Types of Available Air Cleaners

Air cleaning devices are manufactured by many companies in the United States and they 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 Units
Until recently, small, inexpensive, table top appliance-type units have typically been quite popular in terms of unit sales (Clean Air Device Manufacturers Association [CADM], spoken communication). They generally contain small panels of dry, loosely packed, low-density fiber filters upstream of a high-velocity fan. Table top units may also consist of a fan and an electronic or other type of filter. Small table top units generally have limited airflow and inefficient panel filters. Most reviews have shown these table top units to be relatively ineffective (Consumer Reports, 1992 and Fox, 1994). The combination of low filter efficiency and low airflow in these units causes them to provide essentially no cleaning when assessed for impact on the air of the entire room (Nelson, et al., 1988).
Performance tests of 12 table top units were reported by Rodale's New Shelter magazine (1982). Tests were conducted in a 1200 cu.ft. room in which cigarette smoke was mechanically generated. Nine of 12 units reduced smoke levels by less than 24%, compared to a 17% decline that occurred by settling in the absence of the air cleaners. The researchers concluded that effectiveness of table top units in removing smoke particles was marginal at best and differed little from using no device at all. Similar results were reported by Offerman, et al. (1985).
Consumer Reports magazine (1992) also tested nine table top units and found that, because they can move only small amounts of air, they suffice only for a very small room or a portion of a room. Two of the table top units tested were ozone generators. Both of these units were judged "not acceptable" because they produced harmful levels of ozone and did not have automatic controls to limit ozone output.
Room Units
Another major type of residential air cleaner is the larger, yet portable, devices designed to clean the air in a specific size room. Due to their larger and more effective filters or collecting plates, these larger portable room air cleaners are considerably more effective in cleaning the air in a room than the table top units (oonsumer Reports, 1992; Fox, 1994) and have become increasingly popular in the past several years, exceeding the table top units in consumer sales (CADM, spoken communication). Room-size air cleaners are generally utilized when continuous, localized air cleaning is necessary (Consumer Reports, 1992 and Fox, 1994). Most units may be moved from room to room to reduce pollutant concentration levels as needed. Similarly to the table top units, room units also incorporate a variety of air cleaning technologies.
Rodale's New Shelter (1982) reported significantly more effective performances for 15 larger HEPA and/or electronic portable air cleaners. The time required to reduce smoke levels by 95% varied from 26 to 120 minutes.
Offerman, et al. (1985) tested a variety of portable air cleaners using tobacco smoke as a source of particles. The test protocol involved turning cleaners on for a three-to-five hour period following a period of decay and mixing followed by a 6-8 hour period of natural decay. The highest efficiency based on clean air delivery rate (CADR) was observed in the room unit utilizing a HEPA filter.
Room units tested by Consumer Reports (1992) all moved more air than table top units. The highest efficiency for smoke and dust removal was observed in a room unit utilizing electrostatic precipitation.
Portable room air cleaners are much more effective in rooms where all doors and windows are closed (AHAM).
Central Filtration Systems
Air cleaning systems can also be installed 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. The term "central filtration system" will be used in this document. This type of unit provides building-wide air cleaning and, by continuously recirculating building air through the unit, can potentially clean the air throughout the entire air handling system, duct work and rooms. However, with these types of units, 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. Thus, central filtration systems must be operated with the fan "on" for constant air movement through the HVAC system. Generally, residential HVAC systems operate only on intermittent fan to maintain a comfortable indoor temperature.