If an unconscious person is lying on their back, there are a number of airway hazards.
Blockage by tongue
When a deeply unconscious person is lying on their back, gravity allows the jaw to drop backwards and permits the tongue to block the airway. The airway will also be blocked if the head of an unconscious person falls forward while sitting upright, as may happen in car accidents.
Loss of cough, swallow reflexes
We all continually swallow, whether consciously or unconsciously. For example, while you are reading this paragraph your mouth is continually filling with saliva that you swallow. After a meal you may have regurgitation (gastric reflux) and you immediately swallow to avoid stomach contents entering your airway. A deeply unconscious person may lose their ability to cough and swallow to clear their airway.
This is an active and noisy process, during which muscular action causes the stomach to eject its contents and potentially block the airway. If inhaled, vomit can damage lung tissue. Should this occur, immediately turn the person onto their side into a recovery position to clear.
Regurgitation occurs in an unconscious person when the muscle at the top of the stomach relaxes allowing a slow, silent passive flow of stomach contents to enter the airway which is then inhaled into the lungs. All too often it goes unrecognised until too late. Therefore, an unconscious person must be placed onto their side to protect their airway.
CLEARING THE AIRWAY
Before opening the airway, check for foreign material to reduce the risk of it flowing into the lungs.
Check and remove any visible material from the person's mouth using your fingers, and if necessary place them on their side into a recovery position to assist clearance.
Remove any loose dentures.
Turn the face slightly downward to allow any fluids to drain from the mouth.
If a person is unconscious, has a clear airway and is breathing normally, place them onto their side in a recovery position.