The average person spends 90% of the time indoors, where air pollution levels can be more than 10 times worse than outdoor air. With the amount of information on indoor air quality (IAQ) available today, especially via the Internet, the professional HVAC technician must continue to stay on top of the subject.

Improving IAQ involves three separate and distinct strategies: source control, ventilation, and air cleaning. Today we will discuss air cleaning.


Pollutants can be divided into two categories: particulates and gaseous. The following are examples of each.


  • Animal dander;
  • Mold spores;
  • Fungi;
  • Pollen;
  • Insect parts and feces;
  • Bacteria; and
  • Viruses.


  • Carbon monoxide;
  • Carbon dioxide;
  • Cigarette smoke;
  • VOCs (volatile organic compounds) — cleaning supplies, adhesives, paints, varnishes, personal care products, and building materials.


    Mechanical filters — Often referred to as throwaway filters, they are made of a cardboard frame containing fiberglass material. A 1-inch filter is most common in residential applications.

    This type of filter media is also available in rolls and can be cut to size. Metal frames within the air handler or furnace typically house the throwaway filter. Permanent or washable filters are also in this low-efficiency class. Construction consists of a metal frame surrounding metal baffles.

    A higher-efficiency mechanical filter is the extended surface or pleated type. This filter can remove smaller particulates from the air due to an increased filter surface area and denser media composition. As more dirt and particulate become entrapped (loaded), this filter actually increases in efficiency. Mechanical filters are housed in an air handler or furnace, usually on the return air side of the system. Continuous air cleaning is accomplished by setting the thermostat to the fan “on” position.

    HEPA (high efficiency particulate air) filters — Rated at 99.97% efficiency at removing particles down to 0.3 microns, this filter is commonly used in hospital operating rooms and cleanrooms. Due to the higher density media, HEPA filtration requires a more powerful motor for air circulation. It also is available in self-contained systems.

    Electronic air cleaners — As air passes through system ionizing wires, particulates receive a positive electrical charge. Next, the air passes through the collector area, where alternate parallel plates are charged negatively and positively. The charged particles collect on the plates that have the opposite electrical charge.

    Electronic air cleaners typically have a higher efficiency rating than mechanical filters if the systems are maintained and cleaned according to the manufacturer’s recommendations. These units can be mounted within the ductwork attached to furnaces and air handlers or can be self-contained, stand-alone systems.

    UV (ultraviolet) light — A UV air-cleaning system uses the frequency of light to kill or arrest airborne contaminants. Lamp location within the system or ductwork, air velocity, and temperature are all determining factors in UV light effectiveness.


    Particulate size is measured in microns: 1 micron equals one-millionth of one meter. (See Table 1 for common particulates and their respective sizes.) The following are three tests typically used to determine air cleaner (filter) efficiency.

    1. Atmospheric dust spot method: Measures the staining effect of airborne dirt on clean paper. Air samples are taken both up and downstream of the cleaner. As the air passes through the cleaner, its soiling power is reduced and defined as a percentage of effectiveness.

    2. The weight method: Measures the weight of dust removed by the cleaner as a percentage of the weight of the dust prior to filtering.

    3. Military Standard 282: The chemical dioctyl phthalate (DOP) is uniformly vaporized into particles the size of 0.3 micron. Through microscopic analysis of the sampling, the actual percentage passing through the cleaner can be accurately measured.

    Rothacker is a director of For questions or comments on the Tech Page, contact Rothacker at (e-mail).

    Publication date: 08/19/2002