Hepa filters are an example of mechanical filtration. The advantage to this filtration method is there is no reduction in its efficiency at removing the particulates from the air over time. In fact, a Hepa filter will become more efficient as it collects more particulates, because it is so fine. The only reason you need to eventually change it is because it will start to effect the airflow through the air purifier, which will result in a loss of air cleaning capacity and also may damage the motor because of the heat that may buildup. The disadvantage to this kind of filtration is that these air purifiers tend to be noisy, especially on the higher speeds. Their filters are replaceable and can be rather expensive, especially on the more efficient air purifiers which remove more from the air over a shorter period of time.

The other types of air purifiers are called electronic air purifiers and they utilize a single or combination of technologies to clean the air in a room. An electronic air purifier works by charging the particulates upon entering the machine by an ionizer. This is a wiring mechanism that creates an electrical charge; with the outcome being the particulates are negatively charged.

There are four different forms of electronic air cleaning:

1. The first is called an electrostatic precipitator, which consists of a series of metal collection plates that are oppositely charged to the ionized particles. Therefore, the particles will be attracted to these metal plates like a magnet with the net result being the ability to collect particles as small as 0.1 of a micron. There is a drawback to this type of air purifier, which is as the collection plates collect the particles, and their surface area becomes covered by the particulates, their ability to attract the particles will decrease until the plates are taken out and cleaned. In a good unit you may start out at an efficiency of 80% and degrade down to as little as 20%. Thus, it is imperative you clean the metal collection plates on a regular basis as frequently as daily to weekly. These types of air purifiers will generate a fair amount of ozone definitely enough for you to smell and may be annoying to some people. Most of these machines generate safe levels of ozone.
   
2. The second type of electronic air purifier uses a media charged filter. This means it has a electrostatic filter that will collect the particles by being oppositely charged to the ionized particles and therefore able to collect particles again as small as 0.1 of a micron. This filter will start out at a high efficiency as much as 80 to 90 percent but will decrease as the surface area of the filter becomes clogged with particulates. The biggest decrease will be in the smaller particles, because remember this is not a Hepa filter and it is not able to mechanically remove the microscopic particles, because it is not as fine and thick a filter. It will however maintain its ability to filter out the larger particulates better than the electrostatic precipitator because its filter media is small enough to collect the larger particles without having to rely on the opposite charging attraction needed for the smaller particulates. However, you will still have a decrease in the efficiency to as low as 50% if the filter needs replacing and it is not changed.
   
3. The third type of electronic air purifier is the negative ionizer. This machine attempts to clean the room by basically eliminating the particulates from the air. The particles are not removed from the room just from the air. The way it works is the particles are drawn into the machine where they are ionized and negatively charged. From there they are sent back into the room where they are attracted to the positively charged walls, furniture, positive and neutrally charged particulates where they collide with them until they are heavy enough to fall to the floor. The result is that the air becomes free of particulates and the particle count is reduced. The drawback to this type of air purifier is that all of your particles land up on the floor, walls, and furniture. As the particles lose their charge they will release themselves from the walls falling back into the air. Also, unless you are regularly vacuuming and dusting, the particles will be kicked back into the air when any air movement occurs by a door being opened or someone walking in the room. A major concern is that if an ionized particle is inhaled, it is more likely to stick to the walls of your respiratory system thus increasing the chance of an allergic reaction. The advantage of these kinds of air purifiers is that they will be quiet because there is no resistance to air flow and there are no filters .Also the tendency will be to use them for treating a large amount of air, because if a smaller, quieter fan is used, it will still result in a large amount of air being circulated through the machine. Also, there are no expensive filters to replace such as with Hepa filters and no pre-filters that need to be cleaned. Still, we tend not to recommend negative ionizers except if they are used in conjunction with other air cleaner technology.
   
4. The fourth and definitely the most controversial air purifier is an ozone machine. Its whole premise is based on the use of ozone to clean the air but its claims are very suspect. The ozone machine manufacturers claim the air is cleaned and the gases, chemicals, and odors are removed from the air. However, the ozone reacts with the chemical and it breaks it down into another form of pollutant. The question is whether this new form of pollutant is even worse than the original chemicals. Also, to be effective, it must emit high levels of ozone which have been determined to be detrimental to the respiratory system. Therefore, you have a dilemma with this machine, which is to run it at safe low levels and have little effect, or on high levels, where the concentrations could be dangerous to humans. In addition, the question of particulates in the air is not even addressed, since there are no filters to trap the particles. You may be familiar with their use in the hotels where to prepare a room for a customer, the machine is put in place and the machine turned on high. The windows and doors are closed and the room is left unoccupied while the machine is in operation, which is usually to remove the smoke odors. Once the room is cleaned hotel staff will go into the room, turn off the machine, and open the windows to air the room out and rid the room of the ozone. Ozone is a concern to human health and this has been well documented by the EPA.

There are also air purifiers that use a combination of a pre-filter to collect the large particulates, and a UV light. These air purifiers will emit ozone into the room, which reacts with the gases and odors in the room and chemically breaks them down into another form.

In conclusion, the main advantages to electrostatic precipitators are there are no expensive filters to replace and they are quieter than Hepa filters. The disadvantages are, ozone is emitted and there is a steady reduction in efficiency as the metal plates become dirty, so regular cleaning is imperative.

The main benefit of media charged filter electronic air purifiers are that they will be quiet and clean a large amount of air. The disadvantages are the cost of the replacement filters and there will be a gradual efficiency drop as the filter becomes dirty.

The main advantage to negative ionizers are that there are no filters to clean or change, and they will be quiet. The disadvantage is that the particles are not actually removed from the room, and can work themselves back into the, thus re-polluting the room caused by movement such as a door opening and closing or people moving about. Also, negative ionizers create a good amount of ozone, so the odor and human reaction either to the odor or toxicity needs to be addressed. The advantages to the fourth type of electronic air purifier, the ozone machine, is debatable. They can be dangerous and their air cleaning ability is suspect. However, for smoke odor removal after smoking or a fire they are the air purifier of choice because of the low cost.

With all these types of technologies out there the only ones we would consider would be the media charged filter type such as the Blueair, which uses the ionizer to charge the incoming particulates so the particulates will be attracted to the electrostatic filters. There is no ozone emitted by this air purifier. Another one we would consider would be an electrostatic precipitator as in the Friedrich C90A, which uses the ionizer to charge the particulates, which will be attracted to a series of metal collection plates. It will have a pre-filter to collect the larger particulates and a post filter with some carbon impregnated throughout to remove gases and odors.

An AirFree Sterilizer works by incinerating the particulates at high heat at 400 to 600 degrees. It is effective at removing airborne particulates of size 1 micron and smaller. This would include germs, bacteria, mold, viruses, pet dander, smoke, and house dust mite allergens. Keep in mind, this is not something you would want to use to remove dusts, pollens, and other larger particulates. Also, since it does not use a fan it is not able to circulate the air in a room and attain the proper number of air changes to clean the air quickly of everything in a room. The Airfree sterilizer works by slowly over a 1- 3 week period removing the airborne bacteria, viruses, molds, etc. as stated above and it continued operation will maintain the sterilized environment. It is therefore an ideal machine for virus and spore elimination.

Its most effective use is to prevent mold from reoccurring in the future. Each unit will sterilize an area of 250 square feet with an average ceiling height of 8 feet so it will control an area of 2000 cubic feet. One of the three things necessary for mold to grow is the presence of air born mold spores. Normally, there will always be an abundance of these because they are in great supply in the outdoors and some of them will always circulate into the inside of your house. However, the use of the Air free Sterilizer will eliminate them so mold formation will become very difficult.

The heated sterilization process used by the Airfree Sterilizer is intriguing, and backed up by an unusual number or laboratory studies as to its effectiveness. Particularly interesting about this technology is the measurement of the residual level of contaminants in a room after the testing period which can be as low as 85% reduction from its original levels which has exceeded our results in testing of hepa filters. What is difficult in all the study results we have seen from any manufacturer is the rate of recontamination which is a factor in real life. If you live in a major city, your environment is constantly being recontaminated at a pretty fast clip so it is difficult to ever get completely “pure air” even with large filters going at full speed.