adj.
Natural or mechanically induced movement of fresh air into or through an enclosed space.
The hazards of poor ventilation were not clearly understood until the early 20th century. Expired air may be laden with odors, heat, gases, or dust. Mechanical ventilation systems typically include a fan and filter to remove particles. A mechanically powered inlet of air, when combined with a natural exhaust, tends to cause a slight positive pressure within an enclosed space, so that the air leakage is outward. A mechanical exhaust with a natural air inlet causes a slight negative pressure, so that air moves inward. Such systems are often used to confine fumes or smells to a particular area of a building (e.g., laboratories, kitchens, bathrooms) and exhaust them to the outside.