Slow Suffocation

A Medium creature can breathe easily for 6 hours in a sealed cubic chamber measuring 10 feet on a side. After that time, the creature takes 1d6 nonlethal damage every 15 minutes.

Each additional Medium creature or significant fire source (a torch, for example) proportionally reduces the time the air will last (two Medium creatures will run out of air in 3 hours, and so on). Small characters consume half as much air as Medium characters. A creature stuck in a starship or space station whose life support systems have completely failed will run out of breathable air in a similar fashion; while these structures are often larger than a 10-foot cube, they are also often occupied by several creatures. On average, a crew of four in a Medium starship without a source of fresh air can breathe easily for 20 hours.