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  • Active Air Cleaning Methods(top)
    Technologies which fall into this category perform the air cleaning operation throughout the open air of the room or facility rather than within the physical enclosure of the air cleaner.

    • Negative Ion Generators(top)
      Most people have experienced the phenomenon of static electricity when rubbing their feet on carpet and obtaining a light electrical shock by touching a metal object.   But most people are not aware that static electrical charges are continuously being produced as the air circulates in a room and rubs against the surfaces of the room such as the walls, ceiling, and floors.  The walls, ceiling, and floor of the room are literally loaded with millions of these electrical charges.  The materials used in modern construction of the walls, ceiling, and floors generally cause these electrical charges to be positive.  What does this have to do with cleaning the air?  Well, like charged particles repel one another.  When a particulate such as smoke, pollen, dust, mold spore, bacterium, or large organic molecule comes in contact with the surfaces of the room, it alsoIN00521A.gif (1899 bytes) becomes positively charged.  The result is that these particles remain dispersed and suspended in the air you breath by static electrical forces.   The particles are unable to precipitate out of the air.  Solution: neutralize the static charge in the room so that these particles may fall out of the air to the floor.  Negative ion generators do just that by emitting into the room large amounts of negatively charged particles which circulate in the room and neutralize the existing positively charged pollutant particles and room surfaces.  The reduction of the positive static electrical forces causes pollutants to fall out of the air resulting in clean fresh air for breathing.  In addition to obtaining clean breathable air it has been shown that there are additional health benefits to balancing the indoor electrical charge.  n Click Here - for more benefits.  Negative ion generators work well but are limited in their ability to remove small pollutant gas molecules from the air.  Even when there is no electrical charge very small particles approaching the size of gas molecules will remain suspended by the constant collisions with other gas molecules called Brownian motion.  The primary drawback to this air cleaning technology is that it usually takes longer to experience an improvement in air quality after activating the machine and particles not only fall to the floor but they have a tendency to migrate toward the surfaces of the room and adhere to them.  This may cause slight discoloration of walls, ceiling, and accessories over a long period of time.  Negative ion generators sometimes use a device to produce the negative ions called an ionizer.  An ionizer will produce negative ions but it also produces ozone as a by-product.  We discuss ozone in the next section.  It is possible to create negative ions without producing ozone.  n Click Here - to see ceiling fans that act as air cleaners (passive and active methods).

    • Ozone Generators(top)
      Ozone, sometimes called activated oxygen, is a form of oxygen with three oxygen atoms instead of two.  It is a very strong oxidizer and is unstable.  Every twenty minutes half the ozone in the air naturally decays resulting in half the original level of ozone. For this reason, when ozone is used for air cleaning it must be continuously produced to maintain a sufficient quantity in the air to achieve a desired air cleaning result.  How does ozone clean the air?  Ozone generators emit ozone into the room where it circulates and chemically reacts with pollutants in the air.  In most cases, the chemical reactions result in end products of oxygen, carbon dioxide, AG00011_.gif (7216 bytes)and water molecules, but there are no guarantees.  It is possible that certain chemical reactions could actually produce toxic substances.  In fact, ozone is toxic and hazardous if improperly used.  The U.S. federal government has set safe limits for exposure to ozone and ozone generator manufacturers have calibrated their ozone generating products at the factories to provide safe exposure levels.  Most consumer oriented ozone generating machines do not have ozone sensors that self regulate the ozone output of the machines because of the excessive cost.  Therefore, manufacturers generally designate that a particular model ozone generator be used in a certain size room so the level of ozone will not exceed safe limits.  If the consumer ignores the manufacturers recommendation and uses the ozone generator in a room smaller than intended by the manufacturer, the consumer could be exposing themselves, family, friends, and employees to hazardous ozone levels.  The use of ozone is a controversial subject and the

      U.S. Environmental Protection Agency has enlisted technical staff at a number of federal agencies to provide scientific data and advice on this subject.  In general, the E.P.A. does not recommend ozone for air cleaning in habitated areas.

      Ozone Levels (ppm)
      0.003 - 0.015 Odor Detecting Range for Humans
      0.005 - 0.01 Heavy forest Country air
      0.02 - 0.05 Outdoor air After a Lightning Storm
      0.02 90% Reduction of Indoor airborne Bacteria
      0.05 U.S. FDA Limit for Medical Devices
      0.04 U.S. OSHA Limit for Indoor Air
      0.12 U.S. EPA Limit for City Air
      1.0 Human Tolerance Level

        In fact, the evidence indicates that ozone has little air cleaning influence when used at safe levels. 

    Ozone is the only air cleaning method which has the ability to attack pollutants which are not airborne.  Ozone is not recommended for people with allergies, asthma, or chemical sensitivities. 

    What is a PPM?
    PPM = Parts per Million
    1 ppm = 1 inch in 16 miles
    1 ppm = 1 minute in 2 years
    1 ppm = 1 penny in $10,000


    • Q1: Are there any detrimental effects of having too much ozone in the home?

      A1: According to the EPA; ( http://es.epa.gov/techinfo/facts/ozone3.html )

      * When inhaled, ozone can damage the lungs.

      * Relatively low amounts of ozone can cause chest pain, coughing, nausea, throat irritation, and congestion. It may also worsen bronchitis, heart disease, emphysema, and asthma.

      * Healthy people as well as those with respiratory problems, experience breathing problems when exposed to ozone.

    • Q2: What are considered safe limits?

      A2: According to the EPA; ( http://www.epa.gov/iaq/pubs/ozonegen.html#TOC )

    Ozone Health Effects and Standards

    Health Effects Risk Factors Health Standards*
    Potential risk of experiencing:

    Decreases in lung function

    Aggravation of asthma

    Throat irritation and cough

    Chest pain and shortness of breath

    Inflammation of lung tissue

    Higher susceptibility to respiratory infection
    Factors expected to increase risk and severity of health effects are:

    Increase in ozone air concentration

    Greater duration of exposure for some health effects

    Activities that raise the breathing rate (e.g., exercise)

    Certain pre-existing lung diseases (e.g., asthma)
    The Food and Drug Administration (FDA) requires ozone output of indoor medical devices to be no more than 0.05 ppm.

    The Occupational Safety and Health Administration (OSHA) requires that workers not be exposed to an average concentration of more than 0.10 ppm for 8 hours.

    The National Institute of Occupational Safety and Health (NIOSH) recommends an upper limit of 0.10 ppm, not to be exceeded at any time.

    The Environmental Protection Agency (EPA)’s National Ambient Air Quality Standard for ozone is a maximum 8 hour average outdoor concentration of 0.08 ppm.

    (* ppm = parts per million)
    • Q3: How does one determine the levels in the home?

      A3: According to the EPA; ( http://www.epa.gov/iaq/pubs/ozonegen.html#why is it difficult to control ozone exposure with an ozone generator )

      The ability to detect ozone by smell varies considerably from person to person, and one’s ability to smell ozone rapidly deteriorates in the presence of ozone. While the smell of ozone may indicate that the concentration is too high, lack of odor does not guarantee that levels are safe.

      At least one manufacturer is offering units with an ozone sensor that turns the ozone generator on and off with the intent of maintaining ozone concentrations in the space below health standards. EPA is currently evaluating the effectiveness and reliability of these sensors, and plans to conduct further research to improve society’s understanding of ozone chemistry indoors. EPA will report its findings as the results of this research become available.

    For more information on ozone see our air quality links page.

    • Choosing the proper number of Active Air Cleaners for your Application(top)
      Generally active air cleanersSO01512_.wmf (10272 bytes) are factory rated for a certain size area (with 8 foot ceilings).  However, when more than one level is involved such as a two story home, it is necessary to have at least one machine on each level.  This is true even though a single machine may be rated high enough to clean the air in the entire area encompassed by both floors of the home.  The actual coverage of active air cleaner technology may vary depending on the degree of pollution, space configuration and internal air flow.

  • Dust Fire and Explosion Hazards(top)
    Under certain conditions dust from materials may be a fire or explosion hazard.


    Minimum Ignition Temperature (Cloud) Degrees C

    Minimum Explosible Concentration mg/litre




    Citrus Peel



    Coffee, instant






    Fumaric acid






    Milk, skimmed






    Wheat flour



    Wood, dust



  • Reduction of Airborne Heavy Metals (top)
    By regulation, a HEPA quality air filtration system must be implemented to protect against airborne heavy metals.

    • Heavy metals generally include:
      • Cadmium
      • Berylium
      • Lead
      • Zinc, usually from galvanized metal
      • Mercury
  • Noise Figures
    The following chart is helpful to compare noise figures for machines listed on this site.





    Threshold of pain


    Siren at 100 feet



    Jet plane at 50 feet




    Auto horn at 3 feet or Rock & Roll bar




    Chain Saw




    Heavy city traffic


    Rotary mower





    Inside a car at 50 m.p.h. or inside a busy office




    Vacuum cleaner




    Normal speech




    Private office


    Living room, suburban area




    Soft whisper




    Bedroom at night


    When comparing noise measurements, be certain that the measurements were all made with the microphone in the exhaust and placed the same distance from the air purifier in each case. 

    Noise measurements made in a regular room will be higher than noise measurements made in a anechoic chamber (special room with material on the walls to absorb sound) because the noise measurements in a regular room will contain a noise component due to reflections of sound from the walls.

    Also, keep in mind that sound level measurements generally do not take into consideration the frequency spectrum of the sound.  It is possible for the noise from a machine which has a lower sound level to be more annoying than the noise of a machine with a higher sound level because of the distribution of the sound frequencies.

    A change of 3dB represents a factor of 2 difference in the level of noise.

  • Chemical, Biological, & Radiological Protective Measures

  • Radon Gas

    Click Here for more information


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Many, but not all models are displayed.  Air Purifiers are available with a variety of electrical options to meet the power standards of most countries.

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