Answer to Question #6865 Submitted to "Ask the Experts"

If a regulation requires 300 square centimeters of a surface to be swiped, does the swipe need to actually contact 300 square centimeters of surface?

I have always believed that 300 square centimeters of surface would need to be contacted ("wiped"). However, I have visited several facilities that use Q-tips or a similar-sized absorbent object to perform their swipes. The swipes are performed "over" 300 square centimeters of surface but (due to the small surface area of the Q-tip) do not actually contact 300 square centimeters of surface. I have observed this with all types of wipes: package, work surface, and sealed sources. For sealed sources, Q-tips may be appropriate. For larger areas (e.g., 100 and 300 square centimeters), Q-tips seem inadequate. At the extreme, I have observed an entire nuclear medicine facility swiped with one Q-tip. There are, of course, several objections to this approach. I prefer commercially available absorbent circular discs that present a larger contact area with the surfaces being swiped.

Your interpretation of what is intended when a particular area is specified for wipe-testing is appropriate. Decision levels for release of materials or areas or for taking other specific actions related to surface contamination are typically based on specific contamination levels—e.g., in dpm/100 cm². The implication of this is that wipe (or swipe) tests should be capable of providing results that can be compared to the decision criterion. Thus, if the criterion specifies a given activity per 100 cm², the usual area to be covered by a single wipe is 100 cm², often approximated by a 4" x 4" square of surface area. The same consideration would apply to a 300 cm² specified area.

Under some circumstances it may be necessary or desirable to cover a smaller or larger area with a given wipe, and regulatory allowances may be made for such; in such an event the area covered must be known and results can then be adjusted to yield the activity per the criterion-specified area (e.g., 100 cm²). The use of a Q-tip to wipe a surface to evaluate removable surface contamination using a fixed area criterion would generally not be appropriate for the reason you have noted—namely, that the Q-tip does not sample the entire specified area, and no legitimate conclusion can then be drawn as to the actual extent of contamination per specified area.

Exceptions apply, as you have again noted, to leak-testing of sealed sources for which the decision criteria as to whether a source is leaking are not specified on the basis of area contamination but simply on the basis of positive removable activity above a specified level. Similar criteria may apply to other objects whose surface areas are small and for which area criteria are not especially useful.

In summary, keep doing what you're doing.

George Chabot, PhD, CHP