system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do we not run out of Oxygen in closed rooms?

Are there any small ventilation holes to let oxygen in buildings and cars? Do architects need to take this into account?
gpt
Generally, nothing is going to be airtight unless *it's specifically designed to be*.  Making stuff airtight is very difficult, and very expensive.  It requires special materials and precision. 

There are lots of places for air to get in, such as a window frame, underneath a door, vents (even if the furnace isn't running).  

If you were in a room sealed well enough to prevent much air from entering, an average size room has *a lot* of oxygen.  It would take many days, even weeks, to use it all up and asphyxiate.  Chances are you will have left by then.