A confined space like a cabin in a car, truck, aeroplane, or a ship's bridge has to be well-ventilated to keep personnel alert. If you close a car's ventilation (e.g. going through a tunnel or if you don’t like the fumes from a badly tuned truck), it takes about 5-10 minutes (depending on how many people there are in the car) to raise the level of CO₂ to 2,000-3,000 ppm, a level where your decision-making ability could become impaired.

A field where alarms really come in handy is when CO₂ is used as a cooling media in air conditions. You can find more information about cabin environment under Automotive.