Generally the problem is not suitable space, but staff and equipment. At least in developed countries.
....... and disease control.
Standard box air-conditioners or HVAC systems used in large commercial buildings are not suitable for ICU applications. HVAC systems for medical facilities have additional requirements to remove odors, limit cross filtration (to other areas), filter airborne micro-organisms and viruses and control humidity, heating and cooling.
Aircraft HVAC systems generally meet the above criteria in a diverse range of environments (hot and humid / cold and dry, etc.).
As aircraft are generally have enough systems in place to be self sufficient, the ease in which an aircraft can be mobilized and modified for such a role should be logistically a straight forward process.
I believe that you are very wrong on your concept that Aircraft HVAC systems generally meet the criteria for ICU air filtration. Having looked at the specifications for hospitals I cannot imagine that an aircraft has the room necessary for the high air volume HEPA and Activated Carbon filters required for ICU. I doubt that Airliners even have the level of HEPA and Activated filters I have in my house due to my pollen/mold allergy issues (and I know how big those are). What makes flying aircraft safe from me is that there is almost no atmospheric pollen & mold above 20,000 ft.
It would require for each ICU bed (or perhaps pair of beds) removal of 2 window and installing extra air movement and filtration equipment; and then powering them - along with powering all the other ICU equipment.
Now for non ICU cases... perhaps it makes sense to use a large aircraft body to set up quick space (once the seats are removed) in places with minimal buildings.
Have a great day,
I am designing some HVAC changes for an 16 year old outpatient surgery center to meet the new codes that have come into effect. The big item on this project is incorporating humidification and dehumidification that was not put in initially as the 30% to 60% RH usually doesn't require treatment outside of shutting down the economizer. Getting out of range even briefly gets written up, being out of range for a few days could result in suspension of the operating license. The requirements are many steps above normal HVAC.
Why? Controlling infections and contagion is critical for proper medicine. One can operate on a picnic table, but the infection rate will be sky high. To comply with the US regulations for an operating room the air is in specific downflow pattern over the exact table location so the air above the patient is direct and prevents air around the staff from flow toward the patient. OR's require 20 air changes per hour of which 25% is filtered outside air, the return air ducts are at the corners of the room 6" up from the floor. A 3 OR surgery center with recovery, waiting room, prep spaces, staff, linen storage, cleaning, etc takes 12,600 CFM of conditioned air, 100 kW of cooling, 150#/hr of humidifier steam, and 500#/hr dehumidifier capacity. The system must operate 24/7 at the stated design conditions except in off hours the outside air may be reduced from 4,200 CFM to only 1,800 CFM. Changing temperatures and/or humidity in the off hours may shorten the life of supplies, cause damp surfaces (anything over 60% RH is a germ farm) and most importantly violate the room differential pressures. Alarms on the doors go off if the door is open over 1 min, if positive pressure to the corridor is lost, the OR needs to be shut down and sterilized. All the OR surfaces need to be able to be sterilized in about 15 minutes between each procedure. Surfaces need to be designed to be scrubbed down constantly.
Isolation rooms for contagious diseases need 10 air changes per hour with 100% of air exhausted thru MERV14 filters (next step up is HEPA) to prevent the virus from entering the neighborhood. They require alarmed doors with specific negative pressure relative to the corridor. This covers the typical isolation room, far higher standards needed for ebola and the really nasty diseases.
The particle size of viruses makes them almost weightless. The OR requires the uniform high rate downflow to eliminate 99% of the particles after 1 hour at 20 air changes. The waiting room 6 air changes requirement gets 90% of the particles removed.
Another issue is the med gasses - flammables along with oxygen, also lots of electricity, metal, fluids, and contaminants. All must be considered. The HVAC unit filters both the outside air and return air thru MERV7 filters (standard commercial - residential varies between MERV2 and 7). The cooling coil cannot cool the air below about 56 degrees to limit the duct RH to 90%, which has to be monitored. The last step in the air handler is the MERV14 filters which cannot exceed 300 FPM so the filter bank is 8'x8' with 12" deep filters. Only ductwork, sensors and dampers are allowed downstream of the filters.
A proper ICU outside of the 3rd world takes a state of the art facility, it is far better to do the hospital tents within a warehouse or hanger than to try and do it in the confines of the airplane.
Using the old motel with exterior unit doors and below the window AC units actually does reasonably good viral isolation. Fire codes require rated walls without venting connections, so transmission between rooms is almost nil. Staff are outdoors between units so less PPE is needed.