame the sinuses, and make breathing difficult. Numerous studies have shown that contaminants in the air are a contributing factor to a range of respiratory and pulmonary disorders.
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Air Cleaning
The benefits of clean air are most directly the elimination of the various pollutants which can either cause or worsen the health issues described above. Individuals who suffer from asthma and seasonal allergies typically see the most readily apparent benefits of air purifiers, as they find that their condition is no longer exacerbated when they are at home. These individuals will still have to be careful when they venture outside the home. However, given that most people spend the majority of their time outside of work at home, having the benefit of clean air in the home can make a major difference in the quality of life for individuals who suffer from asthma or allergies.

Testing air quality in the home involves using a test kit. Typically, test kits are formulated to test for specific substances, such as mold or certain VOCs. However, certain companies have kits which can be sent to their laboratories for a full analysis of all the different pollutants present in the air, including mold, dust, pollen, dander, bacteria, soot, asbestos, and other VOCs and carcinogens.

The price of such an air quality test kit can vary from around $20 for a single substance to several hundred dollars for a complete rundown. In either case, it is worthwhile to test the air quality, as this will help to determine whether an air purifier is necessary and which type to get. In addition, by testing the air both before and after the installation of an air purifier, it is possible to determine the relative effectiveness of the unit.

Air purifiers work by driving the air in the home or in a particular room through a cleaning mechanism which works to remove various impurities from the air. There are different types of cleaning mechanisms, including both those which use a passive approach such as filtration or adsorption, and an active approach such as charging the air or exposing it to specialized lighting.

Different types of purification mechanisms provide varying levels of effectiveness against different kinds of contaminants. Filtration is effective against larger particles, but not smaller particles. By contrast, adsorption is effective against smaller particles, but not larger ones. Specialized lighting is effective against microorganisms, but not inert compounds. Charging the air is effective against many particle types, but can lead to the release of unhealthy byproducts. Each mechanism has its own benefits and drawbacks, and these are explored in greater detail in the subsequent section dealing with air purifier types.
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Purifier Types
Air purifiers are somewhat unique in that there are not only many different brands, but also many different types, with each claiming to be better than the others. The different kinds of purification systems vary widely in terms of cost, effectiveness, and health effects. Although research is on-going and new information continues to emerge, there is already a great deal that has been established regarding how well each type of air purifier is able to clean the air and what byproducts it releases in the process of doing so.

As mentioned, HEPA stands for high efficiency particulate air and is a filtration based air purifier, meaning that the air is passed through a filter, which is essentially a fine mesh. Any particles that are larger than 0.3 microns are physically unable to pass through this type of filter. As a result an air purifier using HEPA technology is able to remove 99.97% of the particles that are 0.3 microns or larger from the air. Allergens such as pollen, animal dander, mold spores, and dust get trapped. The only drawbacks of an HEPA air purifier is the need to change the filter on a regular basis and the inability to capture pollutants that are smaller than 0.3 microns in size, which includes many viruses and a number of bacteria, as well as most gases and odors. The main benefits of an HEPA air purifier is that it generally works well for larger particles and can be effective in removing allergens from the air.

Activated carbon is another type of filtration technology which is based on the concept of "adsorption" (not to be confused with absorption). Adsorption refers to the property of materials to attach to one another as a result of chemical attraction. When a specialized plate of carbon is "activated", it is treated with oxygen, which creates millions of tiny pores within the carbon. These miniscule pores dotting the surface of the carbon create an enormous surface area. If all the pores in a single pound of activated carbon were flattened out and laid end to end, the resulting surface area would cover more than 100 football fields. The main advantage of activated carbon is that it can remove the tiniest chemicals, including gases, odors, and volatile organic particles from the air. There are several disadvantages, however, including that the carbon filter must be replaced periodically as it will no longer work once saturated, and that activated carbon is not effective for larger particles. For this reason, activated carbon is often coupled with HEPA filters to create a hybrid air purification system that is able to remove both larger and smaller particulates from the air.

Ionic air purifiers do not use filtration, but rather use a charged electrical field to create charged particles which are referred to as ions. These ions are then released into the surrounding air. When the released ions come in contact with a pollutant in the air, the opposing charges between the ion and the pollutant cause the two to bind together and subsequently to cling to a nearby surface, such as a floor, a wall, or a counter. The benefits of ionic air purifiers are that they can be effective against a broad range of pollutants and that they are relatively inexpensive. The drawbacks are that ion generators can produce dirty spots on surfaces, that they do not actually eliminate impurities, but merely weigh them down and pull them out of the air, and that they emit ozone, which can be a lung irritant and lead to serious health risks, particularly for individuals suffering from asthma or other lung disorders.

Electrostatic precipitators utilize the same basic idea as ionic air purifiers, utilizing opposing charges to attract pollutants. However, the key difference is that electrostatic precipitators actually capture impurities rather than simply weighing them down and pushing them against a nearby surface. These systems work by utilizing specialized collector plates. When the air passes through an electrostatic precipitator, any pollutants are charged and immediately forced onto one of the collector plates. These plates can simply be periodically cleaned and then used again and again. Aside from the benefit of removing impurities rather than simply grounding them, electrostatic precipitators share many of the same advantages and disadvantages as ionic air purifiers, including the fact that most models release ozone into the air.

Ultraviolet air purifiers use a specialized UV lamp to kill organic pollutants, such as germs, viruses, bacteria, and other microorganisms. After being exposed to UV light, these organisms' genetic material is altered to the point to where they cannot reproduce. Air purifiers with UV technology are commonly employed in sterile environments such as hospitals, laboratories, clinics, restaurant kitchens, and such. The main advantages of UV air purifiers is that they are effective against any type of germs, including mold and can be particularly beneficial in a household with one or more individuals who are especially prone to sickness. The drawback of UV air purifiers is that they are not effective against many non-organic allergens, as well as chemicals, gases, and odors.

Photocatalytic oxidation (PCO) is the newest technology with respect to air purification. This technology utilizes a combination of short-wave ultraviolet light with a specialized chemical, such as titanium oxide. The combination creates highly reactive electrons which aggressively bind with any impurities in the air and serve to oxidize these impurities. There have been a number of studies suggesting that PCO air purifiers are the most effective all-around air scrubbing systems. The only concern raised was related to the possibility that PCO systems release formaldehyde and acetaldehyde into the air. However, a number of subsequent studies suggested that the latest PCO technology did not create a statistically larger presence of aldehydes in the air. Although they have not been around as long as other alternatives, many experts consider PCO-based air purifiers to be among the best systems available.

Finally, ozone generators are often marketed as air purifiers or air cleaners, but they only serve to generate and release ozone into the air. The impact of ozone is to oxidize many other chemicals, effectively burning up many of the impurities present in the air. However, ozone is also highly dangerous for humans. The only safe use of ozone is in unoccupied spaces. Commercial uses of ozone include the removal of smoke after a fire, mustiness after a flood, and mold after a long decomposition period. Ozone is generally neither safe nor appropriate for residential use and any air purifiers which work on the principle of ozone generation should be avoided. This site does not endorse either ozone generators or air purifiers which produce ozone in larger quantities than .05 ppm.

In addition to each of these individual types of air purifiers, some companies manufacture hybrid systems which combine two or more different purification technologies in a single system. A common example would be a unit which combines an HEPA filter with active carbon technology. While hybrid systems can be even more effective at scrubbing the air, it is important to ensure that such systems do not sacrifice safety for effectiveness, and that their ozone production levels are well within a safety limit of .05 parts per million.
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Purification Technologies
As mentioned, air purification is somewhat unique in that there is no single gold standard with respect to purification technology on which all of the manufacturers agree. Rather, there are multiple competing methodologies utilized in different air purifiers and by different manufacturers. In certain cases, different methodologies are brought together in a single system in order to enhance the system’s overall ability to remove impurities from the air. As a consumer, it is important to understand the different technologies not only from an efficacy standpoint, but also from a safety standpoint.

Filtration is the easiest to understand and, arguably, the safest technology for the removal of pollutants from the air. The basic idea behind filtration is that air moves through either a single filter or a series of filters. As the air moves through, any particles that are not large enough to pass through the fine mesh of the filter will be trapped and removed from the air. In essence, this works the same way as a sieve. When blueberries are washed in a sieve, the water washes over the blueberries and then seeps out through the holes at the bottom of the sieve. However, the blueberries remain inside the sieve because they are too large to pass through the holes. An air filter is like a sieve, except with holes that are much smaller. An HEPA filter, which is the gold standard for air filtration, has holes that are so tiny that only particles that are smaller than 0.3 microns are able to pass through. This effectively removes dust mites (10 - 40 microns), pollen (10 - 100 microns), plant spores (10 - 70 microns), mold, most bacteria (0.3 - 3 microns), fungi (0.5 -5 microns), and almost all pet allergens (0.3 - 100 microns).

Because filtration is a straightforward process that does not alter the nature of the pollutants that it captures, there are no harmful chemical byproducts. It is also fairly inexpensive, requiring little more than the periodic replacement of filters. The only drawback of filtration is that it does not capture particles which are smaller than 0.3 microns in size. As a result, it is not effective against odors, gases, volatile organic compounds, and many types of microorganisms.

Adsorption refers to the adhesion of gas, liquid, and dissolved solid molecules or atoms to a surface. The process of adsorption creates a film of the adsorbate, which is the gas, liquid, or dissolved solid being accumulated upon the surface of the adsorbent. Much like surface tension, adsorption is the result of surface energy, which occurs because the molecules and atoms at the surface of a substance are not wholly surrounded by other molecules and atoms of that substance and therefore are able to create bonds with other substances, meaning adsorbates.

Carbon is an example of a substance which can serve as an effective adsorbent and this property can be further significantly enhanced by creating millions of tiny pores within a block of carbon. When air passes over a surface of activated carbon, a number of the smaller pollutants, such as odors, gases, and VOCs, may "stick" to the carbon as a result of adsorption. This process does not actually remove the pollutants, but rather converts them from gas form to solid form, pulling them from the air and attaching them to the carbon. When aggravated or disturbed by other particles in the air, these pollutants may become dislodged and reenter the air once again, however. In addition, over time the surface of the activated carbon will become increasingly filled with adsorbates and its ability to remove pollutants from the air will be decreased. Finally, many pollutants present in the air are either too large or lack the necessary chemical configuration to be adsorbed, meaning that active carbon will have no impact on their presence in the air. 

Ionization is a process in which electricity is used to generate electrically charged air or gas ions, which subsequently attach to airborne particles. The electrical attraction between the particles and the ions result is a new compound which is often heavier than the original particle and which may fall out of the air onto a nearby surface. In case of ionizers which use electrostatic precipitators, once the ions bind with airborne particles, they are then pulled to a charged plate. Ionization can often lead to the generation of ozone and other oxidants as byproducts. As a result, some ionizers and electrostatic precipitators may not be safe for residential usage. An alternative to traditional ionizers are liquid ionizers which use charged liquid ions to remove particles from the air. The benefit of this technology is that it does not produce ozone or other negative byproducts.

Irradiation is a process in which a substance is exposed to energetic particles or waves of a particular type. In the field of air purification, the exposure is usually to waves in the ultraviolet (UV) light spectrum. These light waves possess the right amount of energy to break organic molecular bonds, causing irreparable cellular and genetic damage to microorganisms such as germs, viruses, bacteria, fungi, and molds. The combination of irradiation with a semi-conducting material, such as titanium dioxide, has the added effect of causing electrons to be emitted which essentially scavenge any free radicals in the air. This combined effect is referred to as photocatalytic oxidation (PCO) and significantly enhances an air purifier's ability to remove pollutants from the air. With PCO, an air purifier is able to remove both microorganisms and VCOs at much higher levels than using UV light alone.

Oxidation is a process in which an oxidizing agent combines with a reducing agent, which causes both substances to be chemically altered. In air purification technology, oxidation is used by ozone generators as a way of removing airborne particles. Ozone is a strong oxidant gas which can oxidize many other chemicals and therefore is able to neutralize many pollutants present in the air. However, ozone has a major drawback in that it is not safe for humans to breathe in any significant quantities. In addition, the release of ozone into the air will not eliminate all pollutants which are present and its effects can be somewhat haphazard. Both government and consumer organizations have come out against ozone producing air purifiers and our recommendation is to avoid them at all costs.
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