<unk>  is the sensation of the urge to breathe. It is usually caused by the detection of high levels of carbon dioxide in the blood by sensors in the carotid sinus and is one of the body's homeostatic mechanisms to ensure proper oxygenation.   Natural chemicals in the blood such as adrenalin can also induce an urge to breathe by a separate pathway.  Insufficient pulmonary minute ventilation, a sustained breath-hold, constriction of the alveoli of the lungs as in asthma or high ambient levels of carbon dioxide in the<unk>  breathed can cause<unk>  resulting in a respiratory distress condition characterized by dyspnea, labored breathing or gasping.<unk>  can be very distressing and triggers strong reactions to restore breathing.
In mammals (with the notable exception of seals and some burrowing mammals) the breathing reflex is triggered by excess of carbon dioxide rather than lack of oxygen, so asphyxiation progresses in oxygen-deprived environments, such as storage vessels purged with nitrogen or helium balloons, without the victim's experiencing<unk> .
Air hunger
Dental fear
Thirst
Halitosis
Heartburn
Sleep disorders
Trench mouth
Hypnagogia