This paper discusses the rationale behind the proposed revisions of Federal Standard 209B on cleanroom air cleanliness classifications. The proposed revisions were developed in a series of meetings held by the RP-50 committee of the Institute of Environmental Sciences. The major recommended changes are:

1. Class 1 and Class 10 be added

2. The two proposed new classes may be met at particle diameters of 0.1,0.2,0.3, and 0.5 μm, (microns), and Class 100 may be met at 0.2,0.3 μm as well as 0.5 μm, with suitable adjustments in the required concentrations

3. Other classes (e.g., Class 50) be defined as appropriate

4. The number of sampling points be specified differently, and a distinction be drawn between unidirectional and non-unidirectional flow, rather than laminar and turbulent flow

5. Minimum sampling volumes be set, based on measurement errors

6. Verification be done initially and periodically

7. Verification require particle counting

8. Particle counting instruments require periodic calibration

9. An area be deemed to meet the class limit if both:

a) the average particle count at each location meets the class limit and

b) the mean of these averages is sufficiently below the class limit that the population mean is almost certainly (95% confidence level) less than the class limit

The rationale for these proposed changes is discussed.

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