Hydrocarbons produce H2O and CO2 if they are completely combusted; i.e., if there is enough oxygen to combine with both H and C to form the products. If, however, there is a closed container (or an enclosed area), there may not be enough oxygen to combine with all of the C and all of the H so that CO is formed instead of CO2 (or at least some CO is formed). CO, of course is quite dangerous to breathe. (For that matter, even complete combustion in an enclosed area isn't advised because too much CO2 isn't good for the lungs if there is insufficient oxygen to breathe. But CO is much more dangerous because it forms a complex with hemoglobin which isn't easily reversed, if at all.)
