[Greenbuilding] minimum filter MERV rating

[candtcampbell at juno.com]

Is anybody on this list qualified to offer an educated opinion on the minimum filter MERV rating necessary to keep an indoor forced air evaporator coil clean (i.e. mold-free) over its expected lifetime? I was recently surprised to see a clean coil come out of a system that had been protected by the very cheapest spun filters for 10 years.

Tim