A low-cost real-time Respirator Seal Integrity Monitor (ReSIM) was recently developed to monitor a respirator's actual performance at a workplace. The objective of this study was to evaluate the capability of the new ReSIM prototype in manikin-based laboratory experiments to rapidly detect induced leakage of a half-mask elastomeric respirator. Two phases of testing were conducted in this study. First, the accuracy of ReSIM measuring an aerosol concentration was assessed by comparing the outputs of ReSIM against a reference optical aerosol spectrometer (OAS) in a flow-through set-up. Second, the capability to detect a leak was tested using a manikin-based set-up to simulate leaks into a functional respirator. The regression curve of ReSIM versus OAS had an R2 of 0.936, indicating its high accuracy within the targeted particle size range of 0.5-2 µm. The ReSIM provided a leak detection sensitivity (probability of correctly identifying intervals with the true leak) of 98.4% when challenged with a combustion aerosol, compared to 71.8% when challenged with a NaCl aerosol. Its specificity (probability of identifying intervals without a leak) was 99.8% after adjusting for persistent false positives for both types of challenge aerosol. The ReSIM prototype not only can estimate the particle concentration with high accuracy but also can rapidly detect respirator faceseal leakage in real time with sufficient sensitivity and specificity. In addition, it can trigger an alarm when the faceseal integrity is compromised.