Filtered air is supplied to a work area by a forced ventilation system. If the functional area's internal air pressure exceeds the air's density, dust and contamination will not be able to enter. As a result, this system will keep the working environment clean and ventilated. Natural ventilation is rarely satisfactory unless outside circumstances are excellent, which rarely occurs. The natural supply of fresh air is determined by the temperature difference between inside and outside. It is rare for fresh air to flow into an inhabited space unless a breeze is blowing in the right direction.