Pressure from high winds can exceed the pressure provided by HVAC system fans. This can be critical for situations where the fans are expected to maintain a pressure difference between one building space and another. One example of this is the smoke control fans used to remove smoke from large open spaces like atria. High winds can reverse flow through these fans. These winds can also cause a "short circuit", as air enters through one makeup air location and leaves though another. Makeup air will also enter more quickly than desired, knocking over the smoke plume. For some designs, it is not wise to attempt to operate such a smoke control system during a hurricane. With 100 mph (44 m/s) wind gusts outside, it is unlikely that the air speeds at the makeup openings will be kept below 200 fpm (1 m/s). High winds can significantly affect indoor pressures as well, causing doors to either become difficult to open or difficult to close. Revolving doors are not immune to wind related problem, since they are often designed to collapse to allow panicking people to exit in the even of a fire. This "bookfold" mechanism has been triggered by high winds, leading the large glass doors to suddenly collapse, certainly a counterproductive aspect of a safety-related design feature. Wind gusts are critical in situations like these, so it is important to understand the fundamentally unstable nature of wind flow around buildings.