Humans need oxygen. Thanks to it we maintain all vital processes in the body. Oxygen is that which keeps the cells in our body alive. Absence of it, even for just a few minutes, leads to their destruction.
The absence of air is especially disastrous for the human brain. The brain is the greatest consumer of oxygen in the body, using up nearly 20% of all the oxygen, even though it makes up just 5% of our total body mass.
If the supply of oxygen to the brain is completely cut off, a person will lose consciousness within 10 seconds. Left without life-giving oxygen, cells in the brain would die within 4 to 6 min, resulting in death.
Several minutes' lack of air can leave a person in a vegetative state, where bodily functions of the face and certain main reflexes still exist but there is no brain activity. Such patients usually die within a year.
An average person can hold their breath underwater, and therefore survive without air for 2 min. Professional divers and snorkelers can hold their breath for 4-5 min. primarily because their lungs are better developed.
The world record for holding one's breath underwater belongs to Stig Severinsen from Denmark, who held his breath for 22 min. He managed to achieve this feat by breathing pure oxygen immediately prior to his attempt in order to eliminate the majority of carbon dioxide and nitrogen from his body.
Additionally the record was set in a pool where the temperature of the water was 86° F (30° C). As is known, cold water slows vital processes in the human body.