[Reference] To prevent the outdoor non-clean air from disturbing the clean and aseptic state indoors, the most effective method is positive pressure control, i.e., keeping the indoor air pressure greater than the outdoor pressure, so that the indoor air can only flow to the outdoors and reverse flow cannot possibly occur. Nowadays, as control has developed from single-room control to whole-area control, it is necessary to establish an ordered gradient pressure distribution between the inside and outside of the area and between spaces of different cleanliness classes within the area.

[Reference] The clean operating department shall maintain a positive pressure relative to the surrounding area, and between rooms of different cleanliness classes within the department, a reasonable airflow direction and different pressure distributions shall be maintained, so as to avoid the influence of the outdoors on the indoors or of lower-class environments on higher-class ones. To truly and effectively reduce the risk of cross-infection in the operating zone, the only way is to ensure that this ordered gradient pressure distribution within the clean operating department can be maintained unchanged under all circumstances (with particular emphasis on non-design conditions or non-normal operating conditions).

[Reference] The specific functions of positive pressure are reflected in three aspects: ① with doors and windows closed, preventing contamination outside the clean room from infiltrating into the clean room through gaps; ② at the instant a door is opened, ensuring that there is sufficient airflow flowing, so as to reduce the disturbance airflow brought about at the moment the door is opened or a person enters, but without guaranteeing the positive pressure while the door is open; ③ ensuring a reasonable and orderly airflow direction and flow rate within the clean area.

[Reference] Positive pressure control is generally achieved by adjusting the difference between the supply air volume and the return air and exhaust air volumes, in combination with control measures. This air volume difference is called the positive pressure infiltration air volume. In the final analysis, this positive pressure infiltration air volume is borne by the fresh air volume of the system. Positive pressure is, in essence, the resistance to the positive pressure infiltration air volume passing through the gaps of the room. The positive pressure air volume is unorganized airflow that infiltrates everywhere; therefore, the greater the positive pressure infiltration air volume and the smaller the gaps of the room, the greater the positive pressure established, and vice versa.