Indoor Air Quality
Air Cleaning: A Strategy for Reducing Pollutants in Indoor Air?
The three strategies (in order of effectiveness) for reducing pollutants in indoor air are source control, ventilation, and air cleaning. Source control is generally the most effective strategy to reduce pollutants in indoor air since it eliminates individual sources of pollutants or reduces their emissions. Ventilation brings outside air indoors; however, there are practical limits to the extent of ventilation can be used to reduce airborne pollutants. Costs for heating or cooling incoming air can be significant, and outdoor air itself may contain undesirable levels of contaminants. Air cleaning may serve as a supplement to source control and ventilation; however, the use of air cleaning devices alone cannot assure adequate air quality, particularly where significant sources are present and ventilation is inadequate. Since no air-cleaning system is currently available that will remove all pollutants from indoor air, the use of air cleaners should only be considered as a supplement when the use of other methods to reduce indoor air pollutants are not successful in reducing pollutants to acceptable levels (EPA).
The decision whether or not to use an air cleaner is left to the individual. If the choice is made to use an air cleaner, the American Lung Association suggests choosing one that ensures high efficiency over an extended period of time and does not produce ozone levels above 0.05 ppm. If air cleaners are to be used, they should either contain charged or uncharged fibrous media, or they should conform to FDA regulations for medical devices (ALA). The reviewed data provide little reason to endorse the use of inexpensive tabletop, appliance-type air cleaners, regardless of the technology they employ. In general, high-efficiency particle collection requires larger filters or electronic air cleaners (ALA). If portable room air cleaners are used, they should be cleaned and used according to the manufacturer’s directions.
Types of Air Cleaners Available
Air cleaning devices are manufactured by many companies in the United States and vary widely in design, methods of operation, cost, and level of efficiency. Air cleaners can either be incorporated into the central heating, ventilation, and air-conditioning (HVAC) system or moved from room to room as portable units, such as small table top units or larger portable room consoles.
Table top appliance type units generally have limited airflow and inefficient panel filters. Most reports have shown these units to be relatively ineffective (ALA, 1/24/2000).
Room-size air cleaners are generally utilized when continuous, localized air cleaning is necessary. They can be moved from room to room. Those units utilizing either HEPA filters or electrostatic precipitation reflected the highest efficiency and removal of pollutants (ALA). For maximum effectiveness, all doors and windows of a room being treated by a portable room air cleaner should be closed (American Household Appliance Manufacturers [AHAM]).
Air cleaning can also be accomplished in the central heating or air-conditioning systems of a residence or in an HVAC system. These units are commonly referred to as “in-duct” units, although this term is actually a misnomer since they are not located in the distribution ductwork, but rather in unducted return air grilles or ducted return air plenums. For these types of units to be effective, the HVAC fan must be in constant operation for air cleaning to occur since the airborne contaminants must be captured and carried back to the centralized filter for capture and retention. Generally, residential HVAC systems operate only with the intermittent fan to maintain a comfortable indoor temperature. Several studies suggest that a highly efficient room unit will be more effective at removing pollutants in the room where it is located than a central filtration system (ALA).
Types of Air Cleaning Processes
The technology employed to remove various sized particles and/or gases from the air includes mechanical filters, electronic air cleaners, hybrid filters, gas phase filters, and chemical process.
Mechanical Filters
- Flat filters or panel filters usually contain a low packing density fibrous medium that can be dry or coated with a viscous substance, such as oil, in increase particle adhesion.
- Pleated filters – Pleating of filter media increases the total area available for filtration.
- High Efficiency Particulate Air Filters (HEPA) – HEPA filters are a further extension of extended-surface media filters. A HEPA filter has been traditionally defined as an extended-surface dry-type filter having a minimum particle removal efficiency of 99.97% for all particles of 0.3 micron diameter with higher efficiency for both larger and smaller particles.
Electronic Air Cleaners
Electronic filters, generally marketed as electronic air cleaners and formerly referred to as electrostatic precipitators, employ an electrical field to trap particles. Like mechanical filters, they may be installed in central filtration systems as well as in portable units with fans. The simplest form of electronic air cleaner is the negative ion generator.
Hybrid Filters
These filters incorporate two or more of the filter control technologies discussed above.
Ozone Generators
These air cleaners utilize a chemical modification process instead of mechanical or electronic filters to “clean” the air. The high concentration levels required for contaminant control are in conflict with potential health effects as established by authorities. “Because of the documented negative health impact of ozone, especially for persons with asthma, and the lack of evidence for its ability, at low concentrations, to effectively ‘clean’ the air, the American Lung Association suggests that ozone generators not be used” (ALA).
Tips for Air Cleaner Buyers
If you decide to use an air cleaner, before purchasing, consider the following:
- Efficiency – Look for mechanical filters such as HEPA filters and electronic air cleaners that can effectively trap particles.
- Clean Air Delivery Rate (CADR) – The overall effectiveness of an air cleaning device depends on the amount of air drawn through it in addition to its efficiency.
- Air Discharge Patterns – The flow of air produced by the air cleaning device affects the general effectiveness of the device.
- Ozone Production – The American Lung Association suggests that ozone generators not be used.
- Price – Consider the cost of replacement filters and maintenance in addition to the initial purchase price.
References:
- “Residential Air Cleaning Devices: Types, Effectiveness, and Health Impact,” American Lung Association, http://www.lungusa.org/pub/cleaners/air_clean_toc.html.
- “Residential Air Cleaning Devices—A Summary of Available Information,” Environmental Protection Agency publication, http://www.epa.gov/iedweb00/pubs/residair.html.
