How do you think it is dangerous? ACs do not emit any gases themselves, so except for the CO2 produced by the engine to keep it running it should not be dangerous.
CO and CO2 produced by the engine is going to come out of the exhaust at the back. Your AC will take in air from the front of the car.
In a parking lot or other well ventilated areas, I don't see a big cause for concern.
Also modern cars produce orders of magnitude less emissions its actually somewhat difficult to kill yourself with one now, you essentially have to run a hose from the tailpipe to the cabin.
You still have an engine that is producing a certain amount of power, and this is associated with a minimum rate of production of CO2. Some of the other interesting emissions are greatly reduced in modern engines, but the CO2 production hasn't fallen that much.