Final answer:
The main limitation of structural fire fighting PPE includes a balance between protection and mobility, while ensuring that radio communications are not compromised.
Step-by-step explanation:
The design limitation of structural fire fighting personal protective equipment (PPE) often relates to a balance between protection and mobility. While PPE such as fire-resistant materials based on lightweight aerospace technology, like that developed by NASA, provides significant protection against heat and fire, they may also limit a firefighter's range of motion or add to the physical burden during operations.
Moreover, PPE has to ensure that radio communications, critical for coordination during firefighting activities, are not hindered. One example of such equipment includes the Nomex hood, part of the fire-resistant gear that is standard for firefighters, which protects against direct flame but can add weight and insulate, potentially leading to overheating if not properly managed.