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Fit testing should be performed before the use of tight-fitting respirators. However, it may not always be conducted for various reasons, mostly time consuming and costly. This study aimed to shorten the fit testing procedures by improving the instrumental settings, sampling system design, and data analysis protocols.Experiments of fit factor measurements were divided into two parts: constant flow and cyclic flow using a breathing simulator. To simulate leakage, capillaries (10 mm in length, diameter 1.0–1.5 mm) were used to insert on N95 and N100 filtering facepieces. The ratio of total to leak flow was considered the ‘true fit factor, FFt’. Flow rates ranging from 5–50 L/min were employed to study the flow dependency. The measured fit factors were determined by concurrent particle concentration measured by a Portacount and a OPS 3330. The default 1.7 m sampling tube was used to connect filtering facepiece to the aerosol instruments. In addition, the effects of breathing pattern (tidal volume: 0.5–1 L, frequency: 5–20 times/min) and lung deposition (with/without HEPA filter behind the respirator) on in-mask particle concentration during fit testing were analysed, to explore the minimal sampling time that approximated the FFt.The particle measurement response times for Portacount and OPS were approximately 5 and 2 s, respectively. For P100 respirators, most measured fit factors were close to the FFt. Whereas, there was an underestimation while using N95 respirator due to filter penetration. Therefore, N95-companion was necessary while testing N95 respirator. For the cyclic flow tests, the fit factor was overestimated because the sampling tube was connected onto the facepiece where filtered air was partly sampled. The higher the breathing flow rate, the more the fit factor was overestimated. On the other hand, the measured fit factor would be close to the FFt when using the highest concentration during a breathing cycle (FFmin). In theory, it could be decided in only one breathing cycle.With improved design in instrumental setting and operating procedures, a fit test for an individual exercise would take approximately only 12 s. Therefore, the whole fit testing process could be shortened from 7.5 to about 3 min.