air cleaners - high efficiency filter

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  • 5200 Butler Pike, Plymouth Meeting, PA 19462-1298, USA Tel +1 (610) 825-6000 Fax +1 (610) 834-1275 Web E-mail

    UMDNS Information

    This Product Comparison covers the following device term and product code as listed in ECRI Institutes Universal Medical Device Nomenclature System (UMDNS):

    Air Cleaners, Particulate, High-Efficiency-Filter [18-112]

    Air Cleaners, Particulate, High-Efficiency-Filter, Mobile [18-113]

    Air Cleaners, High-Efficiency Filter Scope of this Product Comparison

    This Product Comparison covers high-efficiency-filter air cleaners used in general-use and high-risk

    areas of healthcare facilities. Units that discharge into the general ventilation system for recirculation or

    that solely provide room-air exhaust to the outdoors to create negative pressure are excluded.

    These devices are also called: mobile air filters, medical recirculating air cleaners, negative-pressure

    air filtration systems, high-efficiency particlate air (HEPA) filtration units.


    High-efficiency-filter air cleaners (HEFACs) are

    used to reduce the local area concentration of

    airborne infectious Mycobacterium tuberculosis (TB)

    droplet nuclei. Since 1985, the number of people

    infected with TB has increased significantly,

    particularly in healthcare facilities, homeless

    shelters, and correctional institutions. Long-term

    drug treatment is sometimes successful; however,

    multidrug-resistant strains of TB (MDR-TB) have

    been reported in healthcare facilities, affecting both

    patients and personnel. Further complicating the

    spread of TB in healthcare facilities is the increased number of patients with human immunodeficiency

    virus (HIV). Patients with HIV are more likely than others to contract TB, and certain treatment

    procedures, such as those that induce coughing, can increase the likelihood of nosocomial airborne

    transmission of TB via infectious droplet nuclei.

    One method of dealing with this problem is to institute adequate infection-control programs in

    healthcare facilities, including the use of engineering controls.

    TB is usually transmitted when an infected patient coughs,

    sneezes, or otherwise releases infectious particles into the air.

    Infection can occur when another person inhales the particles

    before they settle or are otherwise removed from the air. After

    release, droplets that are not immediately inhaled begin their

    descent and desicate rapidly. Larger droplets fall to the ground

    or onto another surface before evaporating; other droplets

    evaporate into nuclei of 1 to 5 m in diameter. These droplet

    nuclei settle extremely slowly and can remain airborne on normal

    air currents generated in an occupied space until they are

    removed by some means of ventilation.

    Comprehensive of Biomedical Engineering Site (






  • Air Cleaners, High-Efficiency-Filter, Mobile

    2 2007 ECRI Institute. All Rights Reserved.

    Principles of operation

    HEFACs generally contain one or more of the following components:

    An intake duct

    A HEPA filter or some higher-efficiency filter, such as a super-HEPA or ultra-low-

    penetration air (ULPA) filter

    A prefilter, such as a polyester fiber filter, used to increase the life of the more costly HEPA

    filter by removing large particles before they reach the HEPA filter

    A motor/blower assembly

    An exhaust duct

    A control panel containing an on/off switch, an airflow speed control, and possibly a

    differential pressure gauge (which usually indicates the pressure drop across the HEPA

    filter. This pressure drop can be used as a relative measure to determine whether the filter is

    dirty and in need of replacement) and/or a filter-change indicator (e.g., hour meter, timed

    warning light)

    Essentially, a HEPA filtration system pulls room air into the unit and through the filter to remove

    particles, then exhausts the filtered air back into the room (or to another designated location) at a selected

    flow rate.

    The airflow pattern created by the unit varies with the location of the intake and exhaust and the flow

    velocity. These systems often use either an upflow pattern, with the intake near the bottom of the unit

    and the exhaust near the top, or a downflow pattern, which is the reverse. Several systems contain the

    option for integrated ultraviolet germicidal irradiation (UVGI) lamps and an additional charcoal filter to

    remove odors. Depending on the units design, UVGI lamps are placed either upstream or downstream of

    the HEPA filter.

    Most systems have more than one configuration, allowing them to provide a number of functions,

    such as the following:

    Exhaust of filtered air (either directly to the outdoors or back into the general ventilation


    Recirculation of air, which has been filtered to remove infectious particles, into the room or

    to other areas of the facility (either through existing or prefabricated ducts or directly from

    the units exhaust duct)

    A combination of both exhaust and recirculation (the exhaust option can provide negative

    room pressure, while the recirculation option is intended to remove airborne particles and

    increase the number of room air changes through HEPA-filtered air)

    Depending on the configuration, the system may be wall mounted, ceiling mounted, floor mounted, or

    freestanding on a mobile base.

    The extent to which a room is cleared of infectious particles varies significantly with the volume of the

    room and the airflow of the HEFAC. For a TB isolation room, the Centers for Disease Control and

    Prevention (CDC) recommends the following:

    12 air changes per hour (ACH); 6 ACH for existing rooms

    Air directly exhausted to the outside

    Negative pressure in relation to the surroundings

    The following formula can be used to determine the best air cleaner for a designated room based on

    the rooms volume (expressed in cubic feet) and the manufacturers listed airflow rate (expressed in cubic

    feet per minute):

    ACH (room volume) 60 = cfm

    Comprehensive of Biomedical Engineering Site (






  • Air Cleaners, High-Efficiency-Filter, Mobile

    2007 ECRI Institute. All Rights Reserved 3

    For example, using the CDCs recommended 12 ACH, a 10 x 10 x 12 room (1,200 ft3) will need an air

    cleaner with an airflow rate of at least 240 cfm. A 20 x 30 x 15 room (9,000 ft3) will need an air cleaner

    with an airflow rate of at least 1,800 cfm.

    Some HEFACs have UVGI lamps as an additional method of air cleaning. Although it is controversial

    and is recommended by CDC as a supplemental method only, UVGI is used (1) in ventilation ducts and

    in-room units in a manner similar to the use of HEPA filters or (2) in lamp fixtures for upper-room air


    Reported problems

    Few operational problems have been reported regarding HEFACs; most concerns about the units are

    related to usage. One such concern is the placement of the unit in the room. CDC and ECRI Institute

    caution that the air-cleaning efficacy of these units may vary with room configuration and placement

    within the room relative to ducts and furniture. The airflow patterns created by poorly placed units may

    be more hazardous, pushing contaminated air toward doors leading to corridors or short-circuiting

    airflow, filtering already disinfected air, and leaving rooms with dead spaces that still contain

    contaminated air.

    Proper airflow and negative pressure in a room are crucial factors in maintaining the cleanest air

    possible. Open doors to corridors and bathrooms, open windows, and bathroom exhaust fans can all

    affect the airflow and negative pressure of a room. When the airflow patterns are altered, the risk of

    transmitting infectious particles can increase.

    The noise levels of HEFACs can also present problems. Higher noise levels are associated with a

    higher rate of air flow. Many units can operate below conversation noise levels, but noisy units can

    disturb patients and healthcare workers or mask audible alarms.

    HEFACs with UVGI lamps could expose maintenance personnel to ultraviolet (UV) radiation, which

    causes tissue injury. Some units have a special UV-shielding filter to prevent eye- and skin-damaging

    energy from escaping from the unit. Also, some UVGI lamps generate ozone when new; this can be an

    irritant. UVGI can also damage plastics.

    Purchase considerations

    ECRI Institute recommendations

    Included in the accompanying comparison chart are ECRI Institutes recommendations for minimum

    performance requirements for HEFACs.

    The efficacy of any unit will depend on its airflow rate in relation to the size and layout of the room.

    ECRI Institute recommends that facilities attempt to maintain an ACH of at least 12. However, purchasers

    should be aware that manufacturer-stated airflows are not always accurate; ECRI Institute advises

    facilities to work with ventilation consultants, clean-room certifiers, or industrial hygienists when

    addressing issues of TB infection co


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