What are the best designs in operating rooms and hospitals insulation (absolute filters inside the air handling unit or inside the air outlets and why?

For room insulation, the contractor shall ensure the leakages from the door & any wall opening above false ceiling (for ducts) with good insulation. Normally, an OT has just1 double leaf door with a fixed glass window. An AHU is usually located in the adjacent room to shorten the ducting as possible so that with the recommended velocity, the air changes in minimum time. HEPA filters are employed which are of special paper media, capable of10 microns filtration & recommended for90 fpm velocity as per ISO standard. This facilitates the laminar airflow thereby isolating the projected area (covered by HEPA filter plenum) from the surrounding area. Due to slightest humidity, there are chances of mold to develop on the internal surface of the ducts. Hence the most recommended location for HEPA filter shall be at the air terminal devices. Moreover it is easy to remove & replace. In addition, UV light treatment is used inside the AHU.

There is also one important architectural aspect in OT room design which might be useful. As per new regulations an OT (especially for open surgery) shall be of jointless construction. It means that there should not be any corners in the OT. Even the edges between the walls, floor & ceiling should be coved so that there is not even a tiniest gap for germs to settle.

Inside the air outlets to keep the Operation room clear from any a contaminate and  tiny particles or Bactria

the absolute filters must be in the laminar of Operation room.

but if you want to put it built in AHU, (the duct must be stainless steel & round duct) to keep the Operation room clear from any tiny particles or Bactria .

For hospital we can go with special hygienic Air handling Or Packaged units  

For cleasses A, B it is recommended to install HEPA Filters at terminal outlets, but for Classes C & D you can install HEPA Filters at AHU

note that in hospitals paient rooms are D or C classes and surgury operation rooms are A, B classes