Dear Dr. Pierce, I am writing in response to Comparison of Five Methods for Fit-Testing N95 Filtering-Facepiece Respirators by Coffey et al. (October 2002). This article raises several issues that invite comment and clarification. For example, there are factual and procedural errors that render the article's conclusions questionable, as explained below. Fit-test pass/fail results were compared with fifth percentile simulated workplace protection factors (SWPF) for the same respirator. The SWPF measurements included face seal leakage and filter penetration. The authors state that this SWPF measurement has been correlated to a wearer's actual exposure and is a valid measure of respirator performance. However, in the cited article, filter penetration was not included in the measurements. Fit tests were correlated with Freon 113 exposure in a laboratory. In fact, the SWPF measurement in this study has never been correlated to respirator performance in either the laboratory or the workplace. As a consequence, its use as a reference method is inappropriate, etc. Dear Sir, Thank you for the opportunity to respond to the letter from Mr. Janssen. I appreciate his observations, and have responded to each of his points in the order of their fundamental importance. The most fundamental point concerns what constitutes the most appropriate reference test. Mr. Janssen questions the use of an overall performance test - the simulated workplace protection factor (SWPF) test - as the reference test to judge the accuracy of the five fittesting methods. He states, "The SWPF measurement in this study has never been correlated to respirator performance in either the laboratory or the workplace," but he fails to identify a more appropriate test of respirator performance to use as a reference. My co-authors and I submit that the SWPF test used in this study has been demonstrated to correlate with respirator performance in both laboratory and workplace studies(1-3). Further, when evaluating overall respirator performance, we believe it reasonable and proper to measure total penetration (filter + face seal) rather than only face seal penetration as Mr. Janssen implies. In the cited article, total penetration was measured, but the filter penetration was negligible, etc.