Time for the quarterly replacement: out with the old electrets, in with the new. Looking at the old filter with the naked eye, there is an impressive amount of dirt. Certainly enough to keep the furance clean, but I just have to trust the testing labs that the allergens are removed as claimed. Not that I doubt it, but I certainly understand the difference between what the naked eye can see and what it can't. (Akin to what a nose can smell and how dangerous it is.)
Electrets have always struck me in a funny way. I understand the phyics behind them (the placement of a near permanent charge on a polymer, or most typically a polymer) and having worked in tap, film and netting plants, I know to never, ever touch a roll stored in the warehouse unless someone else have been foolish enough to touch it first (charges on some rolls can reach thousands of volt!). But I have never been able to get a good explanation of why the electret can't be grounded.