Most building codes require very tall buildings to provide smoke proof exit stairs for a means of evacuation for occupants as well as an access point for emergency personnel. In order to comply with International Building Code (IBC), there are three design options: exterior stair balconies, mechanically ventilated stair vestibules or stair pressurization, with the latter being the most common.
In order to keep the stairwells free of smoke, they are pressurized to a point where even when a door is opened to access the stairwell no smoke is able to enter. For this reason, it can require a little more force to open a stairwell door from the interior than if you were opening a regular door in the interior of a building.
The fans and air ducts used to pressurize the stairwell have to be enclosed in non-combustible construction and connected to an emergency power supply. Furthermore, the system must be capable of both automatic activation through smoke and fire detector as well as manual activation from a control panel.