Which mechanical ventilation technique uses large fans to force fresh air into a structure?

Positive pressure ventilation is the correct choice because it involves the use of large fans to introduce fresh air into a structure, effectively pushing out smoke and heat. This technique creates a higher pressure inside the building compared to the outside, facilitating the removal of smoke and improving conditions for both firefighters and potential victims inside.

In contrast, natural pressure ventilation relies on existing environmental conditions, such as wind or thermal buoyancy, to facilitate airflow through the structure. Push-pull ventilation typically involves creating airflow by positioning fans at both entrance and exit points to move air through the space, but it does not specifically use large fans to force air in, like positive pressure ventilation does. Negative pressure ventilation operates on the principle of pulling air from a building rather than pushing it in, relying on exhaust fans to create a vacuum effect. Thus, the use of large fans to create positive pressure distinguishes this technique from the others.