For the people commenting about Carbon Monoxide (CO) and Carbon Dioxide (CO2). You can die from both.
Carbon Monoxide gets you by poisonning you, displacing the oxygen carrying capabilities of your red blood cells and is typically caused by faulty boilers or some form of combustion. Yes, you should have a Carbon Monoxide alarm in your house and test it regulary, and replace when the sensor has expired.
You can also die from Carbon Dioxide. In this occurrence, the cause is typically suffocation from the heavier Carbon Dioxide displacing the Oxygen in a confined space.
When we say confined space we mean any space or volume where the Carbon dioxide gas could build up. This could refer to anything from a building, sump well, ditch, cellar etc. and the source of the Carbon dioxide can vary from rotting vegetable matter, yeast, bottled gas leaking, to chalk stone and acid rain.
To give some examples people have been killed by Carbon dioxide suffocation by walking into barns used to store apples, working on motorway ditches, and pub cellars.