Cleanrooms are certified to a particular class by measuring the airborne particle count. However, the aerospace industry is generally more interested in particles larger than those detected by an airborne particle counter. The amount of large particles in a cleanroom is generally measured by use of fallout plates. The Boeing Company has utilized a Class 100,000 high-bay cleanroom for processing aerospace hardware. The cleanroom has consistently met Class 100,000 airborne particulate requirements, but larger particle fallout has varied substantially in the room. Approximately 4 yr of fallout data have been collected for this cleanroom. By tracking fallout, it was possible to detect actions that either positively or adversely affected the fallout rate. the fallout data, the variables affecting the fallout rate, the type of fallout, and the measurement methodology are presented.

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