Technologies Used To Clean The Air
There are many methods and technologies used to clean the air you breathe. Many terms sound alike and manufacturer specifications and claims can be very confusing and sometimes misleading. We will "clear the air" about these different types of filtration to help you make an informed decision about the type of air cleaner that will work best for your specific need by providing information on each of the several types of filtration and the efficiency level, benefits and drawbacks of each.
Key Factors To Consider:
- Efficiency
- Most often, filters are claimed to be 95%, 99% or 99.97% efficient. The important factor to look for is on what particle size are these ratings for ? A 99% efficient filter may be only 99% efficient on particles 6 microns* in size or larger. By comparison, a 99% filter at perhaps twice the price may be efficient on particles as small as 0.3 microns* in size - almost 20 times smaller than the first filter. Many media filters are rated on Arrestance Efficiency. Arrestance rates efficiency on particulate by weight, not by size. Arrestance efficiency figures are normally very high.
- *One micron = 1 / 24,500th of one inch - for more information, please see
- Air Movement
- To be effective in the space you are trying to clean, the air cleaner you choose should have enough capacity to clean that room at least 2 - 4 times each hour. For more information, please see Determining The Model and Quantity of Air Cleaners for Your Application. Even the most efficient filter does little good in your space if it doesn't move enough air through the filter to clean enough the air in your room to provide a reasonable improvement. For example, a high efficiency air cleaner with a very low cfm rate will not clean much air. The air that it does clean is cleaned very well, unfortunately that still means that not much air is being cleaned in your space. Air movement rate, most often rated in cubic feet per minute (cfm) is an important factor to consider.
- Technology or Filtration Type
- The technology or filtration method used in the air cleaner you are considering is very important based what you want to remove from the air. The following chart explains more about the different technologies and the key benefits and drawbacks of those filtration types. See the definitions of these technologies under the chart below.
| Technology | Efficiency Range | Particle Size in Microns | Key Benefits | Drawbacks |
| Fiberglass Media (example - standard furnace filter) | 3 - 5% | 0.3 - 1 micron | Inexpensive | Very low efficiency |
| Electrostatic Media (no electric charge) | 15 - 30% | 0.3 - 1 micron | Relatively inexpensive
Some brands washable and reusable |
Efficient only on larger particles
Some brands disposable requiring replacement |
| Electret Media (electrically charged media) | 40 - 90% | 0.3 - 1 micron | Relatively high efficiency on larger and medium sized particles | Relatively low efficiency on smaller particulate |
| Medium Efficiency Cotton or Synthetic Media | 35 - 60% | 0.03 - 1 micron | Medium cost makes them a reasonably priced upgrade from standard
media
Relatively low airflow resistance |
Disposable filters require regular replacement |
| High Efficiency Cotton or Synthetic Media | 60 - 95% | 0.03 - 1 micron | Relatively high efficiency on many smaller particulates which may be harmful or trigger allergies | High airflow resistance
Typically expensive replacement filter cost |
| HEPA Media (High Efficiency Particulate Arresting) | 99.97% | 0.3 microns | High efficiency on small particulates which may be harmful or trigger allergies | Expensive filter replacements
High airflow resistance - increases as filter becomes dirty decreasing amount of air drawn through filter to be cleaned |
| Ozone Generator emission of the highly reactive molecute-O3 | (inconclusive scientific testing information) | Ozone is ideal for removing odor after fires-in unoccupied space | No means of particulate collection -
May irritate asthmatics Considered dangerous by E.P.A. and O.S.H.A. |
|
| Ionizer (typically negative ions) | (inconclusive scientific testing information) | 0.3 - 1 micron | Relatively low cost
Efficient on small particulates which may be harmful or trigger allergies |
No means of particulate collection - particles are
molecularly attached to
surfaces and must be cleaned from walls and furnishings
Larger particulate may be reintroduced into breathing air by air disturbances (such as walking through room) |
| Electrostatic Precipitator | (up to) 97% | 0.01 microns | High efficiency on small particulate
Washable and reusable filters - no replacement filter cost |
Filter must be maintained (usually washing) regularly
Efficiency decreases as filter becomes dirty |
Technologies and Filtration Types Defined
Media Filters -
- Media filters, commonly cotton or synthetic woven materials, work by the process of impingement. The fibers of the filter get in the way of particulate in the air being drawn through by the circulating fan. Typically, the higher the efficiency of the media, the higher the "pressure drop", or airflow resistance. Pressure drop is the amount the air being drawn through the filter is slowed by the filter fibers. The tighter the media fibers are woven, the smaller the spaces between the fibers and the smaller particulate it is able to capture. However, the more tightly woven medias with smaller spaces between filter fibers also have much higher pressure drops due to the small spaces in which to allow the air to weave around those fibers and through the filter. HEPA media is very tightly woven - thus it's high efficiency and also high pressure drop. Media filters become more efficient over time as particles clog the spaces between the fibers. Unfortunately, their airflow resistance also increases, decreasing the amount of air being cleaned. Media air filters must be changed regularly to maintain optimum system efficiency.
- Electrostatic media is media that is prone to static charge. Usually made of plastic fiber, these strands generate a static charge as friction is generated by air passing over them. The particulate is caught by impingement and static attraction. Electrostatic media is generally fairly loosely woven giving a low pressure drop, but low efficiency on smaller particulates. Electrostatic media filters must be changed or cleaned regularly to maintain system efficiency.
- The same basis as electrostatic media, but the filter media is supplied with an electrical charge rather than relying on airflow friction to create a charge. These filters obtain a moderate efficiency on smaller particles than the electrostatic media. Electret media filters must be changed or cleaned regularly to maintain system efficiency.
- Ion generators do not utilize filters, but operate on the principle of electrostatic attraction. The unit emits negative ions from a needle-like point usually located near the top of the unit. These negative ions travel through the air, attaching themselves to airborne particulate. The particulate then obtains the negative ion's charge and is attracted to positively charged items or surfaces. Walls and furnishings are inherently positively charged, so the particulate is molecularly attached to these surfaces.
- Ozone generators do not utilize filters. These machines operate by introducing the highly reactive molecule, Ozone (O3) into the air. This process dissipates air borne pollutants comparably the way that bleaching agents are used to sterilize water. The problem is that there is much controversy surrounding the use of these machines as "air cleaners". Ozone is considered a hazardous workplace chemical by O.S.H.A.(U.S. Occupational Safety and Health Administration), and is also considered an air pollutant that requires regulation by the E.P.A. (Environmental Protection Agency. Ozone may also be particularly harmful to people with asthma.
- Electrostatic Precipitators, most commonly referred to as electronic air cleaners, operate on electrostatic attraction to a collecting section usually called a filter cell. The electrostatic filter cell provides a charge to the particulate being drawn through the filter cell by ionizing wires in the front portion of the cell. The collecting plates are charged oppositely and the charged particulate adheres to the collecting surfaces. Electrostatic filters have a very low pressure drop and high efficiency on very small particulate. Electrostatic air cleaners become less efficient as the collecting cell becomes dirty. As the surface of the collecting area becomes heavily coated, the charge to attract the particulate is less effective. The electronic collecting cell must be cleaned regularly to maintain maximum efficiency. For more information, see our How It Works page.
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