This review of 85 farms found that electrostatically charged synthetic media filters showed significant efficiency loss in service, while fiberglass media held its efficiency far longer. The recommendation drawn from it was fiberglass media at MERV 15 or MERV 16.
It is the field counterpart to the laboratory result reported by Raynor and Chae in 2004, where electret filters fell from 85 percent to 45 percent efficiency in service with no accompanying change in pressure drop, and a cigarette smoke study showed a fall from 92.5 percent to 33.3 percent. The unchanged pressure drop is the part that makes this a field problem rather than a specification problem. A filter that loses efficiency while its resistance stays constant gives the operator no indication that anything has changed. The classification standards entry in this library notes that ISO 16890 conditions filters in isopropanol vapour specifically to strip the electrostatic contribution before rating, which is to say the standard was written around this failure mode.
For a producer, the practical consequence is that a rating obtained on a new filter and the protection delivered by that filter in month eighteen are two different quantities, and the gap between them depends on how the media achieves its rating. Media that captures mechanically continues to capture as it loads. Media that captures by charge depends on that charge surviving contact with barn air. Where the media type is not stated on a quotation, it is worth asking, because the specification alone does not distinguish the two.