The respirator fit monitor described herein can be worn continuously by users so as to provide an indication as to how well their masks are fitting during use, thereby providing quantitative, wearable fit testers available for continuous use in real-life situations. The monitor includes a low-cost optical particle sensor assembly and controller unit for performing mask fit tests by comparing particle concentrations inside and outside of the mask. The fit test monitor is low cost and wearable, capable of dual sampling, capable of fit factor ratios well above 100, is battery powered and provides near real time measurements with a means for indicating the fit of the mask. The system includes wired or wireless communications for data logging, analysis and display capabilities.