Editing Cleanroom (section)

=== Cleanrooms of different kinds ===

Some cleanrooms are kept at a [[positive pressure]] so if any leaks occur, air leaks out of the chamber instead of unfiltered air coming in. This is most typically the case in semiconductor manufacturing, where even minute amounts of particulates leaking in could contaminate the whole process, while anything leaking out {{citation needed span|would not be harmful to the surrounding community|date=November 2023}}. The opposite is done, e.g., in the case of high-level bio-laboratories that handle dangerous bacteria or viruses; those are always held at [[Negative room pressure|negative pressure]], with the exhaust being passed through high-efficiency filters, and further sterilizing procedures. Both are still cleanrooms because the particulate level inside is maintained within very low limits.

Some cleanroom [[HVAC]] systems control the [[humidity]] to such low levels that extra equipment like [[Air ioniser|air ionizers]] are required to prevent [[electrostatic discharge]] problems. This is a particular concern within the semiconductor business, because static discharge can easily damage modern circuit designs. On the other hand, active ions in the air can harm exposed components as well. Because of this, most workers in high electronics and semiconductor facilities have to wear [[conductive boots]] while working. Low-level cleanrooms may only require special shoes, with completely smooth soles that do not track in dust or dirt. However, for safety reasons, shoe soles must not create slipping hazards. Access to a cleanroom is usually restricted to those wearing a [[cleanroom suit]], including the necessary machinery.

In cleanrooms in which the standards of air contamination are less rigorous, the entrance to the cleanroom may not have an air shower. An [[anteroom]] (known as a "gray room") is used to put on cleanroom clothing. This practice is common in many nuclear power plants, which operate as low-grade inverse pressure cleanrooms, as a whole.

'''Recirculating vs. one pass cleanrooms'''

[[Recirculating cleanroom]]s return air to the negative pressure plenum via low wall air returns. The air then is pulled by HEPA fan filter units back into the cleanroom. The air is constantly recirculating and by continuously passing through HEPA filtration removing particles from the air each time. Another advantage of this design is that [[air conditioning]] can be incorporated.

[[One pass cleanroom]]s draw air from outside and pass it through HEPA fan filter units into the cleanroom. The air then leaves through exhaust grills. The advantage of this approach is the lower cost. The disadvantages are comparatively shorter HEPA fan filter life, worse particle counts than a recirculating cleanroom, and that it cannot accommodate air conditioning.