It depends on how much air you need and what it is used for. I have seen both piston type compressors and rotary screw compressors in hospitals.

I have also seen both oil flooded and oil-free compressors. Of course, you will need compressed air treatment like a compressed air dryer, filters (water, dust, oil, fumes, etc).

It's about the same story as dental compressors (as used by dentist) please see here: http://www.air-compressor-guide.com/dental-compressor.html

Always check with local authorities what the requirements are (cleanliness of compressed air). It should be up to a certain standard.

Have a specialized company advice you and take frequent air samples to make sure the compressed air is really up to the standards.