When deprived of food, water, or air, the rule of three applies. An adventurer can handle three weeks without food, three days without water, and three minutes without air outside of strenuous situations.
After that, such deprivation is a significant test of a PCs’ stamina. At the end of the time period (three weeks, three days, or three minutes), the character must succeed on a DC 20 Endurance check.
Success buys the character another day (if hungry or thirsty), or round (if unable to breathe). Then the check is repeated at DC 25, then at DC 30, and so on. When a character fails the check, he loses one healing surge and must continue to make checks. A character without healing surges who fails a check takes damage equal to his level.
In strenuous situations, such as combat, going without air is much harder. A character holding his breath during underwater combat, for example, must make a DC 20 Endurance check at the end of his turn in a round where he takes damage.
As with environmental dangers, a character cannot regain healing surges lost to starvation, thirst, or suffocation until he eats a meal, drinks, or gains access to air again, respectively.
A character with 0 or fewer hit points who continues to suffer from one of these effects keeps taking damage as described above until he dies or is rescued.