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Air Cleaners

The Asthma Center, Allergic Disease Associates, P.C.
Professional Arts Building
205 North Broad Street, Ste. 300
Philadelphia, PA 19107-1553

Outdoor air pollution is a concern of many Americans, especially those with respiratory diseases like allergies and asthma. However, the quality of our indoor air is of greater concern since most adults spend up to 90% of their time indoors. In many cases, indoor levels of many pollutants and allergens can be higher than outdoor levels.

In fact, the Environmental Protection Agency (EPA) has identified poor indoor air quality as 1 of the top 5 environmental risks to public health in the United States.Indoor air contains as many as 30 million particulates/cubic foot of air, 99% of which are invisible. Indoor pollutants like smoke, chemicals and allergens like dust, pet dander and mold spores commonly trigger respiratory illnesses. In an attempt to improve indoor air quality and reduce exposure to irritants and allergens, central whole house air cleaners or portable room air cleaners are often recommended.

Furnaces and air conditioners are normally equipped with panel air filters. Their main function is to prevent dust and dirt from causing malfunction of equipment in which they are installed. While they do a credible job of removing some pollutants, they remove only the larger particles, which represent only 15% or less of all indoor particles. The effective range of a typical furnace panel air filter drops off when the particle size of the pollutants is less than 10 microns in size (a hair is about 60 microns in thickness.) Particles 10 microns or larger are usually visible with the unaided eye. Particles less than 10 microns in size are visible only with a microscope. Ninety-nine percent of the particles in indoor air are made up of particles less than 1.0 micron. Of note is that particles less than 3 microns can easily be inhaled deep into the lungs. Examples of pollutants entering the lungs include tobacco smoke, viruses, smog and ozone.

There are several inexpensive furnace filters that can be used to improve air filtration.Self-installed 3M Filtrate filters (Ultra 1250 and Micro 1000) are the most efficient (1-800-388-3458 or They have an extended pleated media with electrostatically charged filters that are very efficient in trapping particles less than 1 micron. They should be replaced every 1 to 3 months depending on their condition.

Some air cleaners go far beyond panel filters in their protection of air cooling/heating system. There are several types of air cleaners available. Central units can be placed in the main supply of forced air heating/cooling systems. These units are operative only when the fan, heating, or cooling is on. Portable room units are also available.These units work best when used in a room with windows and doors closed. They are especially helpful in bedrooms and should be on at least 1 hour before going into the bedroom. One type of portable room air cleaner that has been popularized is the small table top unit that pulls air through a small replaceable filter, typically containing activated charcoal or some other odor removing substance. These units, however, have not been shown to be effective in removing airborne particulates.Ionizer air filters are also inefficient air cleaners. They work by electrically charging particles that then stick to walls and other room surfaces. They often make rooms look dirtier. Efficient air cleaners that remove up to 99% of the pollens and mold spores, and up to 90% of the smaller particulate pollutants, rely on either electrostatic precipitation or high efficiency particulate air (HEPA) filtration. These filtration methods do not remove gaseous pollutants like carbon monoxide or odors (unless equipped with charcoal or chemical absorbing media). Dust mites, which are usually not airborne, are poorly filtered through these devices as well.For individuals with respiratory illnesses, HEPA and electrostatic precipitator air cleaners have been shown in several clinical studies to significantly reduce asthma and allergy symptoms and improve sleep quality.

The electrostatic precipitators function in the following manner.Airborne particles are carried into the cleaner and receive a positive electrical charge as they pass through an electrical field. They then enter into a second electrical field which exists between a series of metal plates where the positively charged particles are attracted to and collect on the negative plates. When this filter is on, a ping is heard as dust is collected on the plates .This sound decreases as the collector plates fill up with particles making the filter less efficient. Accordingly, the collector plates must be cleaned periodically by washing the plates. A charcoal filter or a germicidal lamp is sometimes present for the removal of odor as well. Electrostatic air purification systems produce minute amounts of ozone when operating. As long as the resulting ozone concentration is less than 0.1 parts per million, this does not contribute to respiratory difficulties.The ozone odor, however, may be objectionable to some. Electrostatic precipitators can easily be placed centrally into forced air heating/cooling systems with limited restriction on airflow (Honeywell -, Trane - They are also available as less expensive portable room units (Friedrich -, Hoover -, Honeywell Environizer - There are low maintenance costs associated with electrostatic precipitators.

HEPA filters, on the other hand, are nonelectronic filters and are not associated with the production of ozone during filtration. They are also relatively quiet units when in operation. They rank as the most efficient air cleaners available. These filters have an extended surface composed of microscopic glass and synthetic filters woven into a paper-like media and remove 99.97% of particulate matter that is more than 0.3 microns in diameter. They are available for central forced air systems (e.g. Guardian-Plus Whole House HEPA air system -; Spartan HEPA air systems - are several less expensive room units available that are effective (Whirlpool, which is very quiet -, Honeywell Envirocaire - HEPA filters (whole house or portable) in general are more expensive than other air cleaners, since they require powerful fans. They are also associated with higher energy costs than electrostatic precipitators and the HEPA filter often requires periodic replacement.

An air cleaner is effective only when air is circulated through it. Room air cleaners need to be sized appropriately for the dimensions of the room. Generally, an efficient room air cleaner should be able to exchange air at least 5-6 times per hour for a given space. Portable air cleaners should be certified by the Association of Home Appliance Manufacturers (AHAM - Such labeling identifies the air cleaner maximum coverage area and clean air delivery rate (CADR), a measure of cleaning speed. The CADR measures how many cubic feet per minute (CFM) of air is cleared of a certain size particle. The higher the CADR rating the less time the unit needs to remove a given quantity of particles from a specific sized room.CADR for dust particles (small particle size), smoke (medium sized particles), and pollen (large particles) should be over 150 CFM for rooms 12’ x 18’ or smaller and 350 - 450 CFM for 18' x 24' to 20' x 30’. CADR ratings for specific air cleaners are available from AHAM.

A brochure from AHAM also helps select the appropriate CADR ratings for different sized rooms for removal of dust, smoke and pollen.AHAM certified portable air cleaners are marked with CADR data.

Whole house air cleaners may or may not be tested for effectiveness. A minimum efficiency report value (MERV) may give some indication of efficiency. MERV is a number determined by a test method for filter performance designed by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE - This number can be used to compare filters.Higher MERV ratings imply better trapping of small particles and better filtration.

Consumer Report magazine frequently rates whole house and room air cleaners and is worth checking for current recommendations.

Printed with permission from The Asthma Center, copyright 2008