Electret filters tested under HVAC service conditions fell from 85 percent to 45 percent efficiency, with no change in pressure drop across the same period. A companion cigarette smoke study recorded a fall from 92.5 percent to 33.3 percent. The unchanged pressure drop makes this a silent failure mode: the filter continues to present the same resistance while capturing substantially less.
The mechanism matters for how the result should be read. Electret media achieves part of its rated efficiency through electrostatic attraction, and that charge dissipates in service as the media collects particulate and encounters humidity and organic vapours. What remains is the mechanical efficiency of the fibre structure, which is lower. Osborne found the same pattern across 85 swine farms, with charged synthetic media losing efficiency significantly in service while fiberglass held its efficiency far longer, and recommended fiberglass at MERV 15 or 16. ISO 16890 conditions filters in isopropanol vapour before rating them precisely to strip the charge contribution out of the measurement.
For an operator, the practical difficulty is that the ordinary indicator of filter condition does not detect this. Pressure drop rising over time is how a loading filter announces itself, and it is what maintenance schedules are usually built around. A filter losing charge sends no such signal, so the loss is invisible between one performance test and the next. That argues for specifying media whose efficiency mechanism survives service rather than for monitoring more closely, since the monitoring that would catch it is not routinely available on a barn.