They are sealed to a point. They do a fairly good job, however, there is inevitably a little bit of room air that gets pulled into the exhaust chain. We ALWAYS run a Ozone generator (
http://www.blueairproducts.com) inline of our exhaust.
Now, I knew a guy who created his sealed room using polyfilm then cut a space for the AC to go in, so the front side was inside the room and the backside was out of the room. This way, the majority of the air that it might grab from indoors was already relatively clean. This also prevented his CO2 levels from being affected inside his sealed room. To make sure the air being picked up by the A/C was clean, he also ran a can filter as a scrubber with a 90 degree elbow that pointed TOWARD the back of the AC unit, giving it the cleanest, freshest air to suck up.
We run a dual hose A/C, and no matter where we connected the intake, the machine pulls SOME amount of air from the room slowly lowering our CO2 levels. The AC is able to cool faster than the CO2 generator can create heat so we don't have any issues in the end, but it is not exactly the most efficient method.
So in summary, if you have the hoses and/or the back of the unit inside the room then you will inevitably be pulling some amount air from that space. Your best bet is build it halfway in/halfway out if at all possible. There are also amazing units by IDEAL that actually send the air OUT a tube instead of out of the front of the unit like all consumer types. This allows you to put the A/C OUTSIDE the room completely and just duct the cool-air into the room. Of course, they cost around $1500-$2500, but hell, over time, it's efficiency and CO2 savings could probably pay for itself over time.