Carbon monoxide (CO) is a colourless, odourless and taste-free gas, but also poisonous. Its sources include the incomplete combustion of substances containing carbon (oil, gas and solid fuels, etc.). If CO manages to get into the bloodstream through the lungs, it combines with haemoglobin, thus preventing oxygen from being transported in the blood; this in turn will result in death through suffocation. This is why it is necessary to check that gas burners are functioning safely. This particularly applies to older appliances which do not have a so-called flue gas monitoring device.