Filters and other particle air cleaners are used extensively in buildings to remove particles from incoming outdoor air and from recirculated indoor air. Historically, filters were installed to reduce the accumulation of deposited particles on HVAC equipment which diminished air flow rates and impeded heat transfer. Within the last two decades, the potential benefits to health have been increasingly recognized as a primary purpose of filtration. Other potential benefits of filtration include reducing unsightly soiling of indoor surfaces and reducing the deposition and accumulation of organic matter on surfaces such as HVAC ducts where it can become odorous or provide a substrate for microbiological colonization.

In commercial buildings, filters are typically installed in the supply airstreams of forced-air heating, ventilating, and air conditioning (HVAC) systems that provide heated or cooled air to occupied spaces. In U.S. commercial buildings, this supply airstream is usually a mixture of outdoor air and recirculated indoor air, while in many European commercial buildings the supply airstream is often entirely outdoor air. In U.S. residences, filters are commonly installed in the recirculated airstreams of forced-air heating and cooling systems. Stand-alone, e.g., portable, fan-filters units are also relatively common, particularly in residences.

Filtration of indoor pollutants is dependent upon the pollutant and the filtration system. Sources of indoor particles such as cat dander, dust mites, environmental tobacco smoke and outdoor particles brought inside, all have size distributions. Likewise filters have a distribution of collection efficiency that is size dependent. Thus, one filter may work well at reducing one type of pollutant source but be poor at reducing another source. Below are some examples of pollutant source distributions and filter size distributions.

Figure 1. Measured particle size distribution of mainstream and sidestream tobacco smoke.
 

Figure 2. Particle removal efficiencies used in the calculations. (LBNL-47833)
 

Figure 3. The result of adding high efficiency filtration in a real building. (LBNL-43835)