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 to carry oxygen

Hydrocarbons (from crude oil) + oxygen > carbon dioxide + water

Unless there is not enough oxygen around. Then fuels will combust incompletely:

Hydrocarbons + oxygen > carbon monoxide + carbon + water

Carbon monoxide combines with heamaglobin in red blood cells, meaning they can't carry oxygen around the body.