A positive pressure exists when the pressure inside a house zone is greater than the air pressure outside the zone.

In hemodialysis, referred to as "back pressure" or "venous drip chamber." Pressure exerted on the artificial kidney to cause removal of water from the blood. Increasing the positive pressure increases fluid removal.

Pressure in a space which causes an outflow of air.

Air will move from the space to outside the space where the pressure is lower.

a pressure greater than atmospheric

Condition that exists when more air is supplied to a space than is exhausted, so the air pressure within that space is greater than that in surrounding areas. Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.

Pressure at higher than atmospheric; used in SCBA facepieces and in smoke-proof stairwells to reduce entry of smoke or fumes through small openings.

Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will ingress into the surrounding environment.