Considered permanent because homeowners can rinse it free of particles and reuse it.
Should be rinsed every 30 days. Filter usually lasts 5-6 years.
Some brands have an anti-microbial agent that inhibits growth of mold, mildew, bacteria and fungi in the filter.
An electrostatic charge develops as air passes through the filter. The charge, however, varies with humidity and the furnace or A/C blowing cycles. This means some particles may fall off the filter, pass through it and re-enter the air stream.