Particles on Surfaces: Part 6 – How Clean is Clean?

The surfaces have been wiped and the obvious question is:  How clean are they?

Start with what you see. Do the surfaces look visibly clean?  If not, the wiping activity is not yet done. Wipe to the absence of visible soil on both the surface and the wiper. Keep wiping until the last wiper shows no visible soil after it has contacted the surface. If desired, the last wiper to touch the surface can be a black inspection wiper. This permits light-colored contaminants to be detected, since they will show up on the black wiper. Sort of the reverse of the “white glove test”. Black inspection wipers are usually made from color-fast, knit polyester, so that the wiped surface is not recontaminated in the inspection process.

Visualizing the surface contamination is a problem. The eye can detect particles only down to 50µm in size (approximately 0.002 in). Another visual metric is the inability of the eye to detect the surface residues smaller than about 1-4 µm/cm2. Either way, the eye can do only so much. While some may bemoan our limited visual resolution, this indeed may be a good thing. If you were able to see the very small bacteria on the face of your loved one, the human race may never have procreated!

Illuminating the wiped surface with bright light can help identify areas that need further attention. However, if the residues/particles/soils happen to fluoresce we can illuminate the surface with ultraviolet light (Caution:  Eye damage! Wear protective goggles!). In this case, the offending material will show up as bright spots on a purple background.

These inspection procedures work well for viewing cleanroom surfaces such as counter tops, benches, tables, carts, measurement equipment, etc., but they also work well for examining more critical surfaces such as the interior of processing equipment. In Particles on Surfaces – Part 7 and 8, we will deal with other specialized methods used by the pharmaceutical industries and the microelectronic industries, respectively, to verify surface cleanliness of manufacturing equipment after wiping.

Next up: How Clean is Clean for Pharmaceutical Companies?

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