What are the main sources and the financial impact of microbial contamination in the cleanroom?
http://gsfcc.org/index.php?title=Sources_and_Financial_Impact_of_Microbial_Contamination

Recently, in the Cleanroom Cleaning group an inquiry was posted concerning action and alert levels for environmental monitoring measurements (i.e. measurement of bacterial levels) in ISO Class 7 & 8 environments in a medical device manufacturing facility.
While colony forming unit (CFU) measurements in ”controlled environments” display the typical negative exponential histogram character usually seen for particle measurements, in ”dirtier” environments, the histogram changes to a ”normal” i.e. a Gaussian distribution.
In a controlled environment, one would expect alert levels of 3*mean and action levels of 4.6*mean, In ISO Class 7&8 environments the alert levels change to mean +1.65*std dev’n and action levels change to mean +2.33*std dev’n. This information is described in a 2004 Cleanrooms East Conference paper. copy of Cleanrooms Magazine may be able to supply a copy of the full paper “Establishing and Environmental Monitoring Program”.
Other information sources on this topic include:
1. NSF publication “Environmental Monitoring” available at http://www.cerhb.ufl.edu/pdf/edcenter/environmental_monitoring.pdf/
Action and alert levels for ISO Class 7 & 8 environments are presented on page 25 of this document.
2 “Microbial Limit and Bioburden Tests: Validation Approaches and Environmental Limits” 2nd ed., Lucia Clontz, Taylor and Francis Group, 2009.
3. US-American Pharmacopoeia (USP), “Microbiological Evaluation of Clean Rooms and Other Controlled Environments”, USP 26 .