Three strategies (in order of effectiveness) that may be used to reduce indoor air pollutants are source control [stopping pollution at the source], ventilation and air cleaning. Air cleaning may achieve an additional reduction in the levels of certain pollutants when source control and ventilation do not result in acceptable pollutant concentrations. However, air cleaning alone cannot be expected to adequately remove all of the pollutants present in the typical indoor air environment.
When you choose air cleaning
Air cleaners are usually classified by the method employed for removing particles of various sizes from the air. There are three general types of air cleaners on the market: mechanical filters, electronic air cleaners and ion generators. Hybrid units, using two or more of these removal methods, are also available. Air cleaners may be in-duct units (installed in the central heating and/or air-conditioning system) or stand-alone portable units.
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The effectiveness of air cleaners in removing pollutants from the air is a function of both the efficiency of the device itself (e.g., the percentage of the pollutant removed as it goes through the device) and the amount of air handled by the device. A product of these two factors (for a given pollutant) is expressed as the unit's clean air delivery rate (CADR).
Consider the options
Portable air cleaners vary in size and effectiveness in pollutant reduction capabilities. They range from relatively ineffective table-top units to larger, more powerful console units. In general, units containing either electrostatic precipitators, negative ion generators, or pleated filters, and hybrid units containing combinations of these mechanisms, are more effective than flat filter units in removing tobacco smoke particles. Effectiveness within these classes varies widely, however. For removal of larger dust particles, negative ion generators, without additional particle capture mechanisms (e.g., filters), may perform poorly.
Identify the enemy
Pollutants in indoor air may be divided, for convenience, into three groups: particles, gaseous pollutants, and radon and its [byproducts]. Some air cleaners, under the right conditions, can effectively remove small particles which are suspended in air. However, controversy exists as to the efficacy of air cleaners in removing larger particles such as pollen and house dust allergens, which rapidly settle from indoor air ... in the case of pollen and house dust allergens ... natural settling may be so rapid that air cleaners contribute little additional effect. Animal dander may settle more slowly although, again, the surface reservoir far exceeds the amount in the air. Furthermore, control of the sources of allergens and, where allergens do not originate outdoors, ventilation should be stressed as the primary means of reducing allergic reactions.
Some of the air cleaners containing sorbents may also remove some of the gaseous pollutants in indoor air. However, no air-cleaning systems are expected to totally eliminate all hazards from gaseous pollutants and these systems may have a limited lifetime before replacement is necessary. In addition, air cleaning may not be effective in reducing the risks of lung cancer due to radon.
Before you buy
In choosing an air cleaner, several factors should be considered. These include:
- The potential effectiveness of the device under the conditions it will be used.
- The need for routine maintenance, including cleaning and replacement of filters and sorbents.
- The estimated capital and maintenance cost.
- The installation requirements (e.g., power, access).
- The manufacturer's recommended operating procedures.
- The possible production or redispersal of pollutants, such as ozone, particles, formaldehyde, and trapped gaseous pollutants.
- The inability of air cleaners designed for particle removal to control gases and some odors, such as those from tobacco smoke.
- Possible health effects from charged particles produced by ion generators.
- Possible soiling of surfaces by charged particles produced by ion generators.
- The noise level at the air flow rates that will be used.
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