5.10 understand that incomplete combustion of fuels may produce carbon monoxide and explain that carbon monoxide is poisonous because it reduces the capacity of the blood from air to react, forming nitrogen oxides

First of all, incomplete combustion just means that the substance/thing was burnt without lots of oxygen present. When hydrocarbon fuels (for example gas or petrol) is burnt without oxygen, carbon monoxide is produced (as apposed to carbon dioxide, with complete combustion). This is bad because carbon monoxide is poisonous. This is because it combines with haemoglobin (this carried oxygen in your blood), preventing as much oxygen to get to your cells (as less oxygen can bind with the haemoglobin as there is lots of carbon monoxide combined with the haemoglobin). Therefore, less oxygen is being carried around your body (in your blood), this can lead to fainting, a coma or even death (in severe circumstances) as your cells are effectively being starved of oxygen.