LETTERS TO THE EDITOR

Editor:

I was interested in the face mask contamination study in the March 1998 MICRO, "Evaluating the particle containment effectiveness of face masks and head gear," as we have recently completed a similar evaluation of disposable cleanroom face masks. Although we did observe some masks that generate more 0.1-µm particles than the subject's bare face, most masks did significantly reduce particle counts from this level.

Another factor that must be considered, which was not addressed in the article, is the amount of time that the mask has been worn prior to testing. The performance of this type of mask generally degrades over time, leading to inconsistent results if varying time intervals are used.

The authors should be commended for bringing up an important and neglected issue. However, each organization should evaluate the products they use for suitability to their application, and over the time frame of normal use, to determine if they are acceptable.

Charley Dobson
Samsung Austin Semiconductor

The author replies:

Dobson raises an interesting point, one the authors have considered in detail, "Do face masks increase or decrease contamination?" Both this series of tests and the early series of tests mentioned in the article (done in 1985 at 0.5 µm) suggest that face masks actually contribute to contamination. However, this was not the focus of either series of tests.

The purpose of these tests was to compare contamination levels from a subject wearing active headgear with the contamination levels from the same subject wearing various face masks and also wearing no face masks. The tests show that the contamination levels from a subject wearing active headgear are significantly less than from the same subject wearing either face masks or no face masks. The data showing that face masks produced higher levels of contamination than no face masks was a very interesting observation, but not the major point of the tests.

Had the purpose of the test been to compare face masks with no face masks, a different series of tests would have been done. For example, rather than a single subject, numerous subjects would be used because people vary significantly in the level of contamination from an unmasked face.

Due to the great variety of face masks and the different levels of contamination from different subjects, the authors have no trouble accepting Dobson's statement that a series of tests showed some face masks lowered the level of contamination (versus no mask) and some actually increased the level of contamination. Even these data, showing that some face masks (as opposed to all face masks tested) increase contamination, raises serious issues about standard gowning procedures.

Dan Duggan
Dryden Engineering

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