Positive air pressure pushes air out of a room or container by increasing the rate of flow. This directly contrasts with negative air pressure, where the flow is drawn into the environment. In cleanrooms at hospitals, labs, and other facilities, it can be important to maintain positive or negative pressure, depending on the kind of work performed in those environments. Some facilities have ventilation systems that can switch flow, allowing a room to be used in a positive or negative state for multipurpose reasons.

In a room with positive air pressure, fans create a steady flow of air out of the room, and an intake replaces it. The air, and any particles, are forced out of the room, keeping contaminants out for as long as the intake is properly filtered. This can be useful in an environment where nothing in the room is dangerous and workers need to keep the area as clean as possible. Where computer chips are manufactured and handled, for example, impurities can cause problems with the finished product.

The same techniques can be used to create positive air pressure in a container or other unit. A computer tower, for example, can use its fans to push air out at a continuous rate to limit accumulations of dust and debris inside. This can keep the components clean and functional, and may reduce the risk of a system failure. As with positive air pressure rooms, the goal is to keep contaminants out of the area.

Outflow vents may contain filtration systems to trap any materials that escape, if there are concerns about pollution. Inflows also need to be tightly controlled to ensure that enough clean air is supplied to replace the air as it vents out. Some systems recycle, circulating air through filters to return it to a room after it's vented. In a recycled air system, it is critical to filter properly to reduce the risk that impurities will become concentrated with the recirculation, as this could pose a threat to health and safety.

Rooms with multipurpose ventilation systems allow users to switch between negative and positive air pressure to suit different kinds of work. These rooms may have indicator status lights to let people know which direction the ventilation system is circulating in. Whether air has a net flow in or out, an anteroom may control access to the room to limit contamination and maintain a tight seal on the ventilation. Without such controls, every time someone opened the door, contaminants could rush in or out.