Air Cleaners: The Pros and Cons

Introduction
Three-tiered system for cleaner indoor air
Types of air cleaners:
   Mechanical filtration
   Electronic air cleaners
   Mechanical-electronic hybrid filters
Styles of air cleaners:
   Central filtration
   Console-size units
   Tabletop units
   "Personal" air cleaners
Effectiveness
Ozone generators
What to know if you are thinking about buying an air cleaner
What to do when using an air cleaner
The most important things to know about air cleaners
Other air cleaner-related resources

Introduction

Air cleaners are among the fastest growing household products. Since the early 1990s, sales have tripled, and 14 percent of American households use them. In just 1999 alone, people in the United States spent about $350 million on portable air cleaners in hopes that they could take dust, pollen, tobacco smoke, and pet allergens out of the air.¹ Though some indoor air pollutants can be removed with air cleaners, no air cleaner is designed to remove every type of airborne pollutant. Air cleaning is also never good enough by itself; the sources of pollution should always be controlled first along with good ventilation both in the home and in offices and schools.

Some air cleaners are effective at removing dust and particles from the air. However, most air cleaners do nothing to remove gases or vapors. Typical furnace filters are designed to protect mechanical equipment and are not effective at capturing small particles capable of entering the lungs. People with allergies or asthma may want to upgrade standard furnace filters to higher-efficiency filters or electronic air cleaners. However, before going with a higher-efficiency filter, make sure the furnace has the horsepower to blow through this increased resistance.

Three-Tiered System For Cleaner Indoor Air

In the search for clean indoor air, a three-tiered system is used:

• Control the source of pollutants at their source
  
• Provide adequate ventilation
  
• Clean the air

If all sources of indoor air pollutants are controlled, then there is no need to have air cleaners; controlling the source of the pollutants is obviously the first thing you want to do. However, not all sources can be controlled. Sometimes pollutants get in no matter how hard you try to prevent them. Therefore, the last line of defense is the air cleaner. Because of that, it should always be used in addition to, not in place of, the other steps!

One study found that air cleaners could be beneficial for asthmatic patients with animal allergies who want to keep their pets when air cleaning is used in combination with other measures. This was after a three-month intervention with 20 asthmatic children allergic to animals who had pets at home. They found that the airways of those children who had air cleaners in their living rooms and bedrooms were not as reactive as the airways of those children who had fake air cleaners in their homes.²

Types of Air Cleaners

There are three types of air cleaners: mechanical filters, electronic cleaners, and hybrids that are a combination of these two.

Mechanical filtration: These draw air through a flat, pleated, or high efficiency particulate air filter to trap particles. Some of the purposes of air filters are:

• Protect the ventilation equipment from dust and dirt

• Protect furnishings and décor of the occupied space
  

• Reduce housekeeping and building maintenance
  

• Reduce furnace and heating equipment fire hazards
  

• Protect the general well being of people: in order to do this, a much higher performing filter must be used. It might not always be possible to put high efficiency filters on older ventilation systems so this should be investigated before doing so.

• The efficiency of a filter is the difference between an air cleaner and just an air filter. Air filters are less than 20 percent efficient at trapping or filtering out particles; air cleaners are more than 20 percent efficient. It is important to choose the right type of filter since not all air filters are equal. There are a wide range of filter sizes and thicknesses. The filter found in most home ventilation systems is a flat filter that is one-half inch to one inch thick. Though it is efficient in collecting large particles that might hurt equipment, it will not capture the particles that can be breathed deep into the lungs. Very small particles can be breathed deep into the lungs and cause health problems so high efficiency filters are needed to trap these small particles.

• One of the best ways that a filter's efficiency is increased is by pleating it. Medium-efficiency filters (20 percent to 50 percent efficiency) are usually pleated. This increases the total area available for filtration and extends the life of the filter. These use static electricity created by airflow, but their effectiveness may decline as the static charge decreases over time.

• The most efficient filters are High Efficiency Particulate Air (HEPA) filters. They can remove almost 100 percent (99.97percent) of all particles larger than 0.3 micrometers in diameter. Even though HEPA filters are expensive, they can be maintenance-free up to five years when used with a pre-filter. Most home ventilation systems do not have a powerful enough fan or horsepower to use this type of filter since these filters can be up to 36 inches thick (as compared to the one-half to one inch thickness of most furnace filters). A unit that is powerful enough to blow air through a HEPA filter also uses a lot of energy. Recently filters have appeared that call themselves "HEPA-type" filters that are less efficient (60 percent to 95 percent) but can run on a less powerful ventilation system.
  
  Electronic air cleaners: Electronic air cleaners are effective in removing airborne particles, but not gases. One major drawback to many of them is that they can produce ozone that may irritate the lungs.

• Negative ion generators: These give a charge to airborne particles so that they become attracted to and settle on walls, floors, tabletops, draperies, people, etc. Activity within a room, however, may stir up and release these settled particles back into the air. Some devices have a collector to pull the charged particles back to the unit. If a unit does not have a collector, then this kind of a unit can result in the soiling of walls and other surfaces.
  
  While ion generators may remove small particles (such as those in tobacco smoke) from the indoor air, they do not remove gases or odors. The odor of tobacco smoke is mostly due to gases in the smoke rather than the particles. So even if the particles in tobacco smoke are removed, you may still smell the smoke; and with over 4000 chemicals in tobacco smoke, many pollutants are still probably present. These units may also not do a good job at removing large particles, such as pollen and house dust allergens, since these large particles tend to settle quickly.

• Electrostatic precipitators: These give a charge to airborne particles and trap them on metal plates that are oppositely charged. These plates must be cleaned regularly for these units to be effective since their efficiency declines rapidly as the collector plates become coated with particles. Cleaning the plates should be done every few months, at least. When these units are built into a home or building's ventilation system, there is the advantage of cleaner ventilation equipment resulting in lower energy costs and performance that is more reliable. However, since these units can produce some ozone, you need to make sure that the unit does not emit more than the FDA limit of 0.05 ppm.
  
  As long as the units are the correct size for the room and do not give off ozone, they can be effective. For example, in a study in two Swedish day care centers, the absenteeism rate of the children was recorded over three years. In the second year, electrostatic air cleaners were added. The amount of mold in the air decreased after four to five months and absenteeism rates fell by 55 percent.³
  
  Since many indoor air complaints are related to temperature complaints, one manufacturer came up with an air cleaner that also provides personal heating and cooling. This device uses an electrostatic filter to remove airborne particulates and includes a separate filter for volatile organic compounds (VOCs). The system has been designed to include a range of options such as controls for thermal comfort and lighting. The cooling mechanism operates without the use of CFCs and allows a user to lower the temperature at his or her work station by as much as 10°F or to raise it by as much as 25°F compared to the ambient temperature in the building.⁴
  
  An advantage to these types of units is that there are no long-term filter replacement costs.
  
  The drawbacks are:

• They become less efficient with use

• They require frequent cleaning

• They can produce ozone
  

• They can be expensive if installed into existing ventilation systems
  
  Mechanical-electronic hybrid filters are a combination of the two filters mentioned above. For example, an "electret" filter uses an electrically charged flat or pleated fabric to attract airborne particles.

• Gas phase filters may be attached to air cleaning devices to filter specific gases, vapors, and odors since neither mechanical nor electronic filters can remove gases and odors on their own. These air cleaning units may be equipped with a chemical filter designed to removed pollutant gases from the air; activated charcoal is most widely used. Other substances used in these filters may include silica gel, activated aluminum coated with potassium permanganate (which is fairly effective in capturing formaldehyde from the air), zeolites, porous clay minerals, or molecular sieves. These filter materials may become quickly overloaded and need to be replaced often. As a result, these devices are more expensive to buy and maintain. Gaseous pollutant air cleaners are generally rated in terms of the total amount of a chemical that can be captured.

Styles of Air Cleaners

Like humidifiers, air cleaners also come in styles that are part of the central ventilation system, that are room console units, and that are tabletop units. Here are the advantages and disadvantages of each style:

• Central filtration: These are air cleaners that are built into the home's central heating or air-conditioning system or a building's ventilation system. These are used when the air needs cleaning in an entire house or building. It may be difficult to add these to existing systems since a more powerful fan may be required to move sufficient air through some types of air cleaners. Another drawback is that the ventilation system must be running in order for the air to be cleaned. If you are having one of these systems installed, request one that has low-leakage, is easy to maintain, and is energy efficient. Be sure you go with a company or contractor that is very experienced in designing and installing central filtration systems.

• Console-size units: These are often used when air cleaning is needed in a particular room. They have effective filters or collecting plates. When room units were tested, all of them moved more air then tabletop units. One study even found this type of unit with a HEPA filter effective in removing large pollen and spores.⁵ They usually work better when all the door and windows are closed and are usually the unit of choice if pollutants are confined to particular rooms.

• Tabletop units: These units usually have limited airflow and inefficient filters. Most reviews have shown these units to be ineffective. In a 1200 cubic foot room, 12 units were tested for their effectiveness in removing cigarette smoke. Out of the 12 units tested, 9 of them reduced smoke levels by less than 24 percent compared with a 17 percent reduction when no unit was used at all!

• "Personal" air cleaners: These are units designed to be worn around the neck, on a lapel, or in a pocket. To begin with, they are very dangerous because they emit ozone right at the breathing zone; therefore, they can trigger health problems in people with asthma and others with lung problems. They also are ineffective. When Consumer Reports tested one in a test chamber in January 2000, they said that it "barely reduced" the particle concentration in the surrounding air.¹ In another study by the California Air Resources Board in 1999, they found that both personal air purifiers they studied exceeded the Federal Drug Administration recommended limit of ozone as well as the World Health Organization eight-hour air quality guideline of 60 ppb. They recommended that these devices not be used anywhere near the breathing zone, especially by children and persons with asthma, other respiratory diseases, or allergies.⁶

• Houseplants: Houseplants are generally not effective at removing indoor air pollutants.

Effectiveness

The overall effectiveness of an air cleaner depends on its ability to trap small particles and on the amount of air drawn through it. There are no industry-wide performance standards, but the Association of Home Appliances Manufacturers (AHAM) has developed the Room Air Cleaner Certification Program. This program provides a uniform standard for portable air cleaners called the clean air delivery rate (CADR). The CADR is the amount of clean air measured in cubic feet per minute (cfm) that an air cleaner delivers to a room. AHAM publishes a list of Certified Air Cleaners that meet the standards of the Room Air Cleaners Certification Program. The list contains the CADR ratings for removing dust, tobacco smoke, and pollen by brand name and model number of air cleaners. The CADR ratings vary greatly by the type of pollutant, as well as by brand and model number. For example, if you are concerned about removing pollen from the air (rather than tobacco smoke, for example) then you should choose an air cleaner that is certified by AHAM and has a high CADR for pollen.

Ozone Generators

Some air cleaners (called ozone generators) use an electrical charge to generate ozone in an attempt to clean the air. This has been done for more than 100 years.⁷ Although ozone can be used in reducing odors and pollutants in unoccupied spaces (such as removing smoke odors from homes involved in fires), the levels needed to do this are much too dangerous to breathe. The levels of ozone produced by ozone generators sold as air cleaners are too low to clean the air and too high to be healthy. Scientific research has shown that low levels of ozone will not remove indoor air contaminants.⁷

Before buying any electronic air cleaner, find out if it has been tested for ozone production. Air cleaners that produce ozone are bad for people who suffer from asthma; for children; for the elderly; or for people with emphysema, bronchitis, or other respiratory diseases. Because of that, the Wisconsin Department of Health and Family Services urged people who have chronic respiratory diseases or compromised immune functions to immediately turn off any ozone-generating machine.⁸ Low amounts (above 0.08 ppm) of ozone can cause eye/nose/throat irritation, chest pain, coughing, nausea, throat irritation, and congestion. It may also worsen bronchitis, heart disease, emphysema, and asthma, and can make it harder for the body to fight off respiratory infections. Not only can ozone be toxic to human health, but it can also hurt indoor plants and damage materials such as rubber, electrical wire coatings, and fabrics and artwork containing certain dyes and pigments.

Contrary to the claims of some vendors, no agency of the federal government has approved these devices for use in occupied spaces, and the FDA has set a limit of 0.05 ppm of ozone in indoor air. Even when one follows the manufacturer's instructions, some studies have shown that the ozone concentrations produced by ozone generators can be higher than this 0.05 ppm limit. Because of misleading information being given to people about these machines, the Federal Trade Commission (FTC) has even taken legal action against some manufacturers of ozone generating machines to prohibit them from making unsupported claims about the ability of the machines to clean air or to provide health-related benefits. In 1995, one manufacturer told the FTC they would stop making health claims about their ozone generators, but they continued to make the claims; the FTC sued them in 1997. The case went to court and the manufacturer was found guilty in November 1999 of making unwarranted claims. The manufacturer promised an "intense effort" to modify its sales pitch after that ruling. Within days of the ruling, a representative from that same company sold 11 ozone units to 6 Ohio school districts. Two of these ozone devices were sold to elementary schools after telling them that the machines would help "kill" widespread fungal infestations in their old schools. An injunction was files against them by the FTC in January 2000 to force them again to stop making these claims.⁹

If you are thinking about buying an air cleaner:

• Find out what kind of filter may be the best for you.
  

• Try renting a device and see if any health problems, such as allergies, improve.
  

• Be skeptical of exaggerated claims
  

• Look for the CADR number; a larger CADR is better.
  

• Because larger units are noisier than smaller units are, just buy the size of unit you need for the size of room you need to clean.

When using an air cleaner:

• Install room-size units in rooms where you spend most of your time or have the worst symptoms.
  

• Locate the units as close as possible to any identifiable pollution source
  

• Locate the unit away from doors, windows, and foot traffic, but not close to walls, corners, furniture, or other obstructions. This is so that air may easily reach the air cleaner.
  

• Air cleaners should always be used and maintained according to the manufacturer's instructions.

The most important things to know about air cleaners are:

• They cannot be used by themselves to clean the air. Pollutants have to be controlled at their source, and indoor air has to be well ventilated.
  

• Air cleaners have to be well maintained. Dirty air filters, for example, are not going to help you clean the air.
  

• Buy the right size of unit for the space you want to clean. The airflow should be sufficient to exchange the air in the room five or six times per hour; therefore, the size and efficiency of the filtering device should be determined in part by the size of the room. Do not waste your money buying models that sit on a table or hang around your neck.

• Make sure the unit you buy does not emit more than 0.05 ppm of ozone.

1. Parker-Pope T. Popular air cleaners pull in consumers, but how well do they work? The Wall Street Journal. April 30, 2000.

2. Van der Heide S, Van Aalderen W, Kauffman H, Dubois AE, de Monchy J. Clinical effects of air cleaners in homes of asthmatic children sensitized to pet allergens. J. Allergy Clin Immunol. 1999;104(2 pt 1):447-451.

3. Rosén KG, Richardson G. Would removing indoor air particulates in children's environment reduce rate of absenteeism - A hypothesis. The Science of the Total Environment. 1999;234:87-93.

4. Wagner M. "Personal Best." Interiors. 1993 May:40.

5. Cheng YS, Lu JC, Chen TR. Efficiency of a portable indoor air cleaner in removing pollens and fungal spores. Aerosol Science and Technology. 1998;29:92-101.

6. Phillips TJ, Bloudoff DP, Jenkins PL, Stroud KR. Ozone emissions from a "personal air purifier." Journal of Exposure Analysis and Environmental Epidemiology. 1999;9:594-601.

7. Boeniger MF. Use of ozone generating devices to improve indoor air quality. AIHA Journal. 1995;56:590-8.

8. Derus M. Ozone-making units here draw a warning: State official say devices might aggravate, not relieve, symptoms. Milwaukee Journal Sentinel. March 4, 2000:3D.

9. Stolz M. Injunction filed against supplier of air purifiers. The Plain Dealer. January 24, 2000.

Other Resources

1. The United States Environmental Protection Agency. Ozone Generators that are Sold as Air Cleaners: An Assessment of Effectiveness and Health Consequences [Online]. 2002 May 22 [accessed 2002 May 23]; Available from: http://www.epa.gov/iaq/pubs/ozonegen.html
   

Updated May 24, 2002