In contamination control technology relatively dirty surfaces cannot be specified with the same defining equations as those for "clean" surfaces. The current practices in the aerospace community disregard this inaccuracy in defining and specifying gradations of surface cleanliness. Using a 0.926 slope with an IEST-STD-CC1246D particle distribution to describe cleanliness on dirty surfaces, or surfaces subjected to fallout, is inaccurate, unrealistic, misleading, and often impossible to verify. Most importantly, using this model provides incorrect information to experts who calculate sensor performance: a slope of 0.926 is appropriate only for cleaned surfaces. However, the industry continues to use the same slope to describe all surfaces, clean or dirty. Current measurements taken from Raytheon labs have shown that particle distributions dominated by fallout typically have a slope between 0.3 and 0.5, in concurrence with previous studies. Based on the accumulated data, the authors propose that particle distributions for dirty surfaces be described by a second model, which should be incorporated into the next revision of IEST-STD-CC1246.