I thought about this in reference to the question about Schamburg Airport in Chicago. Is there a particular criteria for an airport, for instance the amount of instrument traffic going into and out of an airport, high density flight operations, etc., which would require the FAA to classify an airport as a controlled environment and surrounded by controlled airspace (Class D, C, or B) all the way to the surface?

I guess by question is targeted at what is the threshold between where the FAA will require an airport to be controlled and where it is allowed to be uncontrolled yet allows for reasonable and safe flight operations in the National Aerospace System. I understand the basic differences between Class D, C, and B airspace designations, but I want to know if that is regularly evaluated and assigned new airspace designations surrounding the airport and control requirements.