Air Cleaners A Technology Overview

I. Introduction - Air Cleaners

At the core of air cleaner technology there are three techniques employed. These consist of filter systems, chemical adsorption systems, and chemical injection systems. As should be expected there are a number of implementations of each technique. The efficacy of each technique will be discussed as it related to home and office use of these air cleaners.

The following chart provides and overview of technologies and techniques and gives examples of products that employ the associated technology.

II. Air Filters as used in Air Cleaners

The term “air filter” is used here to mean any air cleaner technology that attempts to impede impurities in the air while allowing the cleaned air to return to the environment, here assumed to be a room or office. There are three types of air cleaners that fall into this category. They are HEPA (in which we include ULPA and HEPA-like air cleaners), electrostatic or electronic, and hybrid systems which include some aspects of the previous systems.

II.A. HEPA Air Cleaners

HEPA is an acronym for High Energy Particulate Arresting. These air cleaners were first developed for the Atomic Energy Commission as a way to remove radioactive particles from the environment. Since then they have been commercialized for use in clean rooms used by the semiconductor, hard disk, and pharmaceutical industries. The core to the HEPA air cleaner technology is a paper filter that has very small holes that limit the size of any particle than can pass though. Since the air flow though a single sheet of this filter material would be miniscule the filter paper is folded into a zigzag shape that provides a very high overall surface area and thus a reasonable air flow. In most cases HEPA air cleaners have a throughput that is half or less than half of the system’s unrestrained air flow.

The upside to HEPA air cleaners is that they are effective and they do no harm to people or goods in the environment in which they are used. They filter particles in the proper range to be effective removing pollen and many asthma and allergy causing pollutants. The downside to these air cleaners are their cost to operate. The HEPA elements clog with use and must be replaced and the consumption of electricity is fairly high because there is a 50% reduction in flow at the very heart of the air cleaner system. That being said this is probably the safest air cleaner for those with sever problems. As a note HEPA air cleaners alone are not effective in the removal of non-particle pollutants such as chemicals.

II.B. Electrostatic Air Cleaners

Electrostatic or electronic air cleaners remove particulates by charging them and then attracting the charged particles to a reverse charged plate. A significant electrical field is required in order to cause this filtration technique to work in the short time that the air is in the air cleaner. Typically these are cleaners are found in whole house systems in homes as well as in commercial establishments. As the air enters the air cleaner it passes between plates that impart a charge to any particulate in the air stream. A short distance later set of plates that are charged with the reverse polarity is positioned. Therefore, the particles that are charged by the first set of plates are attracted to the second set of plates.

The upside to the electrostatic air cleaning technique is that the air is able to move the system in a fairly unrestrained manner thus making the fan very efficient relative to a HEPA air cleaner system. In addition, the particulate removal rates are impressive based on third party testing of portable units. There is little downside to this technology. However, there are issues that must be considered. First, with the high voltage fields employed there is risk that over time the air cleaner may start to emit ozone. This is particularly a concern without good maintenance and proper replacement of the activated carbon post filters typically designed into portable units. Also, if large particulates enter the system there may be popping sounds. These are not typical but should be considered prior to using these air cleaners in a bedroom or other quiet place. Lastly the filter plates must be cleaner, usually in a dish washer.

II.C. Hybrid Air Cleaners

There is only one example of this technology which is manufactured by Blueair. In this case the manufacturer successfully attempted to overcome the shortcomings of both the HEPA and Electrostatic technologies. A HEPA-like polymer-based filter is employed. This air cleaner has larger than HEPA though holes and on its own would be ineffective. The upside is that the fan is more efficient then in a normal HEPA system due to the larger though holes. To bring the overall system to a HEPA level of particle efficiency there are electrostatic brushes that charge the particulates prior to entering the HEPA-like filter. Since the particles are charged they tend to be attracted to the filter rather then pass though.

The upside of the technology is that it is more fan efficient then the standard HEPA technology. The downside is perhaps in Blueair’s implementation. The quoted filter life of their filters is 6-months. This is the shortest life we know of in the industry. We are not sure if this is due to the hybrid technology or how it was implemented.

II.D. Ionic Breeze Air Cleaners

The Ionic Breeze technology is the creation of and registered by the Sharper Image firm.  It is essentially electrostatic technology but without the fan. There appear to be a set of corona discharge wires to charge the air and a set of plates to attract the charged particles.

There are two issues with the Ionic Breeze technology.  First it appears that it gives off ozone, please see the dangers of this chemical below.  Secondly, while the air does move though the unit enough to move paper and ribbons (as seen in the TV commercials) it does not move anywhere near the air required to be a suitable air cleaner.

III. Chemical Adsorption

With  reference to home or office air cleaners Chemical Adsorption medias are typically used in conjunction with one of the air filter technologies. Chemical adsorption is generally achieved by placing a media in the air flow and allowing the surface of the media to absorb chemicals from the air stream.  The design problem is that a maximum surface area "wants" to be presented to the air and this indicates small particles.  Small particles however result in a packed air cleaner filter and very low air flow.  This then indicates a larger particle or very small particles adhered to a loosely woven mat. It becomes very hard to meet the need of a high surface are and long life. These design problems are consistent regardless of what media is employed.

III.A. Activated Carbon Air Cleaner Media

Activated carbon is a higher grade of the charcoal that is used to cook steaks on the grill.  The only difference is that the activated carbon is made in a more controlled oxygen environment and it is heated to a higher temperature.  The base of the carbon is a plant product like wood or coconut hulls.  This process opens up pores in the carbon material and gives impurities places to chemically attach.  While activated carbon will attract many organic compounds it is not effective in absorbing chemicals like nitrates and sodium.

III.B. Zeolite Air Cleaner Media

Zeolite can be man made but most are mined from volcanic ash.  It has the capacity to absorb ammonia and other nitrogen based chemicals.  In terms of industrial usage it can also be used to absorb ions like strontium and cesium which are found in radioactive waste.  Of interest, Zeolite has an amazingly broad range of application as far from air cleaners as worm growing and sewage plant media.

III.C. Potassium Permanganate Air Cleaner Media

Potassium Permanganate is a very strong oxidizing agent that is typically used as a reagent in chemistry experiments.  As such it oxidizes many organic and inorganic chemicals with which it comes in contact.  To the best of our knowledge IQAir is the only firm that utilizes this chemical in some of their advanced air cleaners.

IV. Chemical Injection in Air Cleaners

All of the technologies that have been discussed in Sections I. and II. are used  in what might be called passive air cleaners.  In these cases the air is "operated upon" within the confines of the air cleaner cabinet.  The use of chemical injection in air cleaners is a very different matter.  In these cases an active agent is injected into the living environment with the intent of interacting with pollutants and changing them chemically.  The concern is twofold, first the chemistry that results may not be optimal, secondly, humans are often in the environment as well and these agents will also react with them.

IV.A. Ozone Air Cleaners

Ozone air cleaners inject the chemical Ozone into the environment.  Ozone is an oxygen molecule with an extra oxygen atom which is want to loose so that it can return to a stable two atom structure.  When Ozone encounters a chemical it gives up its extra oxygen molecule and thus oxidizes the encountered substance.  While this most likely eliminates odors it also creates oxidized chemicals of unknown values in the environment.  Please see the references below.  Ozone while valuable in the atmosphere is considered to be a pollutant in a human environment.  It has been found to kill small animals and cause negative respiratory impact to those that are sensitive.

IV.B. Negative Ion Air Cleaners

Negative ions are molecules that have been charged with an extra electron.  This is done with a high voltage needle.  When the molecule encounters another element it tends to give up its extra electron.  When the encountered (now charged) molecule comes within proximity to an electrically grounded structure like your wall it sticks via the extra charge.

Negative ion air cleaners are used in the poultry industry to keep the dust down in large coops.  They have been reported to be used in US submarines.  The bottom line is that they are moderately effective as air cleaners but they take the dirt out of the air and put it on the walls of the environment.  This is at best disconcerting.

V. Conclusion

Air cleaners are poorly regulated and seem to be the subject of much non-science and hocus-pocus.  Staying with proven, yet boring, technologies like HEPA and electrostatic combined with adsorption features proves to be the best course of action.