How do electrostatic air filters maintain their charge?

Question

3M meets the N95 filtration specification on disposable respirators by constructing them with "electrostatically charged microfiber filter media."

How does air filter fabric stay charged? Fun grade school demonstrations involving everything from balloons to Van de Graaff generators show how rubbing certain materials together can impart an electrostatic charge, and that attractive charge can be observed as hair, paper, and other items are drawn towards the charged material. But the same demonstrations show that the electrostatic charge dissipates on contact with the same items that are attracted to it. In fact, the charge will dissipate over time just through contact with air.

I imagine that charged media can be sandwiched between two layers of electrostatic insulators to prevent discharge on contact with hair, skin, clothing, etc. But given that the sandwich has to be permeable to large volumes of air (for respiration), how is a charge maintained on the media for any significant length of time?

(Related question: By what mechanism does electrostatic media aid in particulate filtration?)

Answer 1

The fibres are made from a high resistivity synthetic polymers called electrets which can maintain a permanent dipole moment both on the surface and in the bulk. In many ways they are the electrostatic equivalent of permanent magnets.
They are charged with excess charge on the surface and by producing permanent bulk dipoles in the manufacturing process whilst they are still molten by subjecting the material to a very high electric field (corona charging) and then allowing the polymer to cool which locks the bulk dipoles into position.
Because the manufactured fibres have such a high resistivity and resistance to adsorption of moisture they can stay "electrolised" (magnetised) for very many years but obviously have a finite lifetime when in use.

The advantage of having the charged fibres is that the gap between fibres can be made larger (but not too large) which allow the freer passage of air through the masks which makes breathing through them easier.