“Electret media will drop in efficiency upon use, sometimes by as much as 50 percent,” she said. However, Owen emphasized, electrets become less efficient over time, often significantly so. As a result, the mat can be more porous, causing better airflow and a smaller pressure drop. The charge helps attract airborne particles (rather than just passively impeding them). 2 So-called electret filters use a mat of fibers that are electrostatically charged. We feel the manufacturers’ concerns are a bit overcautious, in part because a recent innovation in air filters permits high MERV ratings (11 to 13) with low pressure drop. An NIH literature review indicates that filters in this range can significantly reduce indoor fine-particulate pollution and alleviate some respiratory ailments. This MERV range, Owen said, “covers most of the stuff that people are likely to breathe in unless they’re really close to the source.” The EPA says filters rated MERV 7 to 13 “are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles.” The higher end of the range, MERV 11 to 13, removes 20-plus to 50-plus percent of the very smallest, 0.3-to-1.0-micron particulates, and filters in this range are especially popular. (For more detail on these models, see the A perfectly good HVAC filter and Two other filters that are also fine sections.) We focused on filters in the MERV 8 to 13 range because they are good to excellent at removing fine particulates (as small as 0.3 to 3.0 microns in diameter, including smoke and exhaust particles and bacteria), plus anything larger.
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