Air filters cannot control humidity, or bring fresh air and oxygen indoors, so they are not a substitute for a ventilation system. As a rule, effectively dealing with air pollutants is a multifaceted approach involving source control, isolation of pollutants from people, and ventilation. The U.S. EPA expresses the steps to indoor air quality in order of importance as:
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2. Ventilation
3. Air Cleaning or Filtration
The greatest benefit, especially for people with allergic respiratory diseases, generally comes from source control (e.g., keeping allergy and asthma triggers out of the indoor air by eliminating their sources) (Nelson, Harold). Still, filters can help improve air quality, so incorporating one into a house can have certain benefits. Unfortunately, many people believe an air filter will solve all their indoor air quality problems. Not true. Filters definitely aren’t a cure-all, but they can help, especially when used in combination with other pollution-reduction strategies such as source control and ventilation.
Virtually all air filters will remove something from the air, but not all filters are created equal; some are better than others at removing pollutants. Different types of filters remove different types of contaminants.
To understand what a filter can and cannot do, it is helpful to place air pollutants into two broad categories: particles (more correctly called particulates) and gases. Particulates are very small solid (or liquid) substances that are light enough to float in the air. Typical particulates include mold spores, pollen, house dust, dead insect parts, dust mite feces, animal dander, fibers from clothing and furnishings, fragments of larger materials, and dirt. In urban areas, minute bits of tire rubber and asbestos fibers from brake linings are common. Gases include combustion gases, by-products of human and animal metabolism, formaldehyde, and hundreds of other volatile organic compounds (VOCs) floating in the air. Tobacco and wood smoke are composed of both gases and particulates.
Filters that remove gases often do almost nothing to remove particulates, and vice versa. You need two types of filters to remove both categories of pollutants: a particulate filter for particulates and an adsorption filter for gases. Note that adsorption is spelled with a “d” because it is derived from the word adhere. Adsorption filters for gases are used very infrequently due to the difficulties in monitoring their effectiveness as they load and subsequent service costs associated with filter replacement.
Of course, virtually all filters have a limited life and must be maintained or replaced periodically. If not, they can cause excessive resistance to airflow and an increase in electricity consumption. (The use of any filter generally results in some energy usage—either to operate, as with an electrostatic precipitator, or in fan energy to overcome the resistance to airflow.)
Remember that choosing the right air filter is only part of the solution to protecting indoor air quality. Removing sources of pollution and bringing in the right amount of fresh air continually with a mechanical ventilation system is the foundation upon which to build optimum indoor air quality, enabling air filters to do their job best.
Mechanical ventilation is a key part of the indoor-clean-air equation because gaseous and particle pollutants tend to become concentrated indoors (even when using air cleaners), oxygen is depleted through normal human activities, and indoor moisture levels can rise or fall unacceptably if not controlled. Exhaust ventilation can be used to get rid of pollutants by pulling them out of the house while supply ventilation will provide outdoor air to dilute the pollutants. Exhaust-only ventilation will still pull in outdoor air to replace the exhausted air while supply-only ventilation will cause indoor air to be “pushed” out of the building - the building will always leak some (and won’t collapse or explode). Balanced ventilation systems, that may include heat or energy recovery, deal with both exhaust and fresh air supply. Exhausting stale air and bringing in fresh air, while promoting effective moisture exchange or control, often solves the biggest part of the problem.
For help in choosing the right ventilation system to work in harmony with filtration, visit the Home Ventilating Institute (HVI) website at www.hvi.org.
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