ve seen ads offering filters that are 90% efficient. Are they any good?
While there are hundreds of different pollutants, from a filtration standpoint, they can be placed in two categories—particulates (small particles) and gases. You need a particulate filter to remove particulates and a gas filter to remove gases. There isn’t a standardized way to evaluate gas filters, but there’s more than one way to measure particulate-filter efficiency—and this leads to some misleading advertising and a great deal of confusion. Particulate-filter efficiency is most-often measured with either an Arrestance test or an Atmospheric Spot-Dust test. Both are described in detail in a technical standard called ASHRAE 52-76. The Arrestance test really isn’t very useful because it only measures how well a filter captures large particulates—something all filters do fairly well. For example, a standard 1″-thick furnace filter is about 80% efficient when measured on an Arrestance test. For most residential applications, it’s better to use the Spot-Dust test—on which a standard fur
