3 minutes - air
3 hours - shelter
3 days - water
3 weeks - food
Three minutes without air? In air depleted circumstances you would start to feel lightheaded and possibly tired. You would see the edges of your vision starting to go dark. Unconsciousness would preclude death.
Where do you lose air?
Strangulation. Something physical (particularly something around your neck) prevents air from getting to your lungs. Drowning. Water is drawn into the lungs instead of air. Suffocation. Something blocks air from getting to your lungs. It doesn't necessarily have to have direct contact with the throat. Asphyxiation. Air is replaced by another gas like carbon monoxide.
Shelter is not only what is around you i.e. A house or lean-to, but also what you're wearing. Shelter keeps out the elements that can sap your strength and cause your body to shut down. Exposure reduces your survivability to 3 hours.
Clothing. Dress for the weather. It's not cool to become victim to elements. Best idea is to wear three layers. The one closest to your body should be a wicking layer. The best materials are merino wool, silk or polypropylene. The second layer should be loose and breathable. The third layer should be element proof, i.e. Water proof, wind proof, cold proof. This system is particularly important for the winter.
In the summer, layering clothing also helps. The inner wicking layer lifts the moisture off your body and dissipates it through the second layer, actually cooling down the body.
External shelter. In addition to proper clothing, external things can be used to protect the body. Smokers who can't smoke in the building know where to go outside of the building to keep out of the elements. This is shelter. In the woods, finding dead falls and thick groves of bushes can help to create immediate shelters. Those who can tap into their inner boy scout can build shelter from whats available to them in the forest.
The average person can last 3 days without water. Adult humans are about 60% water. "Approximately 85% of your brain, 80% of your blood and 70% of your muscle is water."(Source).
"Water helps remove the dangerous toxins that your body takes in from the air you breathe, the food you eat and the chemicals used in the various products you use on your skin and hair. Other things water does for your body is to cushion your joints. Water carries oxygen and nutrients into all your cells. Water also helps regulate your body temperature.(Source).
Without adequate water, the body begins to dehydrate which starts a process that affects your body functions as they systematically shut down in order to protect the core.
Food provides the essential building blocks for the body. Too much food is stored in the body as fat. Too little food causes the body to break down itself in order to find the nutrients it needs to function. From food the body extracts proteins, carbohydrates, vitamins and minerals, etc. These things provide energy, repair and replace damaged cells, sustain the body, and help fight infection.
Because the body stores extra food, it begins to break down these stores when food supplies are interrupted or stopped. Again the body will also systematically shut down in order to protect the core.