Ventilation

Inside a burning building is a dangerous place to be, for a variety of reasons: 1) heat, 2) smoke, 3) dangerous gases, like carbon monoxide (CO). It's important, also for a variety of reasons, to get all three out of the building.

First situation: food in an oven. This produces lots of smoke, but is often easily contained and extinguished. Still, the air will be filled with smoke. To help the homeowner, we set up a fan in a doorway and push outside air in, to clear the air. That's called PPV, or positive pressure ventilation. You put a PPV fan outside the door, such that the cone of air covers the door, and open a window in the smoke-filled room. (If you put the fan inside the door, some/much of the air you were pulling in at ground level would come right back out the door above the fan.) Assuming that the smoke has gone throughout the house, you open a window in one room at a time until the air is clear throughout the house.

Second situation: building on fire. Heat builds up, making the interior much more dangerous. Heat from the fire (as well as the smoke it gives off) rises, and collects on the upper floors, endangering occupants. While some FFs (from the engine company, often) prepare hose lines to go in and extinguish the fire, other FFs (from a truck company, often) can go onto the roof and cut holes to allow the hot gases to escape. This clears the air inside, and makes it safer for both occupants still in the building as well as the engine co. FFs about to go in. (In other situations, a fire can be "self-ventilating", meaning it burns through either a roof or wall, or breaks a window, allowing the heat/smoke/gases to vent on their own.)

Third situation: building no longer on fire. Even though the fire's out, there's still heat, smoke and gases inside. FFs need to go through the building, remove smoldering materials, and ensure that all hot spots are gone (so they don't reignite in the near future). To do that work, they either continue to wear airpacks, or completely ventilate the building so that it's safe to breath ambient air while they work.

Fourth situation: gas leaks, CO alarms, etc. These situations also require ventilation, in addition to figuring out and dealing with the source of the dangerous gas. FFs have hand-held meters to assist in determining the source of gas and in knowing whether ventilation is successful. (As a side note, sometimes the source is obvious; our instructor told us about a CO call, fortunately not involving fatalities, where residents of a row home decided to roast a pig in their basement. Not good.)

So we ventilate, to make things safer for occupants and FFs, during and after a fire, and to assist occupants in putting things back to normal.