What type of ventilation occurs when clean air is blown into a structure to force fire gases out?

Positive pressure ventilation is the correct answer because it involves using a fan or blower to introduce fresh, clean air into a structure. This influx of air increases the air pressure inside the building, creating a flow that forces out smoke, heat, and fire gases. The primary purpose of this type of ventilation is to enhance visibility and improve conditions inside the structure for both firefighters and potential victims, facilitating rescue operations and fire control efforts.

This technique is particularly effective in combating fire scenarios, as it helps to rapidly reduce smoke and heat, which are major hazards. Furthermore, it can also assist in pushing the fire toward the exhaust points, allowing firefighters to attack the fire more effectively.

Other methods mentioned, such as negative pressure ventilation, involve creating a vacuum effect to draw smoke out, while natural ventilation relies on existing environmental conditions, like wind or temperature differences, to move air. Mechanical ventilation focuses on actively moving air but may not specifically refer to creating positive pressure as part of the ventilation process. Each of these methods has its own advantages and applications, but positive pressure ventilation is specifically characterized by the introduction of clean air to displace smoke and gases.