Georges,

The AC unit is a small, cheap, split-type with no outside vent. I hear they're very common outside the US. It is very quiet and allows me to keep the temperature inside at 20C conveniently to speed processing of prints and film. Fresh air is a different story, as you've discovered. I achieve this semi-passively with a large Doran exhaust fan over the sink and a fresh air entry in the ceiling "piped" to an ouside wall. Air is "pulled" into the room by the exhaust fan. The Doran is extremely noisy so I have installed a variable speed fan switch to control its speed (thus noise) when large air transfer is not required. Another point about the split-type AC is that very few US AC installers know how to install them correctly, so finding appropriate installation can be much more expensive than the cost of the unit.