= Solution
\b[The several methods are natural ventilation by wind or buoyancy, mechanical supply, mechanical extraction, and combinations of these.] Each must provide both a route for clean outdoor <air> to enter and a route for used <air> to leave.
For <natural ventilation>, opening windows on different sides permits <cross ventilation> when the <wind> creates a suitable <pressure difference>. Even one window can exchange <air>, but the amount and distribution are more dependent on conditions. Openings at different heights use the <stack effect>: warmer, less dense indoor <air> tends to rise and leave through an upper opening, drawing replacement <air> through a lower one. A flue or ventilating shaft can extend this route. A fireplace may assist extraction through its chimney, provided replacement <air> enters and combustion products are carried safely away.
Permanent inlets, adjustable window openings and outlet shafts can make these principles more controllable. Their positions should distribute fresh <air> through the occupied part of the ward instead of allowing it to pass directly from inlet to outlet. An open door into an adjoining foul space is not necessarily a source of clean <air>. <Florence Nightingale>'s pre-1864 discussions emphasize outside <air>, openings and the relation between ventilation and warmth; admitting fresh <air> need not mean exposing a patient to an avoidable cold draught.<Notes on Nursing by Florence Nightingale>[Ventilation and warming]{ref}<Notes on Hospitals by Florence Nightingale>[Third edition, 1863, discussion of ward ventilation and window arrangements]{ref}
With mechanical extraction, a fan or other air-moving apparatus removes indoor <air>, while planned inlets admit replacement <air>. With mechanical supply, the apparatus introduces outdoor <air>, and outlets allow displacement of the used <air>. A balanced installation provides both mechanically. Natural openings can also be combined with mechanical assistance. Heating the incoming <air> can reduce chilling without making it necessary to recirculate all the used <air>.
What matters is the actual <airflow> through the room, including its direction. Outlet flow can be investigated with an <anemometer>; short-circuiting between nearby openings and stagnant regions must be considered. Moving the same indoor <air> around does not by itself supply fresh <air>. Nor does a perfume remove a source of contamination. Source removal, sufficient exchange and reasonable temperature control belong together; windows or fans are means to those ends, not proof that they have been achieved.
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