Ventilation is the intentional supply and removal of air within a building. It brings in fresh air, removes stale air, controls moisture, and dilutes pollutants. Without it, indoor air becomes stuffy, humid, and potentially unhealthy. Ventilation is not the same as air conditioning, though the two often work together.

There are two main types. Natural ventilation relies on wind and buoyancy to move air through openings like windows, vents, and chimneys. It is free and sustainable, but it is variable and hard to control. Mechanical ventilation uses fans and ducts to move air. It is reliable and controllable, but it consumes energy.

Building codes set minimum ventilation rates for different occupancies. A classroom needs more fresh air per person than an office. A commercial kitchen needs powerful exhaust. A laboratory needs specialized systems to remove hazardous fumes. The requirements are based on research into indoor air quality and health.

Ventilation is easy to overlook until it fails. A building with poor ventilation feels stuffy and smells bad. A building with good ventilation feels fresh without anyone noticing. The best ventilation is invisible.