Final answer:
Natural ventilation is used when forced ventilation systems are unavailable, and it relies on air movement through openings like windows and doors due to temperature differences and wind.
Step-by-step explanation:
In situations where other types of forced ventilation are unavailable, natural ventilation may be used. This type of ventilation relies on the natural movement of air without the aid of mechanical systems. It includes air movement through opened windows and doors and is driven by air temperature differences between indoors and outdoors, as well as by wind. Mechanical devices like fans and ductwork are not required for this process.