CO is an air pollutant. It is a health hazard being highly poisonous gas.Its presence cannot be noticed easily and readily being colorless and odorless.When it inhaled,it binds with the haemoglobin most strongly than that of Oxygen.Thus,hindering the supply of oxygen in body.
Exposure to higher concentration of CO causes headache and fatigue.If inhaled for a longer time it results in breathing difficulties and ultimately death.
It is the reason burning is not allowed in closed places. 
It is advised to switch off coal or gas heaters,cooking range,etc., before going to sleep.
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